physics capsule

7/23/2019 Physics capsule

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PHY PHY PHY PHY PHY SICSSICSSICSSICSSICS

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1. Units & Measurement ............................................................................................ 5-6

2. Motion ..................................................................................................................... 7-18

3. Work, Energy and Power ..................................................................................19-24

4. Gravitation............................................................................................................25-27

5. Properties of Matter ............................................................................................ 28-35

6. Space Exploration ............................................................................................... 36-43

7. Simple Harmonic Motion .................................................................................44-47

8. Sound .................................................................................................................... 48-52

9. Optics.....................................................................................................................53-65

10. Heat ........................................................................................................................ 66-78

11. Electromagnetic Radiation................................................................................79-83

12. Atomic Physics ....................................................................................................84-93

13. Astronomy .......................................................................................................... 94-102

14. Communication System ................................................................................ 103-107

15. Electromagnetism............................................................................................108-122

16. Physics at a Glance-I ......................................................................................123-152

CONTENTS

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Physics [5]

All the quantities in terms of whichlaws of physics are described and whosemeasurement is essential are called physicalquantities like mass, length, time, light,

current electricity, temperature, etc.

UNIT

The fixed part of a Physical quantity bydint of which comparision could be madeof the magnitudes of the same quantitycontained in different substances is termedas a unit.

Systems of units Length Mass Time

1. C.G.S. System Centimetre Gram Second

2. F.P.S. System Foot Pound Second

3. M.K.S. System Metre Kilogram Second

The measurement of a physical quantityrequires a unit and the comparision of unitto the quantity & the number of the unitsobtained gives the measured value of thequantity e.g. when we say that the length of a rod is 5 metres it means that the length of the rod is 5 times the unit of length, metre.

FUNDAMENTAL UNITS

(i) Metre (m) : 1 metre is equal to thelength of the path travelled by light invacuum in 1/2 99792458 of a second wherespeed of light is 299792458 m/s. Recently,the definition of 1 metre of length is realized by using the iodine stabilized helium neonlasers.

(ii) Kilogram (kg) : A standard block of

Platinum Iridium alloy preserved in the

International Bureau of Weights & Measures

at Sevres, near Paris, France is used as a

prototype of unit of mass and one kilogramis equal to the mass of this alloy.

(iii) Second (s) : The definition of second

is based on an atomic clock which works

on energy radiation from an isotope of

caesium (Cs-133). One second is equal to

the duration of 9, 192, 631, 770 periods of

International System of Units (S.I.)

The system is a modification over the MKS systemand hence, is known as rationalised MKS System.

There are seven fundamental and two supplemen-tary units in S. I. System :

Physical Fundamental Symbol

1. Mass Kilogram kg

2. Length Metre m

3. Time Second s

4. Tempertature Kelvin K

5. Electric-current Ampere A

6. Luminous intensity Candela Cd

7. Quantity of matter Mole mol

Supplementary Supplementary SymbolPhysical quantity Unit

1. Plane angle Radian rad.

2. Solid angle Steradian sr

The units of three quantities viz mass,length and time are called fundamentalunits because all physical quatities can beexpressed in terms of mass, length andtime and remaining all units are calledderived units.

UNITS &MEASUREMENT



[6] General Science

radiation corresponding to unperturbed

transition between the two specific energylevels of the ground gaseous state of Cs-

133 isotope. The caesium atoms in the

atomic clock act like a pendulum in a

pendulum clock. The atomic clock gives

the most accurate time with an error of 1

second only in 5000 years.

IMPORTANT PRACTICAL UNITS

For Lengths : (i) Astronomical Unit

(A.U) : It is equal to the distance betweenthe centre of the earth and the centre of the

sun. One A.U. = 1.5 × 1011m.

(ii) Light Year (ly) : It is equal to the

distance travelled by light in vacuum in

one year. 1 light year = 9.46 × 1015m.

(iii) Par sec. (Parallactic second) : 1

Par sec = 3.1 × 1016m = 3.26 ly

(iv) 1 micrometre or 1 micron = 10–6m

(v) 1 nanometre (1nm) = 10–9m

(vi) 1 Angstrom (1A°) = 10–10m

For Masses : (i) 1 tonne or 1 metric ton

= 1000 kg

(ii) 1 pound (lb) = 0.4536 kg.

(iii) The largest unit of mass is Chandra

Shekher Limit (C.S.L) 1 C.S.L. = 1.4 timesthe mass of the Sun.

(iv) For small masses atomic mass unit

(a.m.u.) is used. 1 a.m.u = 1.6 × 10–27kg.

For Area : (i) 1 acre = 4047 m2.

(ii) 1 hectare = 104m2.



A body is said to be in motion if itchanges its position with respect to itssurroundings as time goes on. A body issaid to be at rest if it does not change itsposition with time, with respect to itssurroundings.

TYPES OF MOTION

(i) When a particle or a body movesalong a straight path, its motion isRectilinear or translatory motion.

(ii) When a particle or a body moves in acircular path, its motion is circularmotion. When a body spins about itsown axis, it is said to be in rotational

motion.

(iii) When a body moves to and fro or backand forth repeatedly about a fixed pointin a definite interval of time, it is said to be in vibrational or oscillatory motion.

The path travelled by an object during itsmotion is called trajectory. The actual pathlength during the motion is called distanceand, the straight distance between the initialand final position of the motion in a particu-lar direction is called displacement.

Let a particle travel, starting from pointA and go to point D along the path A B CD in a given interval of time. The total pathlength ( = AB + BC + CD) is the distancetravelled and the shortest path length (AD)in the direction A to D is the displacementwithin the same time-interval.

SPEED

The time rate of change of position of an object in any direction i.e. the rate of change of distance of an object with respectto time is known as speed.

VELOCITY

The rate of change of displacement of an object with respect to time is known asvelocity.

Let a square OPQR of side length 2metre. A particle travels along its sidestarting from O to R via P and Q. It takes atotal time of 2 seconds. The total distance

travelled is OP + PQ + QR = 2 + 2 + 2 = 6metres whereas the total displacement isOR = 2 metres. Hence

MOTIONS



[8] General Science

Average Speed = 3m/s

Average Velocity

ACCELERATION

The rate of change of velocity with re-spect to time is called acceleration.

When a body completes equal displace-ment in equal interval of time, its velocity isconstant and hence, it does not have anacceleration. When a body shows equalchange in velocity in equal interval of timeits velocity is not constant but it has aconstant acceleration.

EQUATIONS OF MOTION

(i) For a body moving with a uniformvelocity: If a body completes a displace-ment ‘S’ in time ‘t’ with a uniform velocity‘V’, then,

displacement = velocity × time

or ...(i)

(ii) For a body moving with a uniformacceleration : If a body starting with aninitial velocity ‘u’ moves with a uniformacceleration ‘a’ for a time ‘t’ and attains a

final velocity ‘v’ after travelling a displace-ment ‘s’ then,

....(iii)

and ...(iv)

When the velocity of a body increases,

it has a positive acceleration and when thevelocity decreases, it has a negative accel-eration. This negative acceleration is calleddeceleration or retardation.

When a body is released from a height,its velocity increases by 9.8 m/s in everysecond and when a body is thrown abovethe earth’s surface, its velocity decreases by 9.8 m/s in every second. This change invelocity every second is called accelerationdue to gravity which is denoted by ‘g’. Its

average value at the earth’s surface is 9.8m/s2. It is always directed towards thecentre of the earth because of the gravita-tional pull. For a freely falling body, itsacceleration is 9.8 m/s2.

POSITION (DISPLACEMENT)-TIME GRAPHS

(i) For a body moving with a uniformvelocity: This graph comes as a straightline because in a uniform velocity the

particle completes equal displacement inan equal interval of time.

(ii) For the motion of a body thrownvertically upwards : When the body movesup, its velocity continuously decreases dueto gravity and finally becomes zero at the

maximum height. Then, the body falls withan increasing velocity.



Physics [9]

The slope of the position time graph is

equal to the uniform velocity.

VELOCITY-TIME GRAPH

For a uniformly accelerated motion thevelocity-time graph is a straight line.

The area under the velocity-time graph

is equal to displacement.

∴ Displacement = Area under velocity

time graph

= Area of ∆OAB = × AB × OB

Where = Average velocity (Var.)

or

∴ V = u + at

The slope of the velocity time graph isequal to acceleration.

In the figure, slope

and OB = time (t)

or,Example 1 : A car is travelling with a

velocity of 20 m/s at a uniform accelera-

tion of 1 m/s2. Find out its velocity after 10

seconds.

Solution : Given, the initial velocity = u

= 20 m/s. acceleration = a = 1 m/s2. time

interval = t = 10 seconds.

hence, the final velocity = v = u + at

or v = 20 + 1 × 10 = 20 + 10 = 30 m/s.Example 2 : A bus moving at 10 m/s

experiences a constant retardation of 2 m/

s2. due to applied brakes. How much dis-

tance does it cover and how much time

does it take before it stops ?

Solution : Given, initial velocity = u =

10 m/s

Final velocity (when the bus stops) = v = 0

retardation = – 2 m/s2

.

v = u + at or = 5 sec.

It takes 5 seconds to stop.



[10] General Science

= 50 – 25 = 25m

or, v2 – u2 = 2as

= 25m.

It covers a distance of 25 m before it stops.

Example 3 : A train travelling with aninitial velocity of 4 m/s accelerates uniformlyat a rate of 2m/s2. for 20 seconds. Calculate

the distance covered.Solution : Given, the initial velocity = u

= 4m/s

acceleration = a = 2m/s2.

interval of time = t = 20 second

∴ Displacement,

or s = 80 + 400 = 480m.Example 4 : A stone is dropped from

rest from the top of a tower. What will beits velocity after 2 seconds and what is theheight of the tower if the stone falls in 5seconds onto the ground ?

Solution : Here a = g = 9.8 m/s2.

u = 0

Hence, after t = 2 seconds, the stonehas a velocity of v = u + at = u + gt = 0 + 9.8

× 2 = 19.6 m/s.Let height of the tower = h

∴ The distance travelled in time = t = 5sec. will be, s = h

∴ s = ut + at2

or h = gt2

Example 5. A stone is thrown verticallyupwards with a velocity of 98 m/s. What will be its maximum height attained and time takento achieve this height ?

Solution : a = g = – 9.8 m/s2, u = 98m/s2.

At the maximum height (h) = s = h andv = 0

∴ v2 – u2 = 2as

∴ 0 – (98)2 = (–)2 × 9.8 × h

∴ v = u + at

∴ 0 = 98 – 9.8 × t or t = 10 seconds

PHYSICAL QUANTITIES

(i) Vectors : They have a definitemagnitude and a definite direction, e.g.displacement, velocity, acceleration, force etc.

(ii) Scalars : They have definitemagnitudes only and not direction. e.g.distance, speed, work, energy, power,electric charge etc.

(iii) Tensors : They have different

magnitudes in different directions, e.g.Moment of interia, stress etc.

In a motion, a body can have a constantspeed but variable velocity like the motionof a body along a circular path. A particlemay have zero displacement and zerovelocity but non-zero distance and speed.When a body completes one revolution



Physics [11]

along a circular path in a given time period,

the net displacement and velocity of the body will be zero but the distance andspeed of the body must be non-zero.

The velocity and acceleration of a bodymay not necessarily be in the same direc-tion and may not be zero simultaneously.The body in equilibrium may be at rest ormay move with a constant velocity. Whena body is thrown upwards, it will go verti-cally until its vertical velocity becomes zeroand it will return to the ground with the

same velocity with which it was thrown.When a body is thrown horizontally

from a height or dropped from the sameheight in both cases it will be reaching tothe ground simultaneously because in boththe cases the body will be acted upon by thesame vertically downward acceleration dueto gravity (g).

A physical quantity having directionmay or may not be a vector e.g. time, pres-sure, current-electricity, surface-tension etc.

They have direction but are not vectors.

Linear-Momentum : It is the quantityof motion which a body possesses and ismeasured as the product of the mass andvelocity of the body.

Linear momentum = mass × velocity.

Impulse : The total change in momen-tum is called the impulse. If a very largeforce acts for a very small time, the prod-uct of force and the time is equal to the

impulse.Inertia : The inability of a body to

change by itself its state of rest or state of uniform motion along a straight line iscalled inertia of the body. The inertia of a body is measured by its mass. Heavier the body, greater is the force required to changeits state and hence greater is its inertia.

Inertia of a body may be inertia of rest,

inertia of motion or inertia of direction.

NEWTON’S LAWS OF MOTION

First Law of Motion

Every body continues to be in a stateof rest or uniform motion in a straightline, except in so far as it may be com-pelled by force to change that state.’Newton’s first law of motion defines in-ertia.

1. Inertia of Rest : the inability of a body to change by itself its state of rest.

When a branch of a fruit tree is shaken,the fruits fall down. This is because the branch comes in motion and the fruitstend to remain at rest. Hence, they getdetached.

The dirt particles in a durree fall off if itis stricken by a stick. This is because thestriking sets the durree in motion

whereas the dirt-particles tend to re-main at rest and hence fall.

When a train starts suddenly, the pas-senger sitting inside tends to fall back-wards. This is so because the lower partof the passenger’s body starts movingwith the train but the upper part tendsto remain at rest.

If a smooth paper having a coin on itplaced on a table is suddenly drawn,the coin remains at the same place on

the table due to inertia of rest. When a horse starts suddenly, the rider

tends to fall backwards due to inertiaof rest

2. Intertia of Motion : The inability of a body to change by itself its state of uniformmotion.




When a horse at full gallop stops sud-

denly, the rider on it falls forward be-cause of inertia of motion of the upperpart of the rider’s body.

When an athelete takes a long jump, heruns first for a certain distance beforethe jump. This is because his feet cometo rest on touching the ground and theremaining body continues to move ow-ing to inertia of motion.

When train stops suddenly, a passen-ger sitting inside tends to fall forward.It happens because the lower part of the passenger’s body comes to rest withthe train but the upper part tends tocontinue its motion due to inertia of motion.

A person jumping out of a speedingtrain may fall forward due to inertia of motion of his body. Hence, he shouldrun a few steps on the platform in thedirection of motion of train.

3. Inertia of Direction : The inability of a body to change by itself its direction of motion.

The wheels of any moving vehicle throwout mud, if any, tangentially, due tothe inertia of direction. The mud-guardsover the wheels stop this mud, protect-ing the clothes, etc. of the person sittingon the bike.

Use of an umbrella to protect us fromrain is based on the property of inertiaof direction because the rain drops can-

not change their direction of motion. When a bus or a car rounds a curve

suddenly, the person sitting inside isthrown outwards. It happens so be-cause the person tries to maintain hisdirection of motion due to directionalinertia while the vehicle turns.

When a knife is sharpened by press-

ing it against a grinding stone, thesparks fly off tangentially becauseof the inertia of direction.

When a stone tied to one end of a stringis whirled and the string breaks sud-denly, the stone spins off along the tan-gent of its circular path. It happens so because of the pull in the string wasforcing the stone to move in a circle. Assoon as the string breaks, the pull dis-appears. The stone becomes free and in

a bid to move along the straight lineflies off tangentially.

Second Law of Motion

‘The rate of change of linear momen-tum of a body is directly proportional tothe external force applied on the bodyand this change takes place always inthe direction of the applied force’.

The second law gives us a measure of force. When a force is applied on a body,

its momentum and hence, velocity change.The change in velocity produces an accel-eration in the body. The rate of change of linear momentum with time is equal to theproduct of the mass of the body and itsacceleration which measures the magni-tude of the applied force i.e.

= mass × acceleration

or,

When a body is moving with a uni-form velocity along a straight line, it nei-ther experience nor require an externalforce. This is because, the acceleration isdue to change in the velocity of the bodyand the velocity remains constant for a



Physics [13]

body moving with a uniform velocity along

a straight line.

When a body changes its velocity ordirection of its motion, its velocitychanges too. It results in an accelerationwhich is possible only by the action of an external applied force. Hence, an ac-celerated motion is always due to anexternal force.

Application of the change in linearmomentum (impluse) and second lawof motion :

Bogies of a train are provided with the buffers. These buffers avoid severe jerksduring shunting of the train. Since force= change in momentum/time and thetime of impact increases due to pres-ence of buffers. Hence, force during jerks decrease. It results in decrease inthe chances of damage.

Crockery items are wrapped in paperor straw pieces before packing because

paper or straw acts as buffers. Itchanges the time of impact and hence,avoids the chances of damage duringthe jerks.

An athlete should stop slowly, after fin-ishing a fast race, so that the time of impact of his run increases at stop andhence, force experienced by him de-creases.

In cricket, a player lowers his handswhile catching a cricket ball to avoid

injury. In doing so, he increases the timeof impact of the ball which in turn re-duces the effect of the force on his hands.

Shockers in the motor-vehicles re-duce the effect of jerk/force by in-creasing the time of impact of the jerks given by an uneven road.

In a head-on collision between two ve-

hicles, change in linear momentum isequal to the sum of the linear momentaof the two vehicles. Since time impact isvery small, hence an extra large forcedevelops which results in maximumdamage to the vehicles.

When a person falls from a height on aconcrete floor, the floor does not yield.The total change in linear-momentumis produced in a very small interval of time. Hene, the floor exerts a much

larger force and the person receivesmore injury. But when a person fallson a heap of sand, the sand yields. Thesame change in linear momentum isproduced in a much longer time. Theaverage force exerted on the person bythe heap of sand is, therefore, muchsmaller and hence the person is nothurt.

Third Law of Motion

‘’To every action, there is always, anequal and opposite reaction.’’

Here, the action is the force exerted byone body on the other body while the reac-tion is the force exerted by the second bodyon the first.

Significance of Third Law : It signi-fies that forces in nature are always inpairs. A single isolated force is not pos-sible. Force of action and reaction act al-ways on different bodies. They never can-

cel each other and each force produces itsown effect. The forces of action and reac-tion may be due to actual physical contactof the two bodies or even from a distance.But they are always equal and opposite.This third law of motion is applicablewhether the bodies are at rest or they arein motion. This law is applied to all types




of forces e.g. gravitational, electric or mag-

netic forces, etc.

Example and application of the thirdlaw of motion :

A book placed on a table exerts a forceas an action on the table. This action isequal to the weight of the book. Thetable exerts a force of reaction equaland opposite to the reaction to supportthe book.

When a gun fires a bullet, it moves

forward due to a force exerted by thegun. The bullet exerts a reaction due towhich the gun recoils backward.

We can walk on a ground easily if it istough because the ground provides suf-ficient reaction against our push. But itis difficult to walk on sand or ice. Thisis because on pushing, sand gets dis-placed and reaction from sandy groundis very little. In case of ice, force of reaction is again small, because friction

between our feet and ice is very little. When a rubber ball is struck against a

wall or floor, it exerts a force as anaction on the wall. The ball reboundswith an equal and opposite force asreaction exerted by the wall on the ball.

A swimmer pushes the water with aforce of action in backward directionwhile water pushes the swimmer witha force of reaction in the forward direc-tion. Consequently, the swimmer is able

to swim. When a jet-plane or rocket moves in

the sky, the gases produced due to combustion of fuel escape through thenozzle in the backward direction dueto the force of action exerted by theengine. The escaping gases exert a force

of reaction on the jet-plane or rocket in

the forward direction. Consequently,the jet-plane or rocket moves.

PRINCIPLE OF CONSERVATIONOF LINEAR MOMENTUM

The total sum of the linear momen-tum of all bodies in a system remainsconstant and is not affected due to theirmutual action and reaction. It means ina system of the two bodies, the totalmomentum of the bodies before impactis equal to the total momentum of thetwo bodies after impact. The law of conservation of linear momentum isuniversal i.e. it applies to both, the mi-croscopic as well as macroscopic sys-tem.

Some common applications of theprinciple of conservation of linearmomentum:

When a person is lying on a frictionless

surface at rest, his momentum is zero.As soon as he blows air out of his mouthor throws an object, he moves in theopposite direction. The total sum of mo-mentum of the person and air blown orobject thrown remains zero due to op-posite directions.

When a man jumps out of a boat tothe shore, the boat is pushed slightlyaway from the shore. The initialmomentum of the man and boat re-

mains equal to that of the finalvalue.

The gun must be held tightly to theshoulder when the gun is fired. It wouldsave hurting the shoulder

Motion of rocket and jet planes is basedon the conservation of linear momentum.



Physics [15]

Out of the three laws of motions, the

second law is the real law because it in-cludes remaining both the first law and thethird law.

UNIFORM CIRCULAR MOTION

When a body moves along a circularpath or a curve with a uniform circularspeed, the body is acted upon by an in-ward acceleration. This acceleration actstowards the centre of a circular path orcurve and is called as radial or centripetalacceleration which gives rise to the cen-tripetal force. The centripetal force is anessential condition of the circular motion.Centripetal force (F

c) = mass of the body

(m) × centripetal acceleration (ac)

or,

centripetal acceleration (ac) = vw

= rw2.

where v = linear speed, w = angularspeed or, r = radius of circular path orcurve.

The centripetal force acting on a bodyis an action and an equal and oppositeforce called centrifugal force appears as areaction.

Application of centripetal force :

When a bucket containing water iswhirled in a horizontal or vertical di-rection water does not fall down on theground.

In a circus, a motor cyclist is able to

perform the feat of driving the motor

cycle along a vertical circle in a cage.

The motor cyclist does not fall downeven at the highest point.

A pilot of an aircraft can successfully

loop a vertical loop without falling at

the top of the loop being without belt.

Motion of vehicles on a curved road :

(a) Level Curved Road : A level curved

road is constructed where the speed of

the vehicles is slow. Here, the force of

friction between the road and tyre of the wheel of the vehicle provides the

necessary centripetal force.

(b) Banking of Roads : At the highways

where vehicles run fast, the frictional

force is not a reliable source for provid-

ing the required centripetal force to the

vehicle. Hence, at such curved roads, a

safer course of action is to raise the

outer edge of the curved road above

the inner edge. It is known as bankingof roads. The banking of roads pro-

vides the required centripetal force.

A cyclist leans forward while going along

a curve. By doing so, the ground pro-

vides him the centripetal force which he

requires for turning. Hence, the cyclist

leans inwards from his vertical position.

In an atom, the required centripetal force

for an electron in its circular orbit is pro-vided by the electrostatic force of attrac-

tion between the electron and nucleus.

The force of gravitation provides the

essential centripetal force when a satel-

lite revolves around a planet or a planet

revolves around the sun.




ROTATIONAL MOTION

Torque (Moment of Force)

The product of force acting on a bodyand perpendicular distance of line of actionof the force from the axis of rotation iscalled moment of force or torque.

Torque = Force × Perpendicular distancefrom axis rotation

Applications of Torque :

Torque due to a force is maximum, thedistance from the axis of rotation ismaximum. We can open or close a dooreasily by applying force near the edge of the door i.e. at maximum distance fromthe hinges. Hence, a handle or knob isfitted near the free edge of the plank of thedoor. A wrench with a long arm is requiredto unscrew a nut fitted tightly to a bolt.Longer the arm of the wrench, smaller isthe required force to give sufficient turningeffect.

Angular Momentum

It is equal to the product of linearmomentum of a body and the perpendiculardistance from the axis of rotation. It followsthe principle of conservation. It means thetotal angular moment of an isolated systemremains always constant.

Applications of conservation ofAngular Momentum :

(i) The angular velocity of revolution of aplanet around the sun in an ellipticalorbit increases, when the planet comescloser to the sun and vice-versa.

(ii) A circus acrobat performs featsinvolving spin by bringing his armsand legs closer to his body and vice-

versa. It is because in doing so the

angular speed increases.

(iii) Consider a ballet dancer is rotatingwith her arms and legs stretchedoutwards. When she folds her armsand brings the stretched legs close tothe other leg, her angular speedincreases.

(iv) Due to the same reason, the angularspeed of the inner layer of the tornado(whirlwind) is extremely high.

(v) All helicopters are provided with twopropellers. If there was one singlepropeller, the helicopter would rotateitself in an opposite direction inaccordance with the laws of conservation of angular momentum.

FRICTION

When a body moves (slides or rolls) oreven tries to move over the surface of another body a tangential force comes intoaction between their surfaces in contact,against their relative motion. This opposingforce is termed as the force of friction.

The force of friction depends upon themass of the body on a surface androughness of the surfaces in contact between them and the magnitude of friction, which increases with increase inroughness and mass.

When a body is at rest on a surface, the

friction is called static friction which is a self adjusting force. When the body is on theverge to move (slide or roll), the friction iscalled limiting friction but when the bodymoves, it gives rise to dynamic friction. Thelimitting friction is the maximum force of friction and it is always more than static ordynamic friction.



Physics [17]

Usually, smoothness decreases the force

of friction. However, when the surfaces incontact are made too smooth by polishing,the binding force of adhesion increases andhence, the frictional force increases. This iscalled ‘cold welding’.

Friction is a non-conservative forceand hence, the mechanical energy(potential and kinetic energy) is notconserved. In fact, friction convertsmechanical energy partly into heat, light(spark), sound, electricity, etc.

Generally, friction opposes motion.However, in certain cases friction isessential for motion. Without friction,motion cannot be started, stopped ortransferred from one body to the other.Thus, friction is a necessary evil.

Advantages of Friction

When a person pushes the ground back-ward, the rough surface of the groundexerts a forward force due to friction. Itmakes possible a person to move on theground. Due to lack of sufficient fric-tion on ice or wet soil, it is difficult towalk.

Two bodies stick together due to fric-tion.

The working of the brakes of vehicles ispossible due to friction only.

The friction between the tyres of vehicles and road makes the motion of the vehicles possible.

The cleaning action of sand-paper oc-curs due to friction only.

In absence of friction, we would not beable to hold a pen and if we could, thepen would not write on paper.

Writing on the black board with a chalkis possible due to friction.

The transfer of motion from one part of

a machine to the other part through belts is possible by friction.

The working of nuts and bolts for hold-ing parts of machinery together is basedon friction.

The knots in woven clothes are possibledue to friction.

Disadvantages of Friction

Since, the force of friction opposes therelative motion between any two bod-ies in contact, hence, extra work is doneto overcome the force of friction. It in-volves extra loss of energy. About 20%of the petrol used in an automobile isused up to overcome the force of fric-tion in the engine and in driving.

The force of friction results in heatingthe working parts of the machinery thatmay damage the parts.

Friction causes wear and tear of theparts of the machinery.

The force of friction can be reduced :

By poli shing : polishing causes smooth-ness.

By l ubr icat i ng : The lubricants, oils,grease etc. fill up the irregularities of the surfaces and hence, the surfaces become smoother.

By using ball-bearings

The force of friction can be increased : by applying sand on the sl ippery

ground.

by applying sand on the road coveredwith snow.

by making depressions, projections, etc.in the tyres during manufacturing.




Order of the magnitude of the

force of friction is:

Rolling friction < sliding friction < lim-iting friction.

Owing to least value of rolling frictionwheels are used in vehicles.

TRIVIA

When a particle goes from one pointto another, the actual length of thepath is called distance covered. The

average speed is defined as the dis-tance covered per unit time. In theabove case, the straight line distance between the initial and final positionsis called magnitude of displacement.The average velocity is defined as mag-nitude of displacement per unit time.

Distance covered is always a + VE quan-

tity. And it never decreases with time.

For a moving particle it cannot be zero.

Distance covered ≥ magnitude of dis-placement.

Speed is always a + VE quantity.However, it can increase or decreasewith time.

When a particle returns to the startingpoint its average velocity is zero butaverage speed is not zero.

For uniform motion distance covered =

magnitude of displacement. The mo-tion is along a straight line and its di-rection cannot change.

If a body covers first half distance withspeed v

1, and the second half distance

with speed v2, then average speed

= Harmonic mean.



Physics [19]

WORK

When a force is applied on a body and

a displacement is carried out in any

direction except in a directionperpendicular to the direction of the force,

an amount of work is done by the force.

The amount of work done is equal to

the product of the force and the distance

travelled in the direction of the applied

force i.e.

or,

Unit of work is Joules(s) 1 joule =Newton × 1 metre.

Work done by a force may be zero,

positive or negative depending upon the

direction of the applied force and

displacement.

When a coolie is carrying some load on

his head and is waiting for the arrival of

the train, he is not doing any work. No

mechanical work is done by a teacher

teaching a class. When a body falls freelyunder the action of gravity, work done by

gravity on the body is positive. When a

body is pushed or pulled by a force, work

done is positive. The force of friction acts

against the motion, hence the work done

by the frictional force is negative.

POWER

The time rate of change of work is

power. When a body takes less time to do a

certain work, its power is said to be moreand vice-versa.

Its unit is watt (w). One kilowatt (1 kw)

is equal to 1000 watt. One horse power

(h.p.) is equal to 746 watt. Power of an

agent measures how fast it can do the

work. The area under the force versus

distance graph is numerically equal to the

work done by the agent.

Work = Force × Distance

The area under power-time graph gives

the work done while the slope of work

versus time graph gives the power.

WORK, ENERGY

AND POWER




Work = power × time = area under w-t

graph

or,

or, = slope of w-t

graph.

ENERGY

The ability of a body to do work is calledenergy. When a body can do more work, it

is said to have more energy and vice versa.Energy is different from power. Energy refersto the total amount of work a body can doand power determines the rate of doingwork. Both the energy of a body and workdone by the body are equivalent and aremeasured in Joule (J).

Kinetic Energy (K.E.)

It is the energy possessed by the body by virtue of its motion. The kinetic energy

of a body is given as

Where m = mass of the body and v =velocity of the body. Thus, K.E. of a body isequal to half the product of mass of the body and square of velocity of the body.The change in K.E. of a body measures thework done by the body.

or,

Where u and v are initial and finalvelocities of the body of mass m.

When a heavy and a tight body are

moving with same K.E. and same retarding

force is applied on each, both the bodieswill stop after travelling the same distance.

K.E. of a body is also given as :

or,

Hence, when a light and a heavy body

are moving with the same linear momentum,the light body will have more K.E.

Every moving system is associated witha definite amount of K.E. e.g. a movingvehicle, wind, water flow, etc.

Potential Energy (P.E.)

The energy possessed by a body by virtueof its position or configuration is known asits potential energy. The mechanical P.E. isof two types viz., gravitational P.E. and

elastic P.E. The gravitational P.E. of a bodyat a certain height is due to gravity whereasthe elastic P.E. is due to its property of elasticity.

Gravitation P.E. = mass × accelerationdue to gravity × height = mgh

At the surface of the earth, h = 0, ∴P.E. = 0

Different Forms of Energy

(i) Heat : It is the energy possessed by a body by virtue of random motion of the molecules or particles of the body.

(ii) Internal Energy : It is the energy of a body due to the molecularconfiguration and molecular motion.

(iii) Electrical Energy : This energy arises



Physics [21]

due to work done in moving free

charge carriers in a particular directionthrough a conductor.

(iv) Chemical Energy : It is the energypossessed by the body by virtue of chemical bonding of its atoms.

(v) Nuclear Energy : It is the energyreleased during the nuclear reactiondue to conversion of mass into energy.

Mass-Energy Equivalence

Mass and energy are equivalent to eachother and can be interconverted according

to Einstein’s equation or

m = E/c2.

Where E = energy, m = mass and, c =speed of light in vacuum = 3 × 108 m/s.

If mass = 1 kg, the energy released is

E = 1 kg × (3 × 108)2 m/s2 = 9 × 1016

Joule

This law unifies the two laws viz thelaw of conservation of mass and the law of conservation of energy. Most of the energyin the universe i.e. energy in stars isobtained due to conversion of mass intoenergy.

Principle of Conservation of Energy

The total sum of all kinds of energy of anisolated system remains constant at all times.Energy neither can be created nor be destroyed but can be transformed from one form to

another. The amount of energy appearing inone form is exactly equal to the energydisappearing in some other form always.

When a body is at rest at a certainheight above the surface of the earth, itonly has an amount of potential energy.As soon as the body is released from that

height, its P.E. begins to decrease and this

decrease in P.E. appears as an increase inits K.E. When the body arives at the ground,its P.E. becomes zero and this appears asK.E.

Sources of Energy

The sources of energy are immense andthey are divided into two groups viz.

(i) Renewable Source : These sourcesof energy are inexhaustible and are beingcontinuously supplied by nature e.g. wind,

flowing water, the sun, ocean tides, biogas,plants and vegetable waste, etc.

(ii) Non-renewable Source : They areexhaustible and have been formed in naturelong ago e.g. coal, petroleum, natural gas,fissionable materials like uranium.

Wind Energy

Air in motion is wind which has kineticenergy called wind energy.

Causes of W ind

The equatorial region receivingperpendicular sun-rays has warmer airthan that in the polar region where sun-rays are slanting. The warm air rises up inthe equatorial region. The air from polarregions rushes towards the equatorialregion to fill the gap. This causes the air tomove. Moreover, local weather conditionsalso contribute to the movement of air. If the speed of air is small, then the wind is

known as breeze. On the other hand, if thespeed of air is very large, then the wind isknown as storm or tornado.

Uses of W ind Energy

(i) Sail Boats : A boat can be rowedwith oars using muscular energy which isa tiresome job for a longer distance. Hence,




a sail is used by utilizing K.E. of the wind

to move the boat. A sail is a large sheet of thick cloth spread over a boat in such away that maximum wind falls on it. Thesail boat moves due to wind. The speedand direction of motion of the soil boatdepend mainly upon the wind.

To move the boat in any desireddirection, two sails at right angles to eachother are attached to the same pole fixedon the pole . A steering device known asrudder is used to turn these two sails

through any angle. One of the sails is usedto direct the wind and the other sail collectsthis wind. The kinetic energy of the windcollected by the second sail (propeller)moves the boat in th desired direction.

(ii) Wind Mill : A device used to convertwind energy into the mechanical energy of the machine is called wind mill.

Wind energy is pollution free andeconomical. However, it is limited to certainplaces where wind is in plenty and blows

most of the time. In India, there are somehigh wind energy regions like Islands of Bay of Bengal and Arabian Sea, coastalparts of Gujarat and Tamil Nadu, someparts of Rajasthan, Karnataka and WesternParts of Madhya Pradesh. Government of India has proposed to install a number of projects to tap the wind energy. In Gujarat,two wind power energy stations areoperating. One is at Lamba in Porbandardistrict. This power station is producing

about 2000 million units of power everyyear. The second power station operatingwith wind energy is in Okha.

Tidal Energy

The alternate rise and fall of water of theocean twice in nearly 24 hours is known as atide. The tides are produced due to the

gravitational force of attraction exerted by

moon and to some extent by the sun on thewater of the ocean. At the time of new andfull moon, when the sun and the moon are ina straight line, tides are very high. When thesun and the moon are at right angle from theearth, tides are low. The K.E. of the waterduring tides is used to produce electricity.

Tidal power plant is made near narrow bays. During tides, the gates of the damare opened. The rising water is allowed tofall on the turbine of the generator which

produces electricity and the K.E. of thewater is converted into electrical energy.During low tides, gates of the dam areclosed and hence the water level behindthe dam rises. This raised water has highpotential energy. Again the gates areopened and the water falls on the turbine.Thus, the electricity is producedcontinuously.

France and Canada are the leadingcountries which harness the tidal energy.In India, three sites viz. Gulf of Kutch

(Gujarat), Gulf of Cambay (Gujarat) andSunderbans (West Bengal) have beenidentified to construct tidal power plants.

Ocean Thermal Energy

It is heat energy obtained due to thetemperature difference between thedifferent layers of water in the ocean. Thesurface temperature of the ocean is muchmore than the inner deep ocean. Due to

this temperature difference, heat energycan be drawn to produce electricity.

The vegetation and biomass found atthe sea bed can be used to produce heat.Different oceans have differentconcentrations of salt. Hence, electricitycan be produced at the site where differentoceans meet.



Physics [23]

Geothermal Energy

The inner part of the earth is very hotand hence, hot underground water comesautomatically out of the earth’s surface incertain regions in the form of fountainsknown as hot water springs or geysers.The steam of these springs under highpressure can be used to rotate the turbinesof the generator to produce electricity.

Biogas

Biogas is a mixture of gases like methane(65%), CO

2, H

2 & H

2S formed when the

animal dung mixed with water is allowedto ferment in the absence of air (Oxygen)and it is produced in a biogas plant. Biogasis a very good fuel to produce heat.

The arrangement of producing biogasfrom animals dung, human excreta andindustrial and domestic wastes is knownas biogas plant. There are two types of biogas plants which have been used in ourcountry. These are :

(i) Fixed-dome type biogas plant.

(ii) Floating gas holder type biogas plant.

Fixed-Dome Type Biogas Plant : Itconsists of a well like underground tankmade of bricks and cement. This tank iscalled digester and has inlet and outletvalves. The roof the tank is dome shaped.There is a gas outlet pipe at the top of thedome. The dome of the digester acts as astorage tank of biogas. Thus, the gas-holder

(i.e., dome) is fixed. There is a mixing tankmade above the ground level. On the otherside of the digester, a rectangular tankcalled outlet chamber is constructed with bricks and cement. This outlet chamber isconnected to the overflow tank whichcollects the used slurry.

Floating Gas Holder Type Biogas

Plant : It consists of a well shaped tankinside the ground called digester. Digesteris divided into two chambers with apartition wall. A drum shaped gas holdermade of steel in the inverted position overthe mouth of the digester acts as a storagetank for biogas. This tank moves up anddown over the slurry in the digester tankand hence this gas plant is called floatinggas holder type. The motion of the drumis controlled by a pipe.

Mixing tank is connected to the digesterwith the help of the inlet pipe. The overflowtank used to collect the used slurry is alsoconnected by a pipe. The biogas collectedin the floating tank is taken out with a pipehaving a gas valve.

MACHINE

Machine is a device to overcome load

or resistance applied to it at some point bya relatively small effort applied to it atsome convenient point.

Lifting machine : It is a device to liftheavy load by applying comparativelysmaller force like lever, wheel and axle,inclined plane, etc.




Effort : The force applied to a machine

to overcome load (resistance) is calledeffort.

Load : The resistance (force) to beovercome by a machine is called load.

Mechanical Advatnage (M.A.):

Velocity Ratio (V.R) or Ideal MechanicalAdvantage (IMA)

Efficiency of a machine

Efficiency is always less than one orless than 100%. Ony for ideal machines,the efficiency is one or 100%.

V.R. or I.M.A. for some simplemachines—

(i) Lever : I.M.A.

(ii) Whel nd Ale :

CONSERVATION

Conservation laws can be used todescribe the behaviour of a mechanicalsystem even when the exact nature of the forces involved is not known.

Although the exact nature of the nuclearforces is not known, yet we can solveproblems regarding the nuclear forces

with the help of the laws of

conservation.

Violation of the laws of conservationindicates that the event cannot take place.

Work done depends on the frame of reference.

Work done is path independent only ina conservative field.

Stopping distance of a body

When the water is flowing through apipe with speed v, then its power isproportional to v3.

If kinetic energy of body is doubled, the

its momentum becomes times.

If an elastic collision of two equalmasses, their kinetic energies areexchanged.

If a light and a heavy body have equalmomenta, then lighter body has greaterkinetic energy.

Potential energy of a system increases whena conservative forces does work on it.

If a body of mass ‘m’ moving withvelocity ‘v’, collides elastically with arigid ball, then the change in themomentum of the body is 2 mv.

Work done by a centripetal force is

always zero. When the momentum of a body

increase by a factor n, then its kineticenergy is increased by factor n2.

If the speed of a vehicle is made n times,then its stopping distance becomes n2 times.



Physics [25]

The force of attraction between any two bodies in the universe is called gravitation.The force of gravitation is the weakest forcein the nature. However, it is the most

important force in the universe because itplays an important role in planetary motion,the birth of a star, etc.

NEWTON’S LAW OFGRAVITATION

The gravitational force of attraction between any two bodies in the universe isdirectly proportional to the product of theirmasses and inversely proportional to thesquare of distance between them.

Let two bodies of masses m1 and m

2

respectively having a separation of r between them.

Then from the Newton’s law of gravitation, we get the gravitational force between them

Where G is the universal gravitational

constant. G = 6.67 × 10–11

Nm2

kg–2

. Thevalue of G remains constant anywhere inthe universe.

The Newton’s law of gravitationexplains the rotation of the moon aroundthe earth or earth around the sun. Itcorrectly predicts the solar and lunar

eclipses, and the orbits and time period of the modern artificial satellites. The cause behind the tides in the oceans is mainly thegravitational force of attraction between

the moon and ocean water.

GRAVITY

It is the force of attraction exerted bythe earth towards its centre on a bodylying on or near the surface of the earth. Itis called earth’s gravitational pull. Thus,the gravity is a special case of gravitation.

The force of gravity acting on a body isthe measure of weight of the body. The

force of gravity produces an accelerationin a body and it is known as accelerationdue to gravity. It is denoted by ‘g’. If a bodyis of mass ‘m’, then its weight at the surfaceof the earth is mg.

Weight = mass × acceleration due togravity.

The acceleration due to gravitydecreases above and below the earth’ssurface. Hence, the weight of a bodydecreases with increase in height abovethe earth’s surface or increase in depth below the earth’s surface. The value of ‘g’is maximum at the poles and minimum atthe equator of the earth. At the centre of the earth ‘g’ is zero.

Gravitation




Acceleration due to gravity is

independent of mass, shape, size, etc. of afalling body i.e. a light and a heavy freelyfalling body will attain an equalacceleration which is equal to theacceleration due to gravity (g). The averagevalue of ‘g’ at the surface of the earth is 9.8m/s2. Heavier a planet or satellite, higheris its ‘g’. The value of ‘g’ at the surface of the moon is 1/6th of the value at the earth’ssurface. The value of acceleration due togravity is minimum at planet Mercury andmaximum at planet Jupiter.

SATELLITE

The body revolving around a planetcontinuously in an orbit is called a satellite.Moon is a natural satellite of the earth.Since 1957, many man made satellites have been put in different orbitals around theearth. They are called Artificial Satellites.

A satellite revolves around the planetin an orbit under the effect of the centripetal

force provided by the gravitational force of attraction between the satellite and theplanet. The minimum velocity with whicha satellite is put into its orbital is calledorbit velocity which is given as

Where g = acceleration due to gravity,R = radius of the earth and h = height of

the satellite above the earth’s surface. If h<< R, then = 8 km/s.

(i) Geostationary Satellite : It appearsalways at a fixed location above the earthin its orbit. Its time period of revolutionaround the earth is 24hrs and its height is36000 km above the equator of the earth. It

rotates around the earth from west to east,

similar to that of the earth. Its speed is 3.1kms/s. Its orbit is coplaner to the equatorialplane of the earth.

It is also known as a geosynchronoussatellite.

(ii) Polar Satellite : It revolves aroundthe earth in a polar orbit which is at 90° tothe equatorial plane. It passes over boththe north and south poles of the earth onceper orbit. Hence, every location on earthlies with the observation of polar satellitetwice each day. It is also known as the sunsynchrononous satellite.

In order to monitor 100% earth’ssurface, three geostationary satellites arerequired while a single polar satellite cando the same job.

ESCAPE VELOCITY

The minimum velocity required tothrow a body so that it becomes free fromthe earth’s gravitational pull is called escapevelocity. It is given as = 11.2 km/

s for the earth where g is acceleration dueto gravity and R, the radius of the earth.

EFFECT OF GRAVITY ONWEIGHT OF A BODY

When a body falls freely, its weight becomes zero. This situation is known asweightlessness. In a spacecraft or satellite,

the effective weight of a body is zero becausethe gravitational force is balanced by thecentrifugal force.

When a lift is at rest or is moving up/down with a uniform velocity, the weightof a body inside it remains unchanged.

But, when the lift accelerates upwards,



Physics [27]

the weight of a body in it increases while

downwards the weight of a body decreases.

KEPLER’S LAWS OF PLANETARYMOTION

(i) The First Law : Every planet revolvesaround the sun in an elliptical orbit.

(ii) The Second Law : The average radiusof the elliptical orbit of the planetsweeps out equal areas in equalintervals of time.

(iii) The Third Law : The square of the timeperiod of revolution of a planet isdirectly proportional to the cube of average radius of its elliptical orbit i.e.T2 µ R3.

TRIVIA

Planets revolves around the sun in el-liptical orbits with the sun at one of thefocii.

The gravitational force is much smaller

than the electrical force because thevalue of G is very small.

Gravitation is a conservative force.

Gravitation is a central force.

G is a scalar quantity.

Work done in moving unit mass frominfinity to any point in space is calledgravitational potential at that point.

Gravitational potential is a scalar quantity.

Gravitational potential due to the earthat its surface is given by : V

e = – GM

e|R

e

where Me = mass of the earth and R

e =

radius of the earth.

Unit of gravitational potential is J/kg.

The gravitational intensity inside a hol-low spherical shell is zero.

Variation of g inside the earth is linear.

That is, it increase in direct proportion tothe distance from the centre of the earth.

Decrease in acceleration due to gravityat a depth d is given by : gd/R, where R= radius of the earth.

Orbital velocity very near the surface of the earth is 7.92 km/s.

Greater the height of the satellite smalleris the orbital velocity.

Time period of the satellite very nearthe surface of the earth is about 84.6minutes.

No energy is dissipated in keeping thesatellite in orbit round a planet.

If the total energy (KE + PE) of a satel-lite is negative, its orbit is either circularor elliptical. In this case PE > KE.

If the total energy of the satellite is zero,it escapes away along a parabolic path.

In this case PE = KE. When the total energy of a satellite is

positive, it escapes along a hyperbolicpath. In this case KE > PE.

The escape velocity from the surface of the earth is 11.2 km/s.

If a body falls freely from infinite height,then it reaches the surface of the earthwith velocity 11.2 km/s.

Kinetic energy of escape for a satellite =

2 × kinetic energy of orbital motion.

A body in gravitational field has maxi-mum binding energy when it is at rest.

A hydrogen baloon released at a heighton the moon will fall with an acclerationnearly 1.6 m/s2.




Matter is broadly divided into threecategories, viz. Solid, Liquid and Gas. Dueto the strongest intermolecular force of attraction in solids, they are tough andhave a definite shape and size. This force isrelatively weak in liquids and so the shapeis easily changed but liquids have a definitevolume. In gases, the intermolecular forceof attraction is minimum and hence, theydo not have a definite shape, size andvolume.

There is a fourth state of matter calledplasma state in which matter exists inionised state. Plasma state is common instars.

SOLIDS

In solids the constituent particles (atoms,molecules or ions) are held strongly at theposition of minimum potential energy. Solidsare of two categories :

(i) Crystalline Solids : They have a regularpattern of constituent particles in threedimensional space. Hence, they have adefinite external geometrical shape anda sharp melting point. They are an

isotropic i.e. their physical propertieslike conductivity (thermal andelectrical), refractive index, mechanicalstrength etc. have different values indifferent directions e.g. rice, sugar,quartz, diamond, rocksalt, most of metals & their compounds, etc.

(ii) Amorphous or Glassy Solids : Theyhave irregular arrangement of particles.Hence, they do not have a definiteexternal geometrical shape and sharpmelting point. They are isotropic i.e. theirphysical properties have the same valuein all directions e.g glass, cement,rubber, paraffin, plastic, etc.

LIQUIDS

Pressure due to a liquid column depends

upon its density and height

Where h = height of liquid column andd = density of liquid. If equal amounts of aliquid are kept in two containers one with broad base and another with a narrow base, the height of liquid column will bemore in the container with the narrow base and hence, the liquid pressure exertedwill be more on the narrow base than thaton the broad base.

Pascal’s Law : ‘If the effect of gravity isneglected the pressure at every point of aliquid in equilibrium of rest is the same.’’ Itmeans, in a liquid content, the effect of pressure is equally trasmitted through outthe liquid system.

Hydraulic lift, Hydraulic press or Brahmapress, etc. work on the Pascal’s law.

Pressure is equal to the force acting

per unit area i.e. pressure

PROPERTIES OFMATTER



Physics [29]

Let a liquid system having two open

surfaces one with a narrow cross sectionalarea and other with a broad cross sectionand provided with pistons. Since pressureremains the same in every section of theliquid system, hence, by putting a smallload on the piston of small cross section aheavy load placed at the piston of the broadcross-section can be uplifted.

Buoyancy : Whenever a body isimmersed in a fluid, the displaced fluid hasa tendency to occupy the original position.

Hence, an upward force is experienced bythe body. This upward force acting on the body inside a fluid is called bouyancy or buoyant force or upthrust.

Archimedes Principle : ‘When a bodyis partially or completely immersed in afluid, it loses some of its weight which isequal to the weight of the fluid displaced by the body.’’

Law of Floatation : A body will float ina liquid, if weight of liquid displaced by the

immersed part of the body is at least equalto or greater than the weight of the body.

Applications are discussed as below :

A wooden block or a big ship can float but an iron pin sinks in water becausethe weight of water displaced by awooden block or a ship is either equalto or more than their respective weightswhereas the weight of water displaced by the pin is less than the weight of the

pin. Due to the same reason, a metallic bowl can float on the water surface.

An iceberg having an iron pin floats onthe water surface but the pin sinks inwater after melting the iceberg. This is because the density of ice is less thanthat of water and the weight of waterdisplaced by the iceberg is more than

the weight iceberg containing an iron

pin but the weight of water displaced by the iron pin is less than the weight of the pin.

Initially, a balloon filled with helium gasrises in the air because the weight of theair displaced by the balloon is greaterthan the weight of the balloon. As it goesup, the weight of air displaced decreasesdue to decrease in the density of air andhence, the balloon halts at a heightwhere the weight of air displaced by the

balloon becomes equal to the weight of the balloon.

ATMOSPHERIC PRESSURE

Air content above the earth exerts forcedue to its weight on the ground. This forceon the ground gives rise to the atmosphericpressure. At the sea level the value of atmospheric pressure is one atmosphere.

1 atmosphere = 760 mm of Hg = 76 cm

of Hg = 1.013 × 105

N/m2

= 1.013 × 105 pascal = 760 torr.

Barometer is a device that measurespressure. Mercury barometer is the mostcommon type.

Pressure : Pressure is equal to thenormal force acting per unit area. If thehandles of the bags and suitcases are broad,the pressure on the hand carrying themwill be small.

Railway tracks are laid on large sizedwooden, iron or cement sleepers so thatthe thrust due to weight of train is spreadover a large area. This reduces the pressureon the ground which would prevent theyielding of ground. Pins and nails arepointed so that on pressing on a surfacethey exert high pressure on the surface




and hence, easily penetrate the surface.

The pointed end has least the surface areaand it provides greater pressure on applyinga normal force because pressure is equal toforce devided by area. Due to the samereason, it is painful to walk bare footed ona road covered with edgy pebbles.

Vander Waal Force of Attraction : It isthe minimum force of attraction betweenany two neutral particles (atoms ormolecules). It may be of two types viz.cohesive force (between similar molecules)

and adhesive force (between dissimilarmolecules).

ELASTICITY

The ability of a body to regain its originalconfiguration after removal of an applieddeforming force is called elasticity and the body is termed elastic. Quartz andphospher bronze are nearly perfectly elastic bodies. Steel is more elastic than rubber.Most of the metals are elastic.

When a body does not regain its originalconfiguration at all on the removal of thedeforming force, it is called a plastic body.e.g. plastic paraffin, putty, etc.

When a deforming force is applied onan elastic body, its configuration changesand after the removal of the force the bodyregains the original form. The force appliedper unit area is called stress and the ratioof the change in configuration to theoriginal configuration is called strain.

Hooke’s Law : Within the elastic limit,the stress developed is directly proportionalto the strain produced in a body i.e. stress a

strain or stress = E × strain or

Where E is a constant for a given body

and known as the co-efficient of elasticity

or modulus of elasticity. The modulus of elasticity is a measure of elasticity of a body.

Ductile Materials : These materialsshow a large plastic range beyond theelastic limit.They are used in makingsprings and sheets e.g. coppers, iron, silver,aluminium, etc.

Brittle Materials : These materialsshow a very small plastic range beyondelastic limit e.g. glass, cast iron, etc.

Elastomers : These materials have noplastic range. In such materials even asmall stress can produce a large strain e.g.rubber, the elastic tissue of aorta in thehuman blood circulatory system, etc.

Safety factor

When a material is used in a certainconstruction, the working stress is keptmuch lower than that of breaking stress sothat the safety factor may have a largevalue.

The metallic parts of the machinery arenever subjected to a stress beyond the elasticlimit, otherwise they will get permanentlydeformed.

The bridges are designed in such a waythat they do not bend much or break underthe load of heavy traffic, force of strongly blowing wind and its own weight. In order

to minimise the depression in the beam of given length and material the depth of the beam is kept large as compared to its breadth.However, too large a depth of the beam maycause bending, called buckling. Hence, acompromise between breadth and depth of a beam is made by using an ‘’I shaped girder’’with a large load bearing surface.



Physics [31]

A hollow shaft is found to be stronger

than a solid shaft made of the same equalmaterial.

The material used to construct a springshould be of higher elasticity. Hence, steelhaving higher elasticity is used to construct aspring. Rubber is of lower eleasticity andhence, it cannot be used to construct a spring.

If a material is subjected to repeatedstrains, it losses its elasticity over a longperiod of time. After a long use, the bridges

are declared unsafe. The modulus of elasticity has the same

units and dimensional formula as thatof strees or pressure.

The material is more elastic if its valueof modulus of elasticity is large.

Young’s modulus of elasticity-Y andmodulus of rigidity h exist only forsolids as liquids and gases cannot bedeformed along one dimension only and

also cannot undergo shear strain.However, the bulk modulus of elasticityK exists for all the three states of mater;viz; solid, liquid and gas.

The solids are more elastic and gases areleast elastic. It is so because for the givenstress applied the gases are morecompressible than that of solids.

Young’s modulus of the material of awire is numerically equal to the stresswhich will double the length of a wire.

TRIVIA

Young’s modulus is defined only for thesolids.

Bulk modulus is defined for all types of materials : solids, liquids and gases.

Reciprocal of bulk modulus is called

compressibility.

Hooke’s law is obeyed only for smallvalues of strain (say of the order of 0.01).

Higher values of the elasticity (modulus)means greater force is required forproducing a given change.

The materials which break as soon asthe stress goes beyond the elastic limitare called brittle.

The materials which do not break well

beyond the elastic limit are called ductile.

The deformation beyond elastic limit iscalled plasticity

Rubber sustains elasticity even whenstretched several times its length.However it is not ductile. It breaks downas soon as the elastic limit is crossed.

Breaking stress does not depend on thelength or area of cross section of thewire. However it depends on the

material of the wire. Breaking force depends on the area of

cross section. Breaking stress of a wireis called tensile strength.

Elastic after effect is temporary absenceof the elastic properties.

Temporary loss of elastic properties dueto continuous use for a long time is calledelastic fatigue.

Thermal stress in a rod is independent

of the area of cross section or length of the rod.

When a body is sheared, two mutuallyperpendicular strains are produced.They are called longitudinal strain andcompressional strain. Both are equal inmagnitude.




Quartz is the best available example of

perfectly elastic material. The pressure is perpendicular to the

surface of the fluid.

The upthrust on a body immersed in aliquid does not depend on the mass,density or shape of the body. It onlydepends on the volume of the body.

The weight of the plastic bag full of airis same as that of the empty bag becausethe upthrust is equal to the weight of air enclosed.

The wooden rod cannot float verticallyin a pond of water because centre of gravity lies above the metacentre.

The cross-section of the water streamfrom a tap decreases as it goes down inaccordance with the equation of continunity.

We cannot sip a drink with a straw onthe moon, because there is noatmosphere on the moon.

The line joining the centre of gravity andcentre of buoyance is called central line.

The point where the vertical line throughcentre of buoyancy intersects the centralline is called metacentre.

The floating body is in stable equilibriumwhen the metacentre is above the centreof gravity. (Centre of gravity is belowthe centre of buoyancy)

The floating body is in unstableequilibrium when the metacentre lies below the centre of gravity. (Centre of gravity is above the centre of boyancy).

The floating body is in the neutralequilibrium when centre of gravitycoincides with the metacentre. (Centreof gravity coincides with the centre of buoyancy).

If a body just floats in a liquid (density

of the body is equal to the density of liquid) then the body sinks if it is pusheddownwards.

The hydrometer can be used to measuredensity of the liquid or fluid.

When a gale blows over a roof, the forceon the roof is upwards.

Sudden fall in atmospheric pressurepredicts possibility of a storm.

If two bodies have equal upthrust in aliquid, both have the same volume.

If one floats on one’s back on the surfaceof water, the apparent weight is zero.

If a beaker is filled with liquid of densityp upto a height h, then the meanpressure on the walls of the beaker ishp g/2.

Stress and pressure have the same unitsand dimensions, but the pressure isalways normal to the surface but the

stress may be parallel or perpendicularto the surface.

Isothermal elasticity = pressure (p)

Adiabatic elasticity = Ratio of specificheats × pressure.

Normal stress is also called tensile stresswhen the length of the body tends toincrease.

Normal stess is called compressive stress

when length of the body tends todecrease.

Tangential stress is also called shearingstress.

When the deforming force is inclined tothe surface, both the tangential as wellas normal stress are produced.



Physics [33]

Diamond and carborundum are the

nearest approach to the rigid body.

Elasticity is meaningless for the rigid bodies. That is, the elastically is theproperty of the non rigid bodies.

Compressibility

Breaking stress is independent of thelength of the wire.

Breaking stress depends on the materialof the wire.

Breaking load depends on the area of cross-section of the wire.

Bulk modulus was first defined by Maxwell.

In the stretched spring, tensile strain isproduced.

Breaking stress for a wire of unitcrosssection is called tensile strength.

If a beam of rectangular cross-section isloaded its depression is inversely

proportional to the cube of thickness of the beam.

If a beam of a circular cross-section isloaded, its depression is inverselyproportional to the cube of radius.

If we double the radius of a rope its breaking stress becomes four times, but breaking stress remains unchanged.

When a beam is bent, both extensionalas well as compressional strain is

produced. When there is no external force, the

shape of a liquid drop is determined bythe surface tension of the liquid.

Soap helps in better cleaning of clothes because it reduces the surface tensionof the liquid.

A liquid does not wet the containing

vessel if its angle of contact is obtuse.

In case of liquids which do not wet thewalls of the containing vessel, the force

of adhesion is less than times the

force of cohesion.

The height of the liquid column in acapillary tube on the moon is six timesthat on the earth.

Soldering of two metals is possible because of cohesion.

When the liquid drops merge into eachother to form a larger drop, energy isreleased.

The liquid rises in a capillary tube, whenthe angle of contact is acute.

Surface tension of molten cadmiumincrease with the increase intemperature.

The coefficient of viscosity of gasesincrease with the rise in temperature but

that of liquids decreases with theincrease in temperature.

The streamlined or turbulent nature of flow depends on the velocity of flow of the liquid.

Reynold number is low for liquids of higher viscosity.

Bernoulli’s theorem is based on theconservation of energy.

Bernoulli’s theorem is strictly applicableto non viscous fluids.

Viscosity is due to the transport of momentum.

Maximum possibility of streamlinedflow is for low density and highviscosity fluids.




Gases cannot be liquified above the

critical temperature.

Above critical temperature a substanceis in gaseous state and below criticaltemperature it can be in vapour state.

The branch of thermal physics that dealswith measurement of the amount of water vapours present in theatmosphere is called hygrometry.

Saturated and unsaturated air :

(i) The air is said to be saturated when the

maximum possible amount of watervapours are present in it.

* The pressure of the water vapours inthe saturated air is called saturationvapour pressure.

(ii) If the air contains vapours less than themaximum possible amount possible inthe air, then it (air) is said to beunsaturated.

The humidity refers to the presence of

water vapours in the atmosphere. It isdefined in the atmosphere. It is definedin the following two ways.

* Absolute humidity : It is the amountof water vapour preseent in 1m3 of theair.

* Relative humidity =

* The relative humidity is generallyexpressed in percentage.

Relative humidity may also be definedas :

Relative humidity =

Dew point

It is the temperature at which theamount of water vapour actuallypresent in a certain volume of the air issufficient to saturate that volume of air.

* At the dew point the actual vapourpressure becomes the saturation vapourpressure.

Relative humidity can also be definedas:

Relative humidity =

Relative humidity is low when the air isdry.

Relative humidity is high if the air is

moist.

Hygrometer :

It is the device to measure the amountof water vapours in the air or the relativehumidity.

Saturation vapour pressure changeswith the increase in temperature.

At 0°C, the saturation vapour pressureis 4.6 mm of Hg. Therefore, watervapours are always present around the

ice.

* It increases with the increase intemperature.

When the temperature of theatmosphere is equal to the dew point,the relative humidity is 100%.



Physics [35]

The study of low temperature is called

cryogenics. Maximum value of relative humidity is

100%.

The saturation vapour pressure of waterat 100°C is 760 mm of Hg at sea level.

As the temperature rises, the absolutehumidity may increase and the relativehumidity may remain constant or mayeven decrease.

Let the saturation vapour pressure at

temperature t°C is p. If the atmosphericpressure is reduced to p, the water starts boiling at t°C.

At the same temperature one feels hotterif the relative humidity is high.

If the air is absolutely dry, the dew pointis not observed.

The dew point does not change if thetemperature of the room is changed.

The relative humidity decreases with the

increase in temperature. If water is sprinkled in the room, both

relative humidity as well as the dewpoint increase.

Dew appears on the leaves of the trees,

etc. due to the condensation of saturatedvapours.

Small droplets of water formed due to thecondensation of water vapours near thesurface of the earth constitute mist or fog.

Mist or fog formed much high above thesurface of earth is called cloud.

Human beings feel comfortable when therelative humidity is between 50% to 60%.

When the air is saturated with water

vapours, the relative humidity is 100%and dew point is equal to the roomtemperature.

When the relative humidity is 100%, thereading of the dry and wet bulbs issame.

Smaller the difference in thetemperature of dry and wet bulbs, largeris the relative humidity.

Water vapours are always seen around

the ice, because the temperature aroundit is less than the dew point.




SURFACE TENSION

The molecules at the surface of a liquidhave a higher potential energy than thoseof the inner molecules. P.E. = mgh wherem is mass of a molecule, g is accelerationdue to gravity and h is the height of the

molecule above the bottom of the liquid.Due to maximum height the surfacemolecules of the liquid have maximum P.E.Hence, in order to have higher stability,the liquid surface tries to minimize its P.E.In doing so, it tends to contract to haveminimum surface area, number of molecules and hence, minimum P.E. andmaximum stability. Consequently, theliquid surface behaves as if convered witha stretched membrane. This property of

the liquid by virtue of which the free surfaceof the liquid has a tendency to contract sothat it can have minimum surface area iscalled surface tension.

The surface tension is measured as theforce acting per unit length at the liquidsurface.

Surface tension

The surface molecules of the liquid are

always under the effect of an inward forcedue to force of cohesion between the surfacemolecules and inner molecules. Thatimparts surface tension. The surface tensionof a liquid decreases with rise intemperature and becomes zero at the boilingpoints.

Some common examples of surfacetension :

The hairs of a shaving brush when takenout of water are pressed together. In-

side water, hairs spread out. On takingout the brush from water, the waterfilm forms between the hairs while tend-ing to make its surface area minimumdue to surface tension brings the hairscloser to each other.

In soldering, addition of flux reducesthe S.T. of molten tin, hence, it spreads.

The liquid drop has a tendency to haveminimum surface area due to the sur-face tension of the liquid and hence, it

becomes spherical.

The surface tension of oil is less thanthat of cold water hence an oil dropspreads on cold water while the sur-face tension of oil is more than that of hot water, hence oil drop remains as adrop on hot water.

In order to spread more, the surface ten-sion of lubricants and paints is kept low.Bees like insects can float on water surface

because the weight of the insect is bal-anced by the force of surface tension of water. Due to the same reason, a greasediron needle can float on water surface.

Stormy waves at sea are calmed bypouring oil on sea water because oilhas low surface tension.



Physics [37]

When a wire loop is dipped into a soap

solution and taken out, a soap film is formedon the loop due to surface tension.

The bits of camphor are seen dancingon the surface of water. The bits of camphor reduce the surface tension of water where they are dipped. Due toirregular shape of the camphor bits,unequal forces of surface tension act onthem. Hence, they move erratically onthe surface of water.

It is better to wash clothes in hot soap

solution. Hot solution has lower sur-face tension. Hence, the solution spreadsover a larger area or clothes and clean-ing action increases.

Action of detergent or soap solution :When detergents or soaps are dissolvedin water, the surface tension of waterdecreases. Hence, a detergent/soap so-lution spreads more quickly over theclothes. Consequently, more dirt comein contact with the solution and cleans-

ing action enhances. Further, hotsoution has lower surface tension thana cold one which gives extra cleaningability to the detergent/soap solution.

Warm food taste better on our tongue because the warm food on chewingspreads more on the tongue due tolower surface tension. Consequently,more food content comes in contactwith the taste buds on the tongue.Hence, warm food is tastier than the

same food when cold.

CAPILLARITY

The phenomenon of natural rise or fallof liquid column in a narrow tube is calledcapillarity and the tube is known as thecapillary tube. The capillarity increases with

increase in surface tension of the liquid.

Examples of capil l ary act i on :

The tip of of the nib of a pen is split inorder to provide a capillary which helpsthe ink to rise to the end of the nib andenables it to write continuously.

When a chalk piece is dropped intowater, air bubbles come out. The poresin the chalk containing air act as nar-row capillaries. Hence, water entersthese pores due to capillary action and

air bubbles come out. A blotting paper has fine pores that

act as capillaries. Hence, ink rises inthem leaving the paper dry.

The capillary action of narrow capillar-ies in the threads of a towel enables thetowel to soak water.

The wooden door swells in the rainyseason because of the capillary actionof moisture available in the air through

the pores in the wood. Molten wax in candles or oil in lamps rises

through threads due to capillary action.

Clay soil has capillaries but sand doesnot have capillaries. Hence, under-ground water rises in a clay soil whilesand remains relatively dry.

In order to preserve moisture in thesoil, ploughing of fields is essential.Ploughing breaks the fine capillaries in

the soil. It prevents the rise of waterand reduces evaporation of water con-tent in the soil.

The transport of water and mineralsfrom the soil to plants is favoured bythe capillary action in the conductiontissues called Xylems in the plants.




VISCOSITYIt is fluid friction that arises due to inter

molecular forces which are effective whenthe different layers of the fluid are movingwith different velocities. It gives rises to a backward dragging force between the fluidlayers moving with respect to each other.Viscosity of a liquid decreases with increasein temperature but viscosity of gasesincrease, under similar conditions.

The phenomenon of viscosity plays an

important role in the circulation of bloodthrough arteries and veins of human body.

At railway terminals, the liquids of highviscosity are used as buffers.

Water flows faster than honey becauseviscosity of water is lower than that of honey hence, a small dragging forceappears during the flow of water.

In an industry, the measurement of

viscosity and its variation withtemperature are useful in judgingwhether a given lubricant oil is usefulfor a machine or not.

Chemists use the knowledge of viscosityto determine molecular mass and shapeof large organic molecules like proteinsand cellulose.

FLOW OF FLUID

The rate of flow of a fluid through atube is inversely proportional to the area of cross section of the tube. It means narrowerthe tube, higher will be the velocity of fluidthrough it. Hence, deep water runs slow ina river. Similarly, the jet of falling water becomes narrow as it goes down becausethe velocity of falling water increases and

hence, its area of cross-section decreases. If

the mouth of the tube used to water plantsin the garden is pressed, the speed andrange of the water flow increases, becauseon pressing the cross sectional area of themouth of the tube decreases. Due to similarreason the velocity of water increases whenwater flowing in a broader pipe enters anarrow pipe.

BERNOULLI’S THEOREM

The total sum of energy of a fluidduring its flow remains constant. A fluidcan have potential energy, kinetic energyand pressure energy during its flow. Itspotential energy depends upon height,kinetic energy on velocity and pressure onthe pressure of the fluid.

Applications of Bernolli’s Theorem :

The size of the needle of a syringe con-trols flow better than the thumb pres-sure exerted by a doctor while adminis-

tering an injection. The velocity of flowis controlled by the size of the needlewhereas the pressure is controlled bythumb pressure. In order to keep totalsum of pressure energy and kinetic en-ergy of the flow constant, velocity in-creases more in accordance with theBernoulli’s theorm.

Two rows of boats moving parallel toeach other and nearby, are pulled to-wards each other. The velocity of wa-ter between the two rows increaseswhich results in increase in K.E. of water in between. Hence, pressureand pressure energy of water on theouterside of the rows increase. Con-sequently, the rows of boats arepushed inwards because the total sumof energy of water in between and



Physics [39]

outside the rows have to be constant

in accordance with the Bernoulli’stheorem.

The roofs of hut or tinned roofs are usu-ally blown off without causing any harmto the huts during cyclones or storms.The wind blows speedly above the roof during the storm whereas air inside thehut remains at rest. It gives higher ki-netic energy and small pressure energyabove the roof whereas below the roof K.E. is small and pressure energy is high.

Due to high pressure and hence, higherpressure below the roof, the roof experi-ences an upward force and is blown off with stormy wind.

The wings of the aeroplanes are de-signed such that it experiences an up-ward pressure and hence, an upwardforce. It is based on Bernoulli’s theo-rem. The upper surface of the wing ismore curved than its lower surface andits leading edge is thicker than its trail-

ing edge. Hence, the velocity of air abovethe wings becomes more than that be-low them i.e. kinetic energy of air abovethe wings becomes greater than that of air below the wings. Consequently, thepressure energy and hence, pressure of air below the wings become greater thanthose of the air above the wings. It givesan upwards push on the wings andprevents the planes from falling down.

When a spinning ball is thrown, it de-

viates from its usual straight line pathand its path becomes curved. This ef-fect is based on Bernoulli’s theoremand is known as Magnus effect. Whena spinning ball is thrown, air moves backward above and below the ball.The speed of air below the ball be-comes more because the spinning ball

throws lower air in a backward direc-

tion. Hence, kinetic energy of lowerair becomes more and consequently,in order to have equal energy, the pres-sure energy of upper air gets to in-crease. It results in increase in pres-sure above the ball. This pressure pro-vides a centripetal force acting at aright angle to the linear velocity of the ball. Consequently, the ball follows acurved path.

TRIVIA Molecular forces donot obey the inverse

square law of distance.

The molecular forces are of electricalorigin.

Work done in forming a soap bubble of raidus R is 8πR2 σ, where σ = surfacetension.

Work done in breaking a drop of radiusR into n drops of equal size = 4 π R2 σ

(n1/3

– 1).Same amount of energy is liberated incombining n drops into a single drop.

Angle of contact increases with rise intemperature. It decreases on additionof soluble impurities.

Angle of contact is independent of theangle of inclination of the walls.

The materials used for water proofingincrease the angle of contact as well assurface tension.

Detergents decrease both the angle of contact as well as surface tension.

Surfaces tension does not depend onthe area of the surface.

Viscosity of liquids decreases with therise in temperature.




Viscosity of gases increases with the

rise in temperature. The rate of flow of liquid in a tube of

radius r length l whose ends are main-tained at a pressure difference p is :

. Here η = coefficient of vis-

cosity where R = 8η l/πr4 is called fluidresitance.

Viscosity is independent of pressure.

When a liquid is in equilibrium, the

force/forces acting on its surface areperpendicular everywhere.

In a liquid the pressure is same at thesame horizontal level.

The pressure at any point in the liquiddepends on depth (h) below the sur-face, density of liquid and accelerationdue to gravity.

It is independent of the shape of the con-taining vessel, or total mass of the liquid.

Bernoulli’s equation is in accordancewith the law of conservation of energy.

It assumes that in the flowing liquid,every particle going across the cross-section possesses the same velocity.

Bernoulli’s theorem doesn’t take theviscosity into account. When the fluidflows along a covered path the en-ergy of centripetal force must also betaken into account.

If a vessel contains liquids upto a

height H and it has a hole in the sideat a height h, then the velocity of

efflux is . The time taken by

the liquid to reach the ground level is

. Horizontal range of the liq-

uid R = 2[h(H – h)1/2. The range isthe same for the hole at a height h

above the bottom or at the depth h

below the surface of the liquid. The range is maximum for h = H/2. It

is given by

Interatomic or intermolecular forces arezero when the separation between themis infinite.

At a certain distance r = r0, the inter-

atomic or inter molecular force is zero.The value of r0 is different for the differ-ent atoms and molecules.

For r > r0, the interatomic/ intermo-

lecular forces are attractive.

For r < r0, the interatomic/intermolecu-

lar forces are repulsive.

At r = r0, the interatomic/intermolecu-

lar potential energy is minimum andnegative. It is the equilibrium position of the atoms/molecules.

Both the interatmoic as well as inter-molecular force vary inversely as thesixth power of distance (F ∝ 1/r6).

Differences between interatomic andinter molecular forces :

(i) Interatomic forces depend only onthe interatomic separation but the in-ter molecular force depends on theintermolecular separation as well asthe orientation of the molecules.

(ii) Interatomic force are 50 to 100 timeslarger than the intermolecular forces.

(iii) Value of r0 for atoms is less than that

for molecules.

(iv) The intermolecular forces may extend beyond their immediate neighbours.This is not true for the interatomic forces.



Physics [41]

Space is a vast and endless area orregion outside the earth’s atmospherewhere the stars, planets and other celestial bodies exist.

Space exploration is the study of collecting and analysing the informationand data about the various heavenly orcelestial bodies in the outer space.

With the advancement in technology, acomplete branch of science known as SpaceScience has been developed to explore theouter space. At present, the spaceexploration is done by using artificial andspace probes.

A small body revolving around a planet

in an orbit is known as a satellite.

TYPES OF SATELLITES

Satellites are of two types :

(i) Natural Satellite : A celestial bodyrevolving around a planet is known as thenatural satellite. For example, the moon isthe natural satellite of the earth.

All planets except Mercury and Venushave natural satellites.

(ii) Artificial Satellite : A man madesatellite that revolves around the earth isknown as an artificial satellite.

ORBIT OF SATELLITE

The closed elliptical path followed by

an artificial satellite around the earth isknown as the orbit of the satellite.

Characteristics of the Orbit of theSatellite:

(a) Apogee : The farthest point on theorbit of a satellite from the surface of theearth is known as apogee.

(b) Perigee : The nearest point on theorbit of the satellite from the surface of theearth is known as perigee.

(c) Inclination : The angle between theplane of the equator of the earth and theplane of the orbit of the satellite is knownas inclination.

A satellite is bound to the earth’sgravitational pull as a planet is bound tothe Sun’s gravitational pull.

DIFFERENT TYPES OFARTIFICIAL SATELLITES

Geo-stationary Satellite : A satellitewhich is at rest with respect to the earth iscalled a geo-stationary satellite. The orbitof such a satellite around the earth isknown as a geo-synchronous orbit.

Geo-stationary satellite plays an importantrole in communication technology. In otherwords, a communication satellite is a geo-stationary satellite.

Polar Satellite : Geo-stationarysatellites, orbiting about 36000 km above

SPACE EXPLORATION




the equatorial plane, are used forcommunication purposes.

On the other hand, low to mediumaltitude satellites are known as polarsatellites. These satellites move over theorbit passing through the north and south

poles of the earth above the polar plane.The orbit in which the polar satellite movesis known as the polar orbit.

Polar satellites are not used forcommunication purposes. They are usedfor remote sensing and hence are knownas remote sensing satellites.

The orbit in which a polar satellite orremote sensing satellite moves is such thatthe satellite always passes over a particulararea of the earth at the same local time.

Such orbit of the satellite is known as sun-synchronous orbit (or polar orbit).

BASIC REQUIREMENTS FORSELF-SUFFICIENCY IN SPACE

TECHNOLOGY

Any country which intends to be self-

Specialised Establishments Operating Under DRDO

Vikram Sarabhai Space Centre (VSSC): Specialised in the development of satellite launch ve-hicles and sounding rockets.

ISRO Satellite Centre (ISAC): The lead centre for satellite development, covering structures, ther-mal systems, spacecraft mechanisms, power systems and satellite integration.

Satish Dhawan Space Centre (SDSC)-SHAR - Sriharikota Space Centre:India's prime launchingpad facility, providing the launch infrastructure as well as solid propellant processing and their test-ing. A second launch pad has been recently built at SDSC-SHAR.

Liquid Propulsion Systems Centre (LPSC) : The lead centre in the area of liquid and cryogenicpropulsion for launch vehicles and satellites.

Space Applications Centre (SAC): Specialised in the development of payloads for communication,meterological and remote sensing satellites; it conducts space applications research and develop-ment.

ISRO Telemetry, Tracking and Command Network (ISTRAC): It provides mission support to Low-Earth orbit satellites and to launch vehicle missions.

Master Control Facility (MCF) : The monitoring and control centre for the geostationary satellites. ISRO Inertial Systems Unit (IISU): Carries out research and development in inertial sensors and

systems and allied satellite elements. National Remote Sensing Agency (NRSA) : An autonomous institution supported by DOS, it is re-

sponsible for acquisition, processing and distribution of data from remote sensing satellites, basedin Hyderabad.

sufficient in the space technology has tofulfill the following requirements:

1. Fabrication of Satellite orSpacecraft : Satellites play an importantrole in the field of space research. Therefore,an expertise is required for planning,

designing and fabricating the different typesof satellites or space crafts.

2. Designing and Fabrication ofLaunch Vehicles : Launch vehicles arerequired to put satellite in the orbit aroundearth and to launch the space probe.

Therefore, these vehicles have to bedeveloped for the successful launching of satellites.

3. Earth Control Station : When satellite

is put into orbit, its all operations have to be controlled and guided by sending propercommand from surface of the earth. Suchcommands are sent from the stationestablished on the earth.

4. Ground Facilities : The arrangements



Physics [43]

are to be made in order to get the benefit of

the information and data sent by the

satellite throughout the country. The signals

re-transmitted from the satellite are received

by the receiving antennas installed across

the length and breadth of the country.

The Indian Space Rearcher Organisation

has set up a large number of organisations

and research centres across the country to

carry out research and developmental

activities in the field of space research andtechnology.

Setting up of Rocket Launching

Facility : The India’s space explorationprogramme began with the setting up of a

rocket launching facility at Thumba, near

Thiruvananthapuram (Trivandrum). India

launched its first rocket RH-75 in 1967

from the Thumba Equatorial Rocket

Launching Station (TERLS). Although this

rocket was very small (diameter = 75 mm),

yet it had all the basic features of a rocket.

Since then India never looked back and

today it has the honour to be the sixth

nation in the world in the field of space

technology.




PERIODIC MOTION

The motion of a body which is repeatedidentically after a fixed interval of time isknown as periodic motion and the fixed

interval of time is known as time period of the motion.

The motion of the hands of a clock is ina periodic motion. The time period of thehour’s hand is 12 hours, of the minute’shand is 1 hour and of the second’s hand is1 minute.

The rotation of the earth about its axiswith a time period of 24 hours and therevolution of earth around the sun with atime period of 1 year are other examples of the periodic motion.

SIMPLE HARMONIC MOTION

It is a special type of oscillation in whichthe particle oscillates in a straight line, theacceleration of the particle is alwaysdirected towards a fixed point on the line& the magnitude of acceleration isproportional to the displacement of theparticle from this point. This fixed point is

called the centre or mean position of oscillation. The simple harmonic motion isan oscillatory motion which in turn is aperiodic motion. However, a periodic mayor may not be an oscillatory motion.

The motion of a freely suspended barmagnet in the earth’s magnetic - field, of the pendulum of a wall clock, of the bob of

a simple pendulum etc. are simpleharmonic motions.

Application of SHM are discussed asbelow :

When the load attached to a spring ispulled once a little from its meanposition & left free, it oscillates in simpleharmonic motion.

The motion of liquid contained in U-tube when it is compressed once in oneof the two limbs & left to itself, is simpleharmonic motion.

If a tunnel is dug along the diameter of earth & a body is dropped in it, the body will oscillate in simple harmonic

motion between the ends of the earth’sdiameter.

When the oscillation of a body in simpleharmonic motion is free from frictionalforces, its oscillations are undamped.The amplitude and energy do notchange with time in an undampedoscillation. The oscillation of a simplependulum in vacuum is an undampedoscillation.

When the oscillation of a body in simple

harmonic motion involves frictionalforces, its oscillations are damped. Theamplitude and energy of the body in adamped oscillation decrease with time.

Most of the oscillations in air or in amedium are damped oscillations likeoscillation of the bob of a simple

SIMPLE HARMONIC

MOTION



Physics [45]

pendulum in air, of the stretched string

in air etc.

When a body oscillates with its ownnatural frequency without involving anexternal periodic force, its oscillationsare called free-oscillations. Theoscillations of the string of a sitar whenplucked once and let free, of the bob of a simple pendulum when oncedisplaced from its mean position andlet free and of the prongs of a tuningfork when one of its prong is struck

once on a rubber and let free areexamples of free oscillations.

When a body oscillates with the help of an external periodic force other than itown natural frequency, its oscillationsare called forced oscillations.

All musical instruments consisting of strings like sitar, violen ReasonantOscillations etc. produce forcedoscillations.

RESONANT OSCILLATIONS

When a system oscillates with its ownnatural frequency, with the help of anexternal periodic force whose frequency isthe same as that of the natural frequencyof the oscillating system, then theoscillations of the system are calledresonant oscillations. The resonantoscillations have very large amplitude. Atresonance, the oscillating systemcontinuously absorbs energy from theagent applying external periodic force.

If the rhythm of pushing against theground is synchromized with the naturalfrequency of the swing, it gives greaterheights due to resonance effect.

All mechanical structures have one or

more natural frequencies. When a

mechanical structure is subjected to a strongdriving force that matches one of its naturalfrequencies, the amplitude of oscillationsof the structure becomes very large.Consequently, the structure may collapse.It is for the same reason the marchingsoldiers break steps while crossing a bridge.In order to avoid effect of resonance theaircraft designers make sure that none of the natural frequencies at which a wingcan oscillate match the frequency of theengines during flight.

In an earthquake, the structures, whosenatural frequency matches the frequencyof the seismic waves, collapse. During anearthquake sometimes, short and tallstructures remain uneffected while themedium height structures fall down. Thishappens because the natural frequenciesof the short structures happen to be higherand those of taller structure lower than thefrequency of the seismic waves.

WAVES

Wave is some sort of disturbance inwhich, information and energy, in the formof signals, propagate from one point toanother without the actual journey of themedium. All our communications dependon the transmission of signals throughwaves. All radiations are waves.

Wave motion is a kind of disturbancewhich travels through a medium on account

of repeated periodic vibrations of themedium particles about their mean positionwithout any net transport of the medium.

Types of Waves : On the basis of medium requirement waves are of twotypes viz. (i) Mechanical or El ast i c w aves :

They require a medium for propagation.




e.g. sound, waves on the liquid surface,

vibration of string, etc. (ii) Elect romagnet i c w aves : They do not require a medium forpropagation e.g light, X-rays, microwaves,infra-red, ultra-violet rays, etc.

On the basis of the mode of vibrationthe waves are of two types viz.

(i) Transverse wave and (ii) Longitudinalwaves

Transverse Wave Motion : It is thetype of wave motion in which the particlesof the medium vibrate about their positionsin a direction perpendicular to thepropagation of the wave. A transversewave is represented in the form of crestand trough.

When a stone is thrown in a pond,transverse waves are formed. The vibrationsof the membrane a of drum, of the string of a sitar, violin etc. produce transverse waves.

Longitudinal Wave Motion : It is thetype of wave motion in which the particlesof the medium vibrate about their mean

positions in the same direction in whichthe wave is propagated.

Sound produced from a source is alongitudinal wave like vibrations of atuning fork, ringing of a bell, etc.

A longitudinal wave is represented inthe form of compression and rarefaction.

Transverse Wave

C

Longitudinal waves

C = Compression

R = Refraction

l (Wave Length) : For the transverse

wave the wave Length is equal to thedistance between the two successive crestsor troughs. It is also equal to the sum of thewidth of a crest and a neighbouring trough.For the longitudinal wave, the wave lengthis equal to the distance between the twosuccessive compressions or rarefactions. Itis also equal to the sum of the width of acompression and a neighbouringrarefaction.

Time Period (T) : It is equal to timeinterval taken to complete one vibration.

Frequency (n) : It is the no. of vibrationsproduced per second. It is equal to thereciprocal of time period i.e.

Frequency

Speed of a wave is given as

Speed = Frequency × Wavelength

A mechanical transverse wave thatrequires a medium can propagate through



Physics [47]

solids and at the surface of liquids, but not

inside liquids and gases. However, alongitudinal wave can pass through solids,liquids and gases.

TRIVIA

A pendulum clock slows down in sum-mer and goes faster in winter.

Sound waves are mechanical waveswhich cannot travel in vaccum.

Sound waves do not propagate in saw-

dust because it is not a continous me-dium.

Mechanical waves propagate energyand momentum, but they do not propa-gate matter.

The sound waves are not dispersed in

air.

The weakest audible sound has inten-sity 10–12 W/m2. Its loudness is equal to1 dB.

If the intensity of sound increases by afactor of 10 then the loudness increase by 10 dB or 1B.

If the loudness of sound is changed by 1dB, then the intensity of soundchanges by 26%.

The prongs of a tuning fork vibratetransversely and the stem vibrates lon-gitudinally with the same frequency.

The point of contact of the stem andprongs of the tuning fork is an antin-ode.




Sound is produced in a materialmedium by a vibrating source. Thesevibrations are carried by air, as a mediumand strike our ear drum. The ear drumvibrates and the message is conveyed toour brain and we hear the sound. A soundheard persists for 0.1 second in brain. It iscalled as persistence of hearing.

Depending upon the frequency range,sound has three categories viz. (i) Infrasonic(ii) Sonic and (iii) Ultrasonic or supersonicsounds.

Infrasonic sound has a frequency lessthan 20Hz. Sonic sound is between 20 Hzto 20,000 Hz. It is the audible range for

human ears. Ultrasonic sound has afrequency greater than 20000 Hz. Both theinfrasonic and ultrasonic sounds are notaudible to human ear. However, a dog canhear sound of frequency upto 50000 Hzand a bat upto 105Hz. Dolphins canproduce and detect sounds of frequencyupto 105 Hz.

Sound is a longitudinal wave. Its speedin dry air is 332 m/s. The speed of anobject greater than the speed of sound is

known as supersonic speed. The speed of sound depends upon elasticity, density,temperature and motion of particles of themedium of propagation. Higher theelasticity of a medium, greater is the speedof sound in it e.g. speed of sound in air is332 m/s and in steel 5000 m/s. If one endof a long steel rod is struck, two distinct

sounds are heard at the other end. The firstsound heard is propagated through steeland the second one is propagated throughair.

An increase in the density of themedium reduces the speed of sound. Moistair has lower density than dry air andhence speed of sound is more than thatmoist air.

The speed of sound increases with risein the temperature of the medium. Thespeed of sound in air increase by 0.61 m/second for every one degree rise intemperature above 0°C.

The speed of sound increases along the

direction of wind velocity and decreasesagainst the wind velocity.

The speed of light is much greater thanthat of sound. Hence, thunder is heardmuch after the flash of lightning is seen.Due to the same reason, spectators hearthe sound of ball on bat a little after theysee the batsman actually striking the ball ina cricket match.

Sonic Boom : Sound produced by asupersonic aircraft is heard as a loudexplosion on the earth. It is known as sonic boom. However, a person inside thesupersonic aircraft cannot hear its sound.

Reflection of Sound : Sound wavesare reflected from the obstacles of size of wavelength of sound and follow laws of reflection similar to those of light. The

Sounds



Physics [49]

reflecting obstacles can be walls, mountains,

clouds, ground, etc. Sound waves can befocused after reflecting from a curved surfacein the same way as light waves.

ECHO

Reflected sound is called echo. An echooccurs when the reflected sound wavecomes back to the listener within a timeinterval of not less than 0.1 second after theoriginal sound wave reaches the listener so

that a distinct repetition of the original soundis perceived. It is so because the persistenceof hearing of sound is 0.1 sec. A sound can be perceived only if stays back at least for0.1 second or more on our ears.

An echo is used to determine the speedof sound in a medium. Exploration of underwater petroleum deposits is done bydetecting echo of shock waves produced by explosions on the water surface.

The wave phenomena like total internalreflection, refraction and interference arealso given by sound waves similar to thoseof the light wave.

SUPERSONIC OR ULTRASONICWAVES

These waves are used :

(i) to determine the elastic symmetries of crystals of solids and to detect flaws inmetals.

(ii) to find the velocity of sound in liquidsthat also informs several physical andchemical properties of the liquids.

(iii) for finding the depth of sea and todetect the submerged rocks,submarines and icebergs.

(iv) to form stable emulsions of immiscible

like water and oil.

(v) to accelerate crystallisation of substances and to produce oxidation.

(vi) to coagulate aerosols i.e. displaced fineparticles of a solid or a liquid in a gas,e.g. dust, smoke, mist, etc.

(vii) to liquify gels in the same manner asthey are liquefied by shaking.

(viii) in getting alloys of uniform

composition.(ix) for washing silken fabrics.

ULTRASONOGRAPHY

It is a technique used to form an imageor picture of a matter by using ultrasonicwaves. This technique is used in medicalscience treatment. The medical sonographyis commonly known as ultrasound. In thistechnique, ultrasound wave passes throughthe organs to be diagnosed. The velocity of wave depends upon the elasticity anddensity of the tissues in the organ and anecho is detected by a specific microphone.The echo obtained from the tissues of theorgan is converted into electric signals onthe screen of ultrasonograph and the imagesare recovered. The pattern of images helpsin diagnosis of the organs like heart, liver,kidney, pancreas, etc.

Sound navigation and ranging : It is

device that transmits ultransonic wavesthrough water and records the vibrationsreflected i.e. echo from an objects. Sonar isused in finding submarines, depth of sea,rocks and mineral deposits, etc.

Biological Effects of Ultrasonic Waves :Ultrasonic waves can kill or injure small




animals like frog, fish, etc. These wavescan destroy micro organisms like bacteriaand yeast.

In dental science, ultrasonic waves areused for extracting the broken teeth, to detectcracks or other defects in homogeneitynoticeable by reflection or absorption.

Vibration of a Source of Sound : Whena source is sounded, it generally vibrates inmore than one mode. When it is sounded

gently, the source vibrates in a simplemanner and produces tones of lowerfrequencies, but when the source issounded rapidly, the vibration becomescomplex and it gives rise to tones of higherfrequencies. The tones of the lowestfrequency are called the fundamental noteand the tones of higher frequencies arecalled over tones. The notes of frequencieswhich are integral multiples of thefundamental frequency are called asharmonics.

DOPPLER’S EFFECT

When a source of sound and a listenerare at rest, the listener receives anunchanged frequency produced by thesource. If there is a relative motion between them, then, the number of waves

received per second or the apparentfrequency of the source changes and isnot the same as that of the source. Thepitch of the note heard appears to rise if they approach each other and appears tofall if they recede away from each other.The apparent change in frequency of asound wave due to a relative motion between the source of sound and thelistener is known as Doppler’s Effect. It isequally observed for light too.

If a railway engine travelling with ahigh speed with its whistle blowing isapproaching a listener, the frequencyappears to rise. The frequency appears tofall just as the engine passes the listener.

Doppler’s effect is used to detect a star,a galaxy etc. It can be used to find outwhether a star/galaxy is approaching usor receding away from us. It favours thehypothesis of an expanding universe.

Dopplers effect is used in ‘speed guns’

used by police to measure the speed of vehicles. This effect can be used to detect amoving object as well.

MUSICAL SOUND

Sounds produced by oscillating strings(sitar, piano, violin etc.), vibrating

Difference between Intensity and Loudness

Intensity Loudness

1. It is a physical quantity. 1. It is not an entirely physicalwhich can be accurately measured. quantity.

2. It does not depend upon the sensitivity 2. It depends upon (i) sensitivity

of the ear. of the ear and (ii) intensity of\ sound.

3. It has an objective existence i.e., it 3. It has got a subjective existenceexists whether there is some i.e, it exists only when some

listener or not. listener is acually present.4. Unit of intensity is Wm –2. 4. Unit of loudness is bel.

In a tape recorder of T.V. bass refers to low pitch and treble refers to high pitch. So when bass is on,

low pitch sounds of tabla and dholak become loud. When treble is on, high pitch sounds becomepredominant.



Physics [51]

membranes (drum), air columns (flute,

organ pipes), wooden blocks or steel bars(marimba, xylophone) etc. produce apleasant effect on our minds and hence,are known as musical sounds.

Intensity, loudness, pitch, quality, etc.are characteristics of musical sounds.

Intensity : Intensity of sound is a purelyphysical quantity, It is equal to the amountof sound energy passing per unit time perunit area around a point in a directionnormal to the area. Intensity is measured

in watt/m2 or watt/(cm)2.

Threshold of Hearing or Zero Levelof Intensity : it is equal to 10–16 watt/cm2

for a normal human ear which can justhear a note of frequency of 1000 Hz.

Loudness : The sensation produced inthe ear that enables us to distinguish between a loud sound and a faint sound iscalled loudness.

Two sounds of equal intensity but

different frequencies may not appear to beequally loud even to the same listener because sensitivity of the ear is different fordifferent frequencies.

Loudness is measured in bel. one bel =10 decibel (db). Both the intensity of soundand loudness increase with increase inamplitude of the sound wave, frequency of the wave, density of the medium andvelocity of sound in a medium. Theydecrease with increase in distance from

the source of sound.

Pitch : It is the characteristic of musicalsound which distinguishes a sharp soundfrom a dull sound. Thus, it represents thedegree of shrillness of a musical note. Pitchdepends on the frequency of sound andDoppler’s effect.

The buzzing of bee or humming of a

mosquito has high pitch, but low intensity,while the roar of a lion has a low pitch, buthigh intensity. Frequency of ladies, voice isgenerally higher than that of gents andhence, a lady’ voice is always sharper thana gent’s voice.

Quality : It is the characteristic of musical sound that enables us to distinguish between the sounds of same intensity andsame pitch produced by two differentsources. Its cause is the difference in the

frequency and relative intensities of overtones produced by two sources of sound.

Voices of two singers singing a duet,voices of our friends, voices of familymembers etc. can be distinguished fromeach other on the basis of difference inquality of the sound produced by them.

REVERBERATION

The persistene of sound after the sourcehas actually stopped producing sound iscalled reverberation of sound and the timefor which the sound persists is called thetime of reverberation.

When a loud sound is produced by asource in a hall it is partially absorbed,reflected and transmitted by the walls,ceiling and floor. If the source of soundstops producing sound, the intensity of sound decreases due to absorption of sound.

However, the sound continues to be heard because of the persistence of the reflectedsound waves which go on traversing thehall a number of times before they reachan intensity below the threshold of hearingi.e. the multiple reflection of sound resultsin reverberation.




A room with zero reverberation time is

called a dead room. An ordinary syllabletakes about 0.2 seconds to decay. For amusical sound, the optimum value of reverberation time may be between 1 to 2seconds. The reverberation time in the hallslike theatres, auditoria, etc. is adjustedsuitably by a specific design so that soundheard is distinct and pleasant.

For obtaining good acoustic propertythe hall should have sufficient soundabsorbing features, like:

(i) A few open windows.

(ii) Sound absorbing soft materials likecloth, asbestos, etc. or heavy curtainsput up in the hall at various places.

(iii) A good audience, because one listener

is equal to 5 square feet of an openwindow.

(iv) Curved walls or corners should beavoided so that sound is not undulyconcentrated and there are no regionsof silence.

The time of reverberation can bedecreased by increasing absorption of sound. Reverberation should be small butnot absolutely zero because then the hallgives a dead effect.

Diatonic Musical Scale : This scaleconsists of 8 notes. From the note (1) to (8)pitch increases due to increase in therelative frequencies.



Physics [53]

Ray Optics : It is based on “therectilinear propagation of light” i.e., lightfollows a straight line motion through agiven transparent medium. It involvesreflection, refraction & dispersion of light.

REFLECTION OF LIGHT

“When a light ray is incident on a sur-face, it returns back into the same medium.”

Laws of Reflection :

(i) The incident ray, normal drawn at theincident point and the reflected ray liein the same plane.

(ii) The angle of incidence is equal to the

angle of reflection. In the given figure

XY = reflecting surface,

AB = incident ray,

BC = reflected ray,

DB = normal drawn atthe incident point.

Angle of incidence

i = ∠ABD and angle of reflection

r = DBC

∴ i = r or, ∠ABD = ∠DBC

Reflection on a Plane Mirror : Theimage formed by a plane mirror is virtual& erect. The image distance is equal to theobject distance from the mirror. The imageis laterally inverted i.e., the right side of the

object appears as the left side of its image& vice-versa. Lateral inversion is due to thefact that the image of an object is as far behind the plane mirror as the object is infront of the mirror.

For a given incident ray, if the planemirror is rotated through an angle ‘q’ thenthe reflected ray rotates through an angleof ‘2q’.

The length of a plane mirror in which aperson can see his/her full image is half the height of the person.

If two plane mirrors facing each otherare inclined at an angle q with each other,then number of images of an object lying between them are given by—

(a) if n = an odd whole no. then ‘n’images are formed

(b) if n = an even whole no., then (n – 1)

images are formed.

e.g. (i) if angle between the two planemirrors is 60°, then

no. of images formed by them

= 6 – 1 = 5.

OPTICS




Image Formation in a Concave Mirror

Position of Position of Nature ofobject image image1. At infinity At the focus Real, Inverted &

diminished.

2. Beyond the Between the Real, Inverted &centre of centre of cur- Diminishedcurvative vature & focus

3. At the centre At the centre Real, invertedof curvature of curvature & same size as

that of the object

4. Between the Beyond the Real, invertedcentre of curvat- centre of & magnifiedure and focus curvature

5. At the focus At inifinity Real, Inverted &

highly magnified

6. Between pole Behind the Virtual, Erect && focus mirror magnified

(ii) If the two mirrors are parallel, thenq = 0°

∴ = µ = infinity.

i.e., infinte no. of images are formed.

Use of Plane Mirror : Plane mirrorsare used as looking glasses, in a solar cooker

to reflect the sun-light, in a Kaleidoscopeto see colourful images, in a periscopeusually used in submarines to see outsidethe water surface, in a barbar’s shop to seethe back portion of the head, etc.

Some important facts regarding theimage formation by a plane mirror :

When we see a series of images in athick plane mirror, out of these the sec-ond image is brightest.

Out of 26 alphabets (e.g., A, B, C etc.)only 11 alphabets show lateral symme-try and the rest 15 alphabets have lat-eral inversion.

If the object is displaced by a distance‘x’ towards or away from the mirror,then its image will be displaced by adistance ‘x’ in the same sense.

If the mirror moves by a distance ‘x’

towards or away from the object, itsimage will move by a distance ‘2x’ inthe same sense.

If the object moves with a velocity ‘v’towards or away from the mirror, itsimage appears to move with a velocity‘2v’ in the same sense. The same is trueif the mirror moves.

Reflection on Spherical Mirrors :Types of spherical mirrors

(i) Convex or Divergent Mirror : It is aportion of a hollow sphere of glass whoseinner surface is silvered. Since rays incidentparallel to the principal axis are divergedaway the principal axis, hence, it is knownas a divergent mirror.

(ii) Concave or Convergent Mirror : Itis a portion of a hollow sphere of glasswhose outer surface is silvered. All raysincident parallel to its principal axis areconvergent at its focus.

The centre of a sphere whose certain

portion is convex or convave, is called acentre of curvature. The middle point on thespherical mirror is called as its pole. The midpoint between the pole and the centre of curvature is the focus. The distance betweenthe pole and the focus is focal-length (f). Thedistance between the pole and the centre of curvature is radius of curvature (R). Theperpendicular drawn at a point where a rayis incident on the spherical mirror must passthrough the centre of curvature.

Convex mirror



Physics [55]

Concave mirror

P = Pole

Px = Principal axis

C = Centre of Curvature

PC = Radius

F = Focus

PF = Focal Length

A & B : Incident points

AC & BC = Normal drawn at incidentpoints.

Image formation in a convex Mirror.

It always produces virtual anddiminished image irrespective of theposition of the object.

Real Image : It is always inverted andin front of the concave mirror.

Virtual Image : It is always erect and behind the concave and convex mirror.

Focal length is half of the radius of

curvature (f = R/2).

Mirror Formula :

Where u = objects distance from thepole of mirror.

v = image distance from the pole of mirror.

f = focal length of the mirror.

R = radius of curvature of the mirror.

Magnification or Linear Magnification (m) -

Spherical Aberration: The inability of a concave mirror of a larger size to convergeall rays parallel to the principal axis at thefocus, is called spherical aberration.

It can be removed by using a parabolicconcave mirror of larger aperture.

Uses of Spherical Mirrors:

A convex mirror is used as a reflector in

street lamps to diverge light over a largearea.

A convex mirror is used as a rear viewmirror or driver’s mirror in cars, scoot-ers etc. for looking at the traffic at therear of the vehicle because this mirrorproduces an erect and diminished im-age of the traffic behind the vehicle. Sincethe image is small in size, hence the fieldview is much wider as compared to aplane mirror or a concave mirror.

A concave mirror is used as a reflectorin search light, head lights of motorvehicles, telescopes, solar cookers, etc.to produce an intense parallel beam of light. A bulb is placed at the focus of the concave mirror. The beam of lightfrom the bulb after reflection from theconcave mirror goes as a parallel beam.

A concave mirror is used by dentistsand ENT specialists to focus light onteeth, eye and throat to examine these

organs. The device consisting of the con-cave mirror used to examine eyes iscalled Ophthalmoscope.

A concave mirror is used as a shavingmirror or make up mirror as it can formerect and magnified image of the per-son within its focus.




ABC of Photometry

S.No.Term Definition Unit1. Luminous flux (f) visible light energy lumen2. Lumnious intensity or luminous flux emitted Candela or C.P.

Illuminating power (l) per unit solid angle3. Illuminance or Intensity luminous flux that passes (i) Lux or metre candle

of illumination of surface (E) through unit area in a (ii) phot or cm candledirection normal to area.

4. Luminance or Brightness (B) luminous flux reflected Lambert from unit area into our eyes.

A spherical mirror can also be used as a

trick- mirror or magic mirror to see dif ferent types of images of the same object or person.

Some important facts regarding thereflection of light:

Image : Size of mirror does not affectthe nature of the image except that a bigger mirror forms a brighter image. Avirtual image cannot be taken on screen.But our eye lens forms a real image of

the virtual image (acting as virtual

objects) on the retina. A virtual imagecan be photographed.

The focal length of a plane mirror isinfinity.

When a spherical mirror (concave orconvex) is placed in a liquid e.g. water,its focal length does not change becausefocal length of a mirror depends uponits construction and not on the externalmedium.

When we read a book, light is scattered by the paper into our eyes. This is calledDiffuse-reflection. A printed paper hasa rough surface and hence, it cannot

produce a regular reflection. That iswhy we read a book because of reflec-tion of light but we do not see even afaint image of ours in its printed pages.

REFRACTION OF LIGHT

When light goes from rarer (e.g. air) toa denser (e.g. glass) medium, it bendstowards the normal to the interface,separating these two media. On the otherhand, when light goes from a denser to

rarer medium, it bends away from thenormal to the interface separating thesetwo media. This phenomenon is known asrefraction of light.

PO = Incident ray, AB = Interface

OQ = Refracted ray, i = Angle of incidence,

r = Angle of refraction.

Distinction between Real Image and Virtual imageReal Image Virtual image

1. It is can be obtained on a screen. 1. Reflection of light occurs from the side which is bul-

ged out.

2. Rays of light from the object, on reflection/ 2. Rays of light from the object, on reflection/ refraction only

refraction, actually meet at the image. appear to meet at the image.

3. It is always inverted. 3. It is always erect.

4. Linear magnification is negative 4. Linear magnification is positive.



Physics [57]

Laws of Refraction: (I) The incident

ray, the refracted ray and the normal tothe surface separating two media all lie inthe same plane.

(II) Snell’s Law: where µ is a

constant. µ is known as the refractive indexof the medium in which refracted raytravels w.r.t. the medium in which theincident ray travels.

Cause Behind Refraction of Light: Due

to change in medium, wavelength of lightchanges. It results into change in the speed

of light in that it causes bending light from

straight path of original light.

Relative Refractive Index (µ)

where 1µ2 = Relative refractive index of

medium 2nd w.r.t. 1st medium,

C1, C

2= speeds of light of the two media.

l1, l

2 = wavelengths of light of the two

media.

µ1 & µ2 = individual refractive indicesof the two media

Difference between Concave and Convex Mirrors

Concave mirror Convex mirror

1. Reflection of light occurs from sunken side of 1. Reflection of light occurs from the side which is bulged

mirror. out.

2. Its focus and centre of curvature are on front 2. Its focus and centre of curvature are at the back of mirror.

side of mirror. f and R both are negative f and R, both are posit ive.

3. Image formed may be erect or inverted. 3. Image formed is always erect.

4. Size of image may be smaller, equal or larger 4. Size of image is always smaller than the size of

than the size of object. object.

5. Field of view is narrow. 5. Field of view is broad.

Deviation (d) of a light during refractionin a medium increases with (1) increase inrefractive index (µ), (ii) increase in thedensity (r) of the medium and (iii) decreasein wavelength (l) i.e.

Principle of Reversibility :

‘If the path of a ray of light is reversedafter suffering a number of refractions

Images Formed by Convex and Concave Mirrors

Mirror Position of Object Position of Image Nature of Image Size of Image

Convex at ∞ at focus erect much smaller

Convex between ∞ and pole between pole and focus erect smaller than object

Concave at µ at focus real and inverted smaller than object

Concave beyond C. between F and C real and inverted smaller than object

Concave at C at C real and inverted same as size of object

Concave between F and C beyond C real and inverted larger than the object

Concave at F at infinity real larger than the object

Concave between F and P behind the mirror virtual and erect larger than the object




and reflections, then in the given figure

a light ray suffers refraction thrice at pointB, C & D. It follows path AB → BC → CD→ DE. If the final ray DE is incident at 90°to a plane mirror M, it retraces the originalpath as ED → DC → CB → BA.

Some Common Effects of Refraction

Stars twinkle due to refraction. The dif-ferent layers of atmosphere are of dif-ferent (a) density, (b) temperature (c)speed and their density, temperaturespeed change continuously. Hence, the

apparanet position of the star changes

continuously. It leads to the twinklingof a star.

A rod appears bent in water due torefraction of light.

The sun is visible to us before actualsunrise & after actual sunset. The at-mosphere has higher density than thatof outer space. Hence, light coming fromthe sun enters outerspace to atmospherei.e., from rarer to denser medium. Itresults in the bending of light. Whenthe sun is below the horizon before thesunrise and after the sunset, its appar-ent position is visible above the hori-zon. Hence, the sun is visible 2 minutes before the actual sunrise and 2 minutesafter the actual sun-set. Thus, the day becomes longer by 4 minutes due toeffect of refraction.

Real and apparent depths of an objectin water or any transparent medium.

Difference between Reflection and Total Internal Reflection(Simple) Reflection Total Internal Reflection

1. Reflection occurs when light is travelling in 1. Total internal reflective occurs only when light isa rarer medium or in a denser medium. travelling in denser medium and tending to go into rarer

medium.2. Reflection takes place at all values of angle 2. It is possible only when angle of incidence in denser

of incidence. medium is greater than critical angle for the pair ofmedia in contact.

3. Some energy gets absorbed or refracted 3. The entire energy is reflected. There is no absorption/

while majority of energy is reflected. refraction.

Difference between convex lens and concave lens

Convex lens Concave lens1. The lens is thick at the middle and thin at 1. The lens is thin at the middle and thick at edges.

edges.2. Image formed may be real or virtual. 2. The image formed is always virtual.

3. Image formed may be equal in size, smaller 3. Image formed is always smaller than the object.or larger than the object.

4. It converges the rays of light. 4. It diverges the rays of light.

5. On moving sideways, image moves in 5. On moving sideways, image moves in same direction.opposite direction.



Physics [59]

An object at O appears at I when it isviewed from air.

AO = Real depth of object in water or atransparent medium.

AI = Apparent depth of object.

If µ = relative refractive index of waterw.r.t.air.

then, µ = or,

Relative Refractive Index =

Since relative refractive index of water

is always greater than that of air (µ } 1),hence, real depth of an object is greaterthen apparent depth.

TOTAL INTERNAL REFLECTION(T.I.R.)

When light is incident from denser torarer medium, it bends away from thenormal drawn at the incident point.

Let us consider two light - rays viz. AO

& CO incident at point O from denser to

rarer medium. Ray AO is incident at anglei1. Its angle of refraction is <NOB = 90°. The

refracted ray OB is parallel to the surface of denser medium. This angle is called asCritical angle (i

1 = c). Ray CO is incident at

an angle i2>i

1. Its refracted ray is OD in the

denser medium. It means when a light rayis incident at an angle greater than thecritical angle, it returns back to the samemedium. This phenomenon of returning back of a light ray in the denser medium isknown as T.I.R. Critical angle (c): The angleof incidence of a light ray in the densermedium corresponding with the angle of refraction in the rarer medium is 90° is calledas critical angle.

Conditions for T.I.R. : (i) Light should be incident from denser to rarer medium &(ii) The angle of incidence should be greaterthan the critical angle (c).

Some common cases of T.I.R. :

The Brilliance of Diamonds : The criti-

cal angle for the rays of travelling fromdiamonds to air is about 24.4°. The dia-mond is cut suitably such that light en-tering the diamond from any face fallsat an angle greater than 24.4°, suffersmultiple total internal reflections at thevarious faces, & remains within the dia-mond. The diamond sparkles due to thelight trapped in it.

Mirage : It is an optical illusion seenusually in deserts by which an inverted

image of a distant object like a tree isobserved along with the object itself asif reflected from the water - surface.

In hot desert, lower atmospheric layersare rarer due to higher temperature thanthose of upper layers. A ray of lightcoming from the tree bends away from




the normal as it travels from denser

upper layer to the rarer lower layer.This process continues till the angle of incidence is less than the critical angle.At a particular layer of air the angle of incidence becomes greater than thecritical angle and it gives rise to theT.I.R. It gives the impression of a waterpond near the tree i.e. mirage.

Air bubbles in water or glass shine :Water or glass is denser w.r.t. air. Whenlight is incident at an angle greater thanthe critical angle, on going from wa-

ter/ glass to air bubble, it suffers T.I.R.Hense the air bubble shines brilliantly.This effect is used in manufacturingdecorating glass objects.

Totally Reflecting Prisms : These prismsare right angled isosceles prisms whichturn the light through 90° or 180°. Thecritical angle for glass-air interface is 42°& in these prisms the angle of incidenceis 45°. Thus, the angle of incidence be-comes greater than the critical angle thatcauses total internal reflection. Such

prisms are used in periscopes.Combination of Colours

(a) Triangle representing primary colours and

complementary colours is :

(i) R + G + B = W

(ii) B + Y = W

(iii) R + C = W

(iv) G + M = W

(b) Triangle representing complimentary

colours and their combination is :

(i) B + Y = G

(ii) R + B = M

(iii) Y + R = O

(iv) Y + M = B + O = R + G = Black

Seven Colours of Visible Light

Colour Range of Range of frequnecy

Violet wavelength (λ ) (× 1014 Hz)Indigo 3900-4550 7.69-6.49

Blue 4550-4920 6.59-6.10

Green 4920-5770 6.10-5.20Yellow 5770-5970 5.20-5.03

Orange 5970-6220 5.03-4.82Red 6220-7800 4.82-3.84

Refraction Through Lenses

Lens is a transparent medium bounded by two refracting surfaces. Out of these

two refracting surfaces at least one isspherical.

A lens can be regarded as combinationof a larger no. of small prisms. When a beam of light is incident on a lens, the raysof light get refraction through these prisms.

Structure of a lens:

(i) Principal Axis : It joins the two centresof curvature of the two sphericalsurfaces of a lens.

(ii) Optical Centre : The central pointinside the lens where the passing rayremains undeviated is called opticalcentre.

(iii) Principal Focus : It is a point on theprincipal axis where the rays of aparallel beam of light meet or appearto meet after being refracted throughthe lens.

(iv) Focal Length : It is the distance between the principal focus & the

optical centre.

Difference between Primary and Secondary Rainbow

Primary Rainbow Secondary Rainbow1. It is much brighter. 1. It is fainter.

2. Outer arc is red and inner arc is violet. 2. Outer arc is violet and inner arc is red.3. Incident ray of light undergoes one total 3. Incident ray of light undergoes two total internal

internal reflection. reflections.



Physics [61]

A biconvex or a biconcave lens has two

centres of curvature and two foci.

Convex Lens

Concave lens

XY = principal axis, C = optical centre,

C1 & C

2 = Centres of curvaturess

F1 & F

2 = foci of lens.

Image formation by a lens :

(1) A convex lens can form both the real aswell as the virtual images dependingupon the position of the object.

(2) A concave lens forms always a virtualimage.

Power of a Lens (P) .

Its S.I. unit is Dioptre (D). ID = 1m–1.Smaller the focal length of a lens higher isits power.

Common Defects of Vision

Long sightedness (Hypermetropia): Aperson suffering from this defect cansee objects far away but cannot see thenearer object. In this defect eye forms

the image of an object behind the retina.For correct vision image should beformed at the retina only. This defect iscorrected by using a convex (converg-ing) lens of suitable focal length.

Short-sightedness (Myopia) : A per-son suffering from this defect can seethe nearer objects clearly but is unableto see objects far away. In this defectthe image is formed in front of retina.This defect is corrected by using a con-cave lens of suitable focal length.

Some facts regarding the Lenses :

(1) If a convex and a concave lens of thesame focal length are put in contact thecombination acts as a glass slab.

Defects of Human Eye

Defect What it is Cause Remedy1. Myopia or Short (i) Eye can see distinctly Focal length of eye lens Using a concave lens of

s ightedness only the near objects, decreases. Rays from µ focal length = distance of(ii) Far point of eye comes focus at a point in front far point from defective eye.

closer than infinity. of the retina.2. Hypermetropia or (i) Eye can see distinctly Focal length of eye lens Using a convex lens of focal

Long sightedness only the far off objects, increases. Rays from near length (f) where

(ii) Near point of eye point focus at a point at Here,

shifts away from the eye. the back of ratina. u = distance of object from eye

lens v = distance of near point of defective eye.




(2) If a convex and a concave lens of the

same focal length are placed co-axiallywith a small separation between them,the combination acts as a convex lens.

(3) If a lens is cut into two equal halveswith a plane perpendicular to theprincipal axis, the focal length of eachpart doubles.

(4) If a lens is cut into two equal halveswith a plane along the principal axis,the focal length of each part remainsunchanged.

(5) An air bubble in water behaves as aconcave lens.

(6) When a lens is placed in a medium of higher density (higher refractiveindex), the nature of the lens reversesi.e. a converging lens behaves as adiverging lens & vice-versa.

(7) When a lens is placed in a medium of

equal density, it acts as a glass slab. Itsfocal length becomes infinity andpower becomes zero.

OPTICAL INSTRUMENTS

Microscope : It is used to magnifyminute objects close to us.

(i) Simple Microscope (Magnifyingglass) : It is a biconvex lens. When anobject is placed within its focus, a

magnified, virtual and erect image isformed.

(ii) Compound Microscope : It consistsof two convex lenses of unequal size placedco-axially. The larger lens is eye-lens to seethe final image and the smaller lens is objectlens in front of which the object is placed.The object lens forms a real inverted image

Difference between Astronomical and Galilean Telescope

Astronomical Telescope Galilean Telescope

1. Used for observing far off objects in the sky. 1. Used for observing far off objects on the ground.

2. Final image is inverted w.r.t. the object. 2. Final image is erect w.r.t. the object.3. Eye piece is a convex lens. 3. Eye piece is a concave lens.

4. 4.

5. Length of telescope tube is greater, L = f 0 + f

e5. Length of telescope tube is smaller, L = f

o – f

e

of the object. This image is formed within thefocus of the eye-lens and acts an object forthe eye-lens. The eye-lens forms a highlymagnified virtual image of the object.

Telescope : It is used to see far off

objects.

(i) Astronomical Telescope : It is usedto see stars, planets etc.

It consists of two convex lenses. Thelarger one is the object lens and the smallerone is the eye lens. The real image formed by the object lens within the focus of the

eye-lens, acts as an object for the eye-lenswhich gives the final, virtual and highlymagnified image.

DISPERSION OF LIGHT

When white light passes through a glassprism, we get seven colours on a whitescreen. This phenomenon is known asdispersion of light. These seven coloursobtained are viz. violet, indigo, blue, green,yellow, orange and red. They can beremembered as ‘VIBGYOR’. Dispersion of



Physics [63]

light occurs due to refraction of light of

different wavelengths contained by whitelight. Smaller the wavelength of light, moreis its deviation. Since wavelength increasesfrom violet to red light, hence, the deviationdecreases from violet to red light. A bandof seven colours of white light obtained bydispersion is known as a spectrum.

Spectroscope (or Spectrometer) : Adevice used to disperse light and to observe

the spectra obtained is called spectroscope.

COLOUR OF OBJECTS

Colour of any object is the same as is

reflected by the object and received by our

eyes. e.g. when white light falls on a red-

rose, it reflects only wavelength of red

colour and absorbs all other colours of white

light. Hence, it appears red.

Pigment : It imparts colour to the

objects. A pigment absorbs most of the

colours of white light and reflects one ormore colours e.g. (i) A red rose appears

black in green light. A red pigment in rose

absorbs the green light and reflects nothing.

So red rose appears black. (ii) A yellow

flower appears red in red light. This is

because yellow pigment in the yellow

flower cannot absorb red light and hence,

reflects the red light.

Primary Colours : They cannot be

obtained by mixing two or more colours.

They are viz. Red, Blue and Green. When

they are mixed together, the result is white

i.e. Red + Blue + Green = White

Secondary Colours : When two

primary colours are mixed, the new colour

is a secondary colour.

They are :

(1) Magenta (M) = Red + Blue

(2) Yellow (Y) = Red + Green

(3) Cyan (C) = Green + Blue

(4) White (W) = Red + Green + Blue.

Complimentary Colours : Any twocolours are said to be a pair of complimentary colours if these coloursproduce white, when mixed together e.g.

(i) Yellow + Blue = White

∴ Red + Green + Blue = White & Red +Green = Yellow.

(ii) Magenta + Green = White

∴ Red + Blue + Green = White & Red +Blue = Magenta.

(iii) Cyan + Red = White

∴ Green + Blue + Red = White & Green+ Blue = Cyan.

Thus, the complimentary pairs are(i) Yellow + Blue, (ii) Magenta + Green &(iii) Cyan + Red.




Primary Pigments : They are Yellow,

Cyan & Magenta. When these threeprimary pigments are mixed in equalproportion a black pigment is obtained.

Yellow pigment + Cyan pigment +Magenta pigment = Black pigment.

Because, this mixture of pigmentsabsorbs all colours of white light and reflectsnothing, so it appears black.

Secondary Pigments : When a pair of primary pigments are mixed together, a

new pigment is obtained called secondarypigment. They are:

Primary Pigments SecondaryPigments

(i) Cyan pigment + Yellow Green Pigment

pigment

(ii) Cyan pigment + Blue Pigment

Magenta pigment

(iii) Yellow pigment + Red pigment

Magenta pigment

Thus, the secondary pigments areGreen, Blue and Red pigments.

The primary pigments are same as thatof secondary colours whereas thesecondary pigments are same as that of primary colours.

SCATTERING OF LIGHT

When light passes through a mediumand falls on the particles of the medium, itgets scattered in all directions. The intensity

of scattered light decreases with increasein the wave length of light i.e., smaller thewavelength of light, more is the intensityof its scattering. e.g.

(i) Blue colour of Sky : Most of the bluecolour of white light coming from thesun gets scattered by atmospheric

particles because blue colour has

smaller wavelength.

(ii) The sun looks red at the sunrise andsunset: The red colour is scattered leastdue to its longest wavelength andhence, the red-light is able to travelmaximum distance in atmosphere. Atthe time of the sunrise and sunset, theposition of sun is lower in sky andhence, it looks orange-red.

(iii) Due to least scattering of red-colour,

it is used as a sign of danger-signals.Rainbow : It is a spectrum of sun’s

light in nature. It occurs in the form of arcsof concentric coloured circles in the sky,when the sun’s light falls on rain drops.Rain-drops act as tiny prisms that causedispersion of light. The essential conditionfor observing a rainbow is that the observermust stand with his back towards the sun.Rainbow is formed due to the total internalreflections and refractions caused by rain

drops.Wave-Optics : Light is a transverse

wave lying under the electromagnetic wavespectrum. Its speed in vacuum is 3 × 108m/s. It can undergo different wavephenomena like reflection, refraction,dispersion, interference, diffraction,polarisation, superposition, etc.

INTERFERENCE OF LIGHT

It involves superposition of two lightwaves at a point in the medium of propagation of light. When these lightwaves superpose crest to crest or trough totrough, the result is constructiveinterference whereas crest to troughsuperposition results in destructiveinterference. Coloured soap bubbles and



Physics [65]

oil film on water surface are observed due

to interference of white light. For thisthickness of wall of the bubble and oil filmshould be of the order of wavelength of visible light (4000A° to 8000A°). Thephenomenon of interference is used inholography (three dimensional virtualimage formation) by using a laser beam.

POLARISATION OF LIGHT

It involves restricting all vibrations of alight wave in a single plane.

Polaroids : Materials that polarise light.Most of them are artificial. Tourmaline is anatural polaroid.

Uses of Polaroids :

In Sun glasses to protect the eyes fromglare.

In head light of motor-car to reduceglare.

In wind shields of automobiles.

In holograhy (3D Motion pictures)

In old oil paintings to improve colour

contrast.

In optical stress analysis.

In calculators and watches, letters andnumbers are formed by LCD (liquidcrystal display through polarisation of light.

Polarisation of scattered sunlight is usedfor navigation in solar compass in po-lar regions of earth, where magneticcompass becomes non-functional.

In CD Players, polarized laser beamacts as needle for producing sound fromcompact disc, which is in encoded digi-tal format.

Polarisation is used to study asymme-tries in crystals and molecules usingthe phenomenon of optical activity.The ability of a crystal or molecule torotate the plane of polarised light,either clockwise or anticlockwise iscalled optical activity.




Heat is the form of energy that flowsfrom a body at higher temperature toanother body at lower temperature, whenthe two bodies are in contact with eachother. Every body is composed of a largeno. of particles which possess certain kineticenergy. The total sum of kinetic-energy of all constituent particles of a body is equalto heat contained by the body.

TEMPERATURE

It is the degree of hotness or coldness of the body. It is just a scale that measures thethermal state of a body. All bodies in thermalequilibrium are assigned equal temperature.

A hotter body is assigned higher temperturethan a colder body. The temperatures iof the two bodies are said to be in thermalequilibrium if no transfer of heat occurswhen they are placed in contact.

Scales of Temperature : Commonscales for the measurement of temperatureare (i) degree centigrade or degree celsius(°C) (ii) Kelvin (K) and (iii) degreeFahrenheit (°F) scales. They are related toeach other as follows :

or,

∴ TK = t°C + 273, t°C & so on.

The temperature of a body is measured by a

Thermometer: There is no upper limitof temperature but there is a fixed lower

limit. The lowest value of temperature iscalled as ‘Absolute Zero’. The absolute zerois equal to zero kelvin (O°K) or (–)273.15°C.

Clinical Thermometer : It is a mercuryin glass type thermometer. Thethermometer scale is marked from 95°F to110°F or 35°C to 43°C within the range of human body temperature. Advantages of mercury in the thermometer-

(i) it is opaque and shining and hence,temperature can be read easily.

(ii) it is a good conductor of heat & hence,measures temperature quickly.

(iii) it does not stick to the wall of glass tube& also does not vaporize much &hence, gives correct readings.

A clinicial thermometer should not besterilized in hot water otherwise mercurywill expand too much and break the glass.Instead of mercury, water cannot be usedinside the clinical thermometer because it

freezes at 0°C whereas mercury freezes at(–) 39°C. Hence, in the cold countries wheretemperature in winter is usually less than0°C, alcohol thermometers are useful. Thetemperatures recorded in the weatherforecasting are carried out by using a specialtype of thermometer called the six’smaximum and minimum thermometer.

HEAT



Physics [67]

THERMAL EXPANSIONThe increase in length, area or volume

of an object due to rise in its temperature iscalled as thermal expansion. The objectshows contraction due to decrease in itstemperature. The expansion/contractionof an object depends upon the change intemperature & its nature i.e. expansivitiy.

Practical applications of thermalexpansion:

• A small gap is always left in betweentwo iron-rails in a railway line.

• An iron rim to be fitted on a woodencart wheel is always slightly small indiameter. The rim on heating expandsand upon cooling gives a strong grip onthe wheel due to contraction.

• Some suitable space is left between thegirders used for supporting bridges.

• A glass stopper jammed in the neck of a glass bottle can be removed by warm-

ing the neck of the bottle.

• The clock pendulums are usually madeof invar (an alloy), which has very lowexpansivity. This enables the clock tokeep correct time in different seasons.

Bimetallic Strip : It consists of a brasslayer & an invar layer riveted together.The expansivity of brass is more than thatof invar. Hence, due to rise in temperaturethe strip expands into a curved shapehaving brass on the convex side. Whentemperature decreases, the strip retainsoriginal shape. Bimetallic strips are used asswitches in thermostats which are used forregulating temperatures of electricallyheated rooms, ovens, toasters, etc.

Unusual Expansion of Water : Thevolume of water decreases & density

increases with increase in temperature from

0°C to 4°C. But its volume increases anddensity decreases with increase intemperature above 4°C. Thus, the densityof water is maximum at 4°C. This unusualexpansion of water has a favourable effectfor aquatic animals. Since the density of water is maximum at 4°C, water at the bottom of lakes, ponds etc. remains at 4°Cin winter even if at the surface it freezes.This allows marine animals to remain aliveand move near the bottom.

SPECIFIC HEAT

The specific heat of a substance is equalto the amount of heat absorbed by its unitmass to raise its temperature through onedegree. Thus, larger the value of specificheat of a substance, smaller is rise in itstemperature inspite of absorbing a largeamount of heat. The specific heat of asubstance depends upon the nature and itsphysical state. As specific heat capacity of

water is large (4200 J kg–1

k–1

), hence, byabsorbing or releasing large amounts of heat,temperature of water changes by smallamounts. That is why water is used in hotwater battles and also as coolant in radiators.

VAPOURS

A vapour is a gas that can be liquified by increasing the pressure withoutchanging the temperature. Gas and vapourare two distinct state of matter. A gas

cannot be liquified by the application of pressure alone, howsoever large thepressure may be. However, a gas can beliquified by applying high pressurelowering the temperature.

When the molecules of a liquid escapethe liquid surface slowly, it results in vapour




above the surface and this process is known

as evaporation. Evaporation lowers theenergy of a liquid and hence, causes coolingeffect. This effect is used in cooling waterin pitchers having porous walls. When aspace contains the maximum possibleamount of vapour, the vapour is calledsaturated. If the amount is less thanpossible maximum amount of vapour, thevapour is called unsaturated. The possiblemaximum amount of vapour depends onthe temperature & can be achieved easilydue to rise in temperature of the liquid.The pressure exerted by a saturated vapouris called saturation vapour pressure (SVP).The temperature at which the saturationvapour pressure becomes equal to thepresent pressure is know as Dew-Point. If the temperature is decreased below thedew point some of the vapour condenses.

HUMIDITY

The amount of water vapour in a unitvolume of air is called the Absolute

humidity of air. Generally, it is expressedin gm/m3. The ratio of the amount of watervapour present in a given volume to theamount of water vapour required tosaturate the volume at the sametemperature is called the relative humidity(R.H.). The R.H. can be given in terms of thefollowing ratios as R.H.

Generally, relative humidity (R.H) is

expressed in a percentage. If the aboveratio is 0.5, the relative humdity is 50%. If the air is already saturated, the R.H. is

100%.

Dew : In winter nights, the atmospheric

temperature goes down. The surfaces of window-panes, flowers, grasses etc. become

still colder due to radiation. The air nearthem becomes saturated & condensation

begins. The droplets condensed on such

surfaces are called as Dew.Fog : In winter, if temperature goes

down even more, the whole atmosphere in

that region may become saturated. Small

droplets then condense on the dust particles

present in the air. These droplets keep

floating in the air & form a thick mist

which restricts visibility. This thick mist is

called Fog.

In winter, glass-walls of a house,

window & car-glass get wet on its outersurface. It occurs due to the condensation

of water-vapour in the atmosphere on the

glass surface as the temperature insidehouse or car is relatively higher than outer

temperature. In summer, body temperatureis regulated by the evoporation of sweat.

However, at higher humidity in air, therate of evaporation from the body slows

down and sweat starts rolling off in streams.

Sitting under a fan then increases the rate

of evaporation due to moving air. Theincreased evaporation produces cooling.

The human body is comfortable at atemperature between 23°C to 25°C at a

relative humidity between 60% to 65%. An

air conditioner regulates these conditions

of temperature and humidity inside a room.



Physics [69]

BOILING OF A LIQUIDThe energy of a certain mass of a liquid

in its vapour state is more than in its liquidstate. When heat is supplied to a liquid, itskinetic energy increases and at a certainstage, the molecules anywhere in liquidcan form vapour bubbles. These bubblesfloat to the surface and finally vapourmolecules come out of the liquid. Thisphenomenon is known as boiling and thetemperature at which boiling occurs is

known as boiling point. At the boiling point,the vapour pressure at the liquid surface becomes equal to the atmospheric pressure.Hence, higher the atmospheric pressure atthe liquid surface higher is the boiling pointof liquid. Water boils at 100°C when theatmospheric pressure is 1 atmosphere. 1atmosphere is equal to 760 mm of mercury.If the pressure becomes 2- atmosphere,water boils at 120°C. In pressure cookerwater boils at temperatures higher than100°C due to highly increased pressure.The increased boiling point allows waterto hold more heat which cooks food-faster.At higher altitudes, atmospheric pressureis reduced. It lowers the boiling point of water and food takes longer time to cook.Hence, a pressure cooker is more useful forcooking on hill-stations.

HEAT ENGINES

A heat engine converts heat into

mechanical work. It takes heat from a bodyat higher temperature, converts a part of heat into the mechanical work & deliversthe rest to bodies at lower temperature.The working substance inside the heatengine comes back to the original state.Thus, a heat engine works in cyclic process.The efficiency of a heat engine is equal to

the ratio of the mechanical work done by

the engine to heat supplied to it i.e.,

Efficiency of a heat engine is alwaysless than 1 or 100% because a heat enginecannot convert total heat supplied to itinto mechanical work.

A heat engine consists of threecomponents viz.

(i) Source of heat : It is at highertemperature from which the engine getsheat.

(ii) Working Substance : It convertsthe heat obtained partly into mechanicalwork & supplies the remaining heat to thesink.

(iii) Sink : It is at lower temperature toabsorb unused heat.

Types of Heat Engine :

(i) External Combustion Engine : In anexternal combustion engine, heat isproduced by the burning fuel in a chamberoutside the body of the engine e.g. steam-engine. Super heated steam at 250°C & 20atmospher is its working substance.Practically, its efficiency is about 12% to16%.

(ii) Internal Combustion Engine : Inthis engine, heat is produced by burning of fuel inside the body of engine e.g. Petrol &diesel engine. Its working substance is amixture of air (98%) and fuel (2%). Petrolengine with a practical efficiency of 26% isused in light vehicles while diesel enginewith a practical efficiency 40% is used inheavy vehicles.

(iii) Refrigerator or Heat Pump : It is aheat engine working in reverse direction.




Its working substance takes an amount of

heat from a cold body, converts it partlyinto work & supplies the heat left to thehot body. Food stuffs, water, etc. inside therefrigerator act as a source of heat. A highlyvolatitle liquid, Freon (dichlorodifluoromethane) is its working substance. The outersurrounding at higher temperature acts assink.

The freezer is surrounded by a coppercoil in which freon evaporates that causescooling effect. The vapour is removed and

condensed to form liquid in the condensercoil, fitted at the back of the cabinet, by acompression-pump or compressor. Thecondenser coil becomes warm due to theliquification of vapour. Again, the liquid begines to evaporate & this cyclic processcontinues. A thermostat switch regulatesthe temperature inside the refrigerator byswitching the compressor on and off atintervals. When the refrigerator isdefrosted, the temperature inside itincreases. Hence inner ice content melts.Defrosting is necessary for better workingof the refrigertor.

HEAT TRANSFER

Conduction, convection and radiationare the three methods to transfer heat fromone point to another. Usually, conductiontakes place in solids, convection in liquids& gases and radiation does not require amedium.

A. Conduction : It involves heat-transferfrom the hot end of a body to its cold enddue to molecular collision to minimise thetemperature difference. However, theconduction does not involve the actualmovement of the molecules of the medium.

In general, solids are better conductors

than liquids & liquids are better conductors

than gases. Metals are much betterconductors than non-metals. Metals havefree moving electrons which carry heatfrom one point to the other in a metal.

For a given body, the rate of flow of heat by the conduction is directlyproportional to the cross-sectional area,proportional to the temperature difference between the ends & inversely proportionalto the distance between the ends. Hence, athick & short metallic rod conducts more

heat in a given time across its two endsthan a thin & longer metal-rod.

Applications of Thermal Conduction:

• Cooking utensils are made of metalswhereas their handles are made of plas-tic or wood because metals are betterconductor and plastic & wood are poorconductors. Hence, hot utensils can beeasily held by their insulating handles.

• In winter, metallic handle of a wooden

door appear colder because it is a goodconductor. Heat flows from our bodyto the handle which gives the sense of cold.

• Eskimos live in double walled snowhuts called igloos. Air enclosed betweenthe walls of ice is a bad conductor andice is also a bad conductor. It preventsthe heat they produce from escapingand keeps them warm.

• Saw dust is a poorer conductor of heat

than the wood from which it is made because of air trapped in the saw dust.

• In winter, birds often swell their feath-ers. Air enclosed between their bodyand feathers does not allow flow of heat from the body of the birds to thecold surroundings.



Physics [71]

• Ice is packed in gunny bags or saw-

dust. Air trapped in the saw dust blocksthe transfer of heat between the coldice and the surroundings. It preventsthe melting of ice.

• A new quilt is warmer than an old one.The new quilt encloses more air thanthe old one. This air, being bad conduc-tor of heat, does not allow heat of our body to flow to the surroundings.

• Two thin blankets together are warmerthan one blanket of their combinedthickness. The air contained in betweenthe two blankets is a bad conductorand it prevents heat from escaping andkeeps the body warm.

• In airconditioned rooms, double lay-ered windows are preferred than thesingle layered windows because a thinlayer of air is contained between thetwo layer of glass panes in the win-dow.

• In winter, a stone floor feels cold to the bare feet, but a carpet on the same floorfeels warm even though they, are at thesame temperature. Since the stone is a better conductor of heat than the car-pet, hence the feet feel cold on the stone but not on the carpet.

• Due to the continuous working of arefrigerator, a thick layer of ice depositson the outside and the inside of thefreezer and hence, it has to be switchedoff for defrosting. The cooling action of

the freezer slows down because ice is a bad conductor of heat. Hence, defrost-ing improves the functioning a refrig-erator.

B.Convection : It involves the transferof heat due to the movement of molecules.The hot molecules have higher kinetic

energy and hence, they move towards the

cold region and in order to occupy theirposition the cold molecules come from thecold region. It results convection currentsand consequently the entire system getsheated. Convection is common in liquidsand gases.

Applications of Thermal Convection:

• Function of ventilation in houses is basedon the convection. During winter, aircomes out from houses because the in-side temperature is high whereas in

summer air comes in because the outertemperature is high. It involves themovement of air from high in tempera-ture to lower temperature by convec-tion currents.

• In refrigerators, freezer is set at the top.Temperature of air at the bottom ishigher than that inside the freezer. Thewarm bottom air moves up and coldfreezer air moves down. It results con-vection current of air in the refrigera-

tors that cools the entire inner space.• In geysers and waters heater, the heat-

ing elements are fitted at the bottom. Itcauses hot-bottom water to move upand cold top-water to move down andthereby setting up a convection cur-rent. Consequently entire water con-tent gets heated.

• In electric ovens, the heating elementsare fitted near the bottom to heat theentire enclosed air by convection.

• Monsoons, trade wind in sea, waterstreams in sea, winds, etc. are due tothe convection.

• The main mechanism for heat transferinside a human body is forced convec-tion through blood which is circulating by the pumping action of heart.




C. Radiation : Heat transfer by

conduction and convection requires amaterial medium whereas heat transfer byradiation does not require a medium. All bodies radiate thermal radiations at alltemperatures. The amount of heatradiation radiated per unit time dependson the nature of the radiating surface,surface area and temperature of the body.Similarly, all bodies absorb a part of thermalradiation incident on them. When anamount of thermal radiation is incident ona body, it is partly absorbed, partly reflectedand partly transmitted. The amount of absorbed, reflected and transmitted thermalradiation depends on the nature &temperature of the body.

When a body absorbs more and emitsless radiation, its temperature goes up; whena body emits more and absorbs lessradiation, its temperature goes down andwhen a body absorbs and emits equalamount of radiations, its temperatureremains constant. A good absorber is a goodemitter and a poor absorber is a poor emitter.A body that absorbs all the radiations fallingon it is called a black body.

Applications of Black Body: Such a body will emit radiation at the fastest rateon heating. Lampblack is close to a blackbody. Bolometer and thermopile areclose to a blackbody. Bolometer andthermopile are instruments used to measurethermal radiation.

• A rough black surface is a good emitteras well as a good absorber of heat-radiation while a bright polished surfaceis a bad emitter as well as a badabsorber. Hence, a bright polished cupkeeps tea or coffee warm for a longertime in comparison a rough black cup.The base of an electric iron is highly

polished so that it does not lose heat by

radiation. White or light colours of houses keep cooler in summer becauselight coloured surface do not absorbmuch solar radiation.

• Thermos Flask : It is a double walledglass bottle having a vacuum betweenthe walls. The inner wall is silveredwhose mouth is closed by a plasticstopper. The vacuum does not allowthe loss of heat by convection and thestopper being insulator does not allow

conduction of heat. The silvered wallprevents radiation as it is a poor emitterand a poor absorber of radiation. Thus,in a thermos flask transfer of heat byconduction, convection and radiationis minimised. Hence, a hot liquid in itremains hot and a cold liquid or ice in itremains cold for a longer time.

• At a point in front of fire, heat is receiveddue to radiation only, while at a pointabove the fire, heat is received both due

to radiation and convection. Hence, itis hotter at the same distance over thetop of a fire than in front of it.

• When animals feel cold, they curl their bodies in a ball so as to decrease thesurface area of their bodies. As totalenergy radiated by a body isproportional to the surface area of the body, the loss of heat due to radiationwould be reduced.

• A box or a house with glass walls or

glass windows, acts as a green-house because it traps heat radiations enteringit through glass. The glass allows theheat radiation inside but blocks the hot-air from comeing out. Hence, a carparked in the sun with its windowsclosed gets terribly warm in comparisonto the outer atmosphere.



Physics [73]

• Cloudy nights are warmer than clear

nights because clouds reflect theradiations emitted by the earth at nightand keep it warm. Thus, clouds act as a blanket.

• Glass, which is ordinarily used as the base in photographic plates, istransparent to light while opaque tothermal infra-red radiation. But certainspecial kinds of glass have beenprepared which are transparent onlyto thermal infra-red. Such glasses, used

in the preparation of photographicplates, have made it possible to obtainlong distance photographs, even undermisty conditions using infra-redradiations.

• The fact that good reflectors are badabsorbers and bad radiators is utilisedin making firemen’s helmets and metallictea-pots highly polished on the outside.

Water can absorb 90% of heat radiation,

which shows the importance of theexistence of water vapour in theatmosphere. The water vapours in theatmosphere protect the earth from themore intense heat rays of the sun duringthe days and from the extreme coldduring the nights. In very dry regions,where there is little water vapour in theair, the days are intensely hot and thenights are extremely cold.

• Gases are poor radiators of heat, hence

in the construction of furnaces, thegases are made to fall on fire-bricks,which radiate heat freely.

• Because black bodies are the bestabsorbers of heat radiations, a blackdress is uncomfortable during the hotsummer, while a white one is cool.

NEWTON’S LAW OF COOLINGIt states that the rate of loss of heat

from a hot body is directly proportional totemperature difference between the bodyand its surroundings.

A hot water during cooling from 100°Cto 90°C takes less time than that in coolingfrom 50°C to 40°C. Hence, when hot waterand fresh tap-water are placed inside arefrigerator, the rate of cooling of hot wateris found to be faster than that of fresh-tap

water.

If hot coffee is served with cream in a cupand without cream in another cup, thecreamed coffee remains hot for a longer time.

LAWS OF THERMODYNAMICS

First Law : The first law of thermodynamics is a particular and morerigorous statement of the principle of conservation of energy. When a substance

absorbs an amount of heat at a constantpressure, then part of this heat is used upto raise the temperature which results inthe increase of internal energy. The rest of the heat is used in doing work in allowingthe external pressure.

Second Law : It is impossible toconstruct a device working in a cyclicprocess whose sole effect is the transfer of heat from a body at a lower temperature toa body at a higher temperature ‘OR’ it is

impossible to construct an engine workingon a cyclic process whose sole effect is theconversion of all heat supplied to it in anequivalent amount of work.

Entropy : It is a measure of randomnessor disorderness of a system. It is a thermalproperty of a body which remains constantduring an adiabatic process.




Third Law : At absolute zero (zero

Kelvin) temperature the total sum of absolute entropy of a crystalline solid isequal to zero.

THERMODYNAMIC PROCESS

Isobaric Process : It involves constantpressure

Isochoric Process : It involves constantvolume.

Isothermal Process : It involves

constant temperature. However, heatchanges.

Adiabatic Process : It involves constantheat. However, temperature changes.

Melting process and boiling process arecommon examples of an isothermal process because they occur at constanttemperatures. Practically, a perfectadiabatic process is impossible. However,its approximate examples are (i) Sudden bursting of the tube of a bicycle tyre (ii)Propagation of sound in air. (iii) Expansionof gas or vapour in a heat engine.

LAWS OF GASES

• Boyle’s Law : For a given mass of a gas,the volume is inversely proportional toits pressure at a constant temperature.

If P = pressure and V = volume of agiven mass of a gas at a constant

temperature then,constant

or, P1V

1=P

2V

2

Where P1 and P

2 are initial and final

pressures and V1 and V

2 are initial and

final volumes of a given mass of a gas at aconstant temperature.

• Charle’s Law : The pressure remaining

constant, the volume of a given mass of a gas is directly proportional to itstemperature in Kelvin scale i.e. V a T or

= constant; where V is volume & T is

temperature in Kelvin.

• Gay Lussac’s Law : The volumeremaining constant, the pressure of agiven mass of a gas is directlyproportional to its temperature inKelvin i.e.

P a T or = constant, where P is pressure

and T is temperature in Kelvin of agiven mass of gas at constant volume.

• Ideal Gas Equation : It is also known

as standard or the perfect gas equationwhich is given as

PV = nRT

where P = pressure, V = Volume, T =temperature in Kelvin, n = number of moles and R = universal gas constant.

KINETIC THEORY OF GASES

The Kinetic Theory of gases explainsthe behaviour of gases. Basic features are:

(i) All gases are composed of molecules.The molecules of a gas are alike anddiffer from the molecules of other gases.

(ii) The molecules are extremely small pointmasses, their dimensions beingnegligible as compared to the distance between them.



Physics [75]

(iii) The molecules are continuously in

motion. They have different velocitiesin different directions. The velocity of gas molecules increases withtemperature.

(iv) The gas molecules collidecontinuously against each other andagainst the wall of container.

(v) The pressure of the gas is equal to thepressure exerted by the gas moleculeson the wall of container.

(vi) The gas-molecules are perfectly elasticspheres and exert negligible force of attraction or repulsion on each otheror on the wall of container.

The kinetic theory of gases explainsmost of the gas laws like Boyle’s law,Charle’s law, ideal gas equation, etc.

TRIVIA

• The net change in the intermolecularkinetic energy is determined by the tem-perature. This is in accordance withthe kinetic theory of gas. That is aver-age kinetic energy associated with in-termolecular forces is directly propor-tional to the temperature of the system.

• The heat capacity or thermal capacitydepends on nature of the substance(specific heat capacity) and mass orquantity of matter of the body.

• Thermodynamics deals with the con-

version of heat into other forms of en-ergy as well as the change in state (solid,liquid, gas) of a system.

• Random motion of the constituents of the system involving exchange of en-ergy due to mutual collisions is calledthermal motion.

• Total kinetic energy, or internal energy

or total energy does not determine thedirection of flow of heat. It is deter-mined by the temperature alone.

• When heat is absorbed by a body but itstemperature does not change, only theintermolecular potential energy of thesystem changes. This is the case withmelting and boiling.

• Systems are in thermal equilibriumwhen their temperature are same oraverage kinetic energy per molecule issame.

• Internal energy consists of energy of translational, rotational as well as vibrational motion of the molecules.

• The ratio of molar specific heat capac-ity depends on the molecular weight of the substance. Because it depends onmass of one mole which is turn de-pends on molecular weight.

• The temperature, volume or pressure

of a system may remain constant whenit absorbs heat.

• The temperature of the system may in-crease when heat is supplied to it orwork is done on the system.

• Whole of the heat supplied can never be converted into mechanical work.This implies that the efficiency of heatengine can never be equal to 100%.

• Thermodynamics deals with the

interconversion of heat energy, internalenergy and mechanical energy or work.

• The physics of heat engine is similar tothat of refrigerator.

• Hydrogen cannot be used as a thermo-metric substance above 500°C, becauseit starts diffusing.




• Pure and dry gas should be used as

thermometric substance.

• Below -200°C, the hydrogen and nitro-gen cannot be used because they startliquifying. Therefore, helium gas is usedfor temperature below - 200°C.

• The platinum thermometer can mea-sure temperature accurately upto 0.1°C.

• The thermoelectric thermometers arevery sensitive and can be used to mea-sure the temperature of insects.

• Radiation Pyrometer : These are thedevices to measure the temperature bymeasuring the intensity of radiationsreceived from the body. They are basedon the fact that the amount of radia-tions emitted from a body per unit areaper second is directly proportional tothe fourth power of temperature(Stefan’s law). These can be used tomeasure temperatures ranging from800°C to 4000°C. They cannot measure

temperatures below 800°C, because theamount of radiations emitted from the bodies is too small to be measured.

• Vapour Pressure Thermometer : Thevapour pressure depends on the tem-perature alone and it is independentof the nature of the vapours. This factcan be used to measure the tempera-ture. It can be used to measuretemperatues near absolute zero byusing different gases as the thermo-

metric substances.• Magnetic Thermometer : The magnetic

thermometer is a device that uses adiabaticdemagnetisation for measuring the tem-perature. They can be used to measuretemperature very near to the absolute zero.

• The mercury thermometer with cylin-

drical bulb are more sensitive than those

with spherical bulb.

• Alcohol thermometer is preferred to themercury thermometer due to the largervalue of the coefficient of cubical ex-pansion.

• Following properties make mercury theideal thermometric substance.

(i) Does not stick to the glass walls.

(ii) It shines.

(iii) Coefficient of expansion is uniform.(iv) Vapour pressure is low.

(v) Low thermal conductivity andspecific heat.

(vi) Available in pure form.

• Gas thermometers have higher sensi-tivity than the mercury thermometers because their coefficient of cubical ex-pansion is much larger and same is forall gases:

(i) Celsius & Fahrenheit at - 40°C = -40°F.

(ii) Fahrenheit and Kelvin at 574.25°F= 574.25 K.

(iii) Fahrenheit and Reaumur at -25.6°F = - 25.6°R

(iv) Reaumur and Celsius at 0°R = 0°C

(v) At no temperature the Celsius scalecan have the same reading as theKelvin scale.

• Six’s thermometer measures minimumand maximum temperature during aday. It uses mercury as well as alcoholas the thermometric substance.

• Let the reading on the faulty thermom-eter be f and the true treading be t, then



Physics [77]

Normal human temperature is 37°C =

98.4°F

• The clinical therometers read from 96°Fto 110ºF.

• Thermo electric therometers can be usedto measure rapidly changing tempera-ture.

• Clinical thermometer are much shorterthan the laboratory thermometers be-cause they are used to measure a lim-ited range (96°F to 110°F) of tempera-

ture.• Platinum resistance therometer can be

used to measure temperature inside amotor engine.

• The radiation therometers can measuretemperature from a distance.

• Adiabatic demagnetisation can be usedto meausre temperatures very near tothe absolute zero.

• The degree Fahrenheit is the smallest

temperature range among all the scalesof temperature.

• Heat : It is a form of energy. The SI unit of heat is joule.

(i) Calorie : It is the amount of heat re-quired to raise the temperature of 1gof water through 1°C. For interna-tional use the calorie is defined asthe amount of heat required to raisethe temperature of 1g of water from14.5°C to 15.5°C. [This definition is

based on the average value of heatrequired to raise the temperature of 1g of water from 0°C to 100°C per°C rise in temperature]

(ii) Kilocalorie (k.cal) = 1000 cal =amount of heat required to raise thetemperature of 1 kg of water by 1°C.

(iii) British Thermal Unit (BTU) : It is

the fps unit of heat = 252 cal =amount of heat required to raise thetemperature of 1 lb of water through1°F.

• 4.2 J = 1 cal

• Specific Heat : It is the amount of heatrequired to raise the temperature of unitmass of substance through 1 degree. Itsdimensional formula is ML2T–2q–1. Itmay be expressed in cal/g°C, kcal/kg°C.BTU/lb°F. J/kgk

• Cal/g°C = kcal/kg°C = 4.2 × 103 j/kgK.

• Molar Specific Heat : The amount of heat required to raise the temperatureof 1 mole of substance through 1 kg iscalled molar specific heat. It is denoted by C. Its unit is J/mol K. It is generallyused for gases.

• For the gases, molar specific heat is de-fined at constant volume (C

v) and at

constant pressure (CP). It is found thatC

P- C

v = R. Where R is molar gas con-

stant.

• Thermal Capacity : It is the amount of heat required to raise the temperatureof substance through 1°C or 1K. Ther-mal capacity = mass × specific heat.The dimensional formula for thermalcapacity is ML2T2q–1. Its unit is cal/°C,kcal/°C, J/K.

• Water Equivalent : It is the mass of water that requires the same amount of heat to raise its temperature through1°C as the substance requires to raiseits temperature through 1°C. Its unit iskilogram or gram. If m be the mass of the body and c be its specific heat, thenwater equivalent is given by W = mc.




• Principle of Calorimetry : The amount

of heat gained or lost when its tem-perature rises or falls by Dq is given by :Q = mc Dq, where m is the mass of thesubstance.

• According to the principle of calorim-etry, when two bodies exchange heat :the heat lost = the heat gained. It is inaccordance with the law of conserva-tion of energy.

• Latent Heat : Heat required to changethe state (from solid to liquid or from

liquid to gaseous, of one gram/kilo-gram of substance at constant tem-perature is called latent heat (symbolL). Its unit is cal/g, kcal/kg or J/kg.

• The amount heat absorbed or given outduring the change of state is given by :Q = ML, where M is the mass of thesubstance.

• Latent heat is of two types : (i) Latentheat of fusion for change from solid to

liquid at the melting point and (ii) La-tent heat of vaporisation - for change of sate from liquid to gaseous state.

• Latent heat of fusion of ice is 80 cal/g.

• Latent heat of vaporisation for water is535 cal/g

• Latent heat is used for doing work in

increasing the distance between the mol-ecules during the change of state. Thatis the latent heat increases the potentialenergy of molecules and their kineticenergy remains constant, therefore, thetemperature also remains constant.

• Sublimation : Direct conversion of solidto vapours is called sublimation.

• HoarFrost : Direct conversion of vapours to solid is called hoarfrost.

• Melting : It is the process in whichsolid is converted into lqiuid. The re-verse is called freezing or solidifcation.

• Boiling : It is the process of conversionof liquid to gaseous state. The reverse iscalled condensation or liquefaction.

• Melting and boiling occur at definite tem-peratures called melting point and boil-ing or liquefaction.

• The liquids boils at a temperature, atwhich its vapour pressure is equal tothe atmospheric pressure.

• Evaporation : Conversion of liquid intogaseous state at all temperatures is calledevaporation. It is a phenomenon thatoccurs at the surface of the liquid.

• The rate of evaporation increases withthe rise in temperature.



Physics [79]

An Electromagnetic radiation is anelectromagnetic wave (E.M.W.) thatconsists of oscillating electric and magneticfields. An accelerated charge (e.g.oscillating charge) produces an oscillatingelectric as well as magnetic field in itsneighbourhood. These oscillating fields actas sources for each other and therebyproduce each other. It results an E.M.W.The accelerated charge reduces its energythat appears as energy of E.M.W. Theelectric-field, magnetic field and speed of the E.M.W. are mutually perpendicular. Itgives transverse nature of the E.M.W. AnE.M.W. is associated with energy &momentum. Hence, it exerts a pressure on

the surface of incidence.

BASIC FEATURES OF E.M.W.

(1) They are produced by accelerated oroscillating charges.

(2) They do not require any materialmedium for propagation.

(3) They travel in a freespace (vacuum orair) with the speed of light i.e. 3X108

m/s because light is also an E.M.W.

When they travel through a medium,their velocity depends entirely on theelectric and magnetic properties of themedium.

(4) They are themselves uncharged andhence, do not get deflected by externalapplied electric and magnetic fields.

(5) They are composed of energy particlescalled Photons. Every photon has a fixedfrequency which does not change whena photon travels through differentmedia. The velocity of the photonchanges in different media due tochange in its wavelength. Photon iselectrically neutral does not have massat rest. In fact, a photon does not existat rest because a photon is an energyparticle of the E.M.W. which in turn isa dynamic state of energy.

(6) The E.M.W. shows all wave phenomenalike light.

ELECTROMAGNETIC SPECTRUM

The orderly distribution of electromagneticradiations according to their wavelength orfrequency is called the electromagneticspectrum.

Only a small part of this spectrum isvisible to human eyes because the radiationsof this part produces sensations on theretina of the eye.

(1) Gamma Rays: They are emitted byradioactive nuclei. They have the shortestwavelength and the highest frequency andhence, the highest energy. The study of γ -rays provides us with valuableinformation about the structure of theatomic nuclei.

(2) X-Rays: They are produced mostcommonly when fast-moving electrons

ELECTROMAGNETICRADIATION




decelerate inside a metal target. They areused -

for detecting explosives: opium, jewelleries, etc. in the body of smug-glers.

In Medical Diagnosis: e.g. for the de-tection of fractures, foreign bodies, like bullets, diseased, organs and stones inthe human body.

for detecting pearls in oysters and de-fects in rubber tyres, gold and tennis balls.

in engineering e.g. (a) for detectingfaults, cracks, flaws and holes in final

metal products (b) for the testing of weldings, castings and moulds.

in radiotherapy to cure untraceable skindiseases and malignant growths. Thephotographic film containing the pho-tographs of the body parts is as knowna Radiograph.

(3) Ultraviolet Rays: They are produced by the excitation of atoms, a spark and arclamp. The sunlight consists of ultravioletrays as a part. Ultraviolet rays are very

harmful to the living organisms. A longand constant exposure of our body to theseradiations causes blindness and cancer. Theozone layer in the upper atmosphereabsorbs most of the ultraviolet rays emitted by the sun. Only a small fraction of theserays reach the surface of the earth.Therefore, the ozone layer protects theorganisms from the ultraviolet rays. Awelder wears coloured glasses to protecthis eyes, from the ultraviolet rays emitted by the welding spark. During the solareclipse, the sun is completely covered by

the shadow of the moon. But the ultravioletrays emitted by it reach the earth. Hence,the direct vision of the solar eclipse throughthe naked eyes is avoided.

They are used: (i) to preserve food stuff & making drinking water free from bacteriaas these rays kill bacteria & germs.

Electromagnatic Spectrum

S.No. Name Wavelength (in m.) Frequency Range (in Hz) Source

1. Gamma rays 6×10 —14 to 1×10 –11 5×1022 to 3×1019 Nuclear origin2. X-rays 1×10 –11 to 3×10 –8 3×1019 to 1×1016 Sudden deceleration of

high energy electrons3. Ultra-violet 6×10 –10 to 4×10 –7 5×1017 to 8×1014 Excitation of electrons, spark

and arc lamp.4. Visible Light 4×10 –7 to 8×10 –7 8×1014 to 4×1014 Excitation of valence

electrons.5. Infra-Red 8×10 –7 to 3×10 –5 4×1014 to 1×1013 Excitation of atoms &

molecules6. Heat-Radiation 10 –5 to 10 –1 3×1013 to 3×109 Hot bodies7. Microwaves 10 –3 to 0.3 3×1011 to 1×109 Oscillating currents

in special vacuum tube.

8. Ultra-high frequency 1×10 –1

to 1 3×109

to 3×108

Oscillating circuit(UHF)9. Very High Frequency 1 to 10 3×108 to 3×107 Oscillating circuit

(VHF)10. Radio-Frequencies 10 to 104 3×107 to 3×104 Oscillating circuit11. Power frequencies 5×106 to 6×106 60 to 50 Weak Radiations from

a.c. circuit.The overlapping in certain parts of above E.M. Spectrums shows that the correponding radiations can be produced bytwo methods.



Physics [81]

to detect the invisible writings, forged

documents and finger- prints in forensiclaboratory.

to study the structure of solids and mol-ecules.

for sterilizing the surgical instruments.

for checking the mineral samplesthrough the property of ultravioletrays causing flourescence.

in burglar alarm.

(4) Visible Light: White-light emitted

by the sun is visible light. It is composed of seven colours. Except the visible light, allthe remaining electromagnetic radiationsare invisible. Besides the solar-radiations,there are a number of sources giving visiblelight e.g. candle, electric-bulbs etc.

(5) Infra-red rays: Infra-red rays areemitted by the atoms and molecules of hot- body. Infra-red rays are absorbed by the body on which they fall. So the body becomes hot. Hence, infra-red rays are also

known as heat-radiations.They are used:

to heat the interior of a car and house,

to cook food using a solar cooker,

to heat water using a solar water heater,

to treat muscular strains,

for drying wet clothes and grains afterharvesting,

in night vision devices during warfare

because infra-red rays can pass throughhaze fog, smoke and mist,

to provide electrical energy to satellite by using solar cells,

for producing dehydrated fruits,

for taking photographs during the con-ditions of fog, smoke, etc.

in green houses to keep the plants warm,

in weather forecasting through infra-red photography, and

in checking the purity of chemicals andin the study of molecular structure bytaking infra-red absorption spectra,

(6) Heat Radiations: They areproduced by any hot bodies. They areused for heating purposes.

(7) Microwaves: They are produced byoscillating electrical circuits. They are used:-

in micro wave ovens.

in RADAR systems and in satellite com-munication.

(8) UHF, VHF and Radiowaves : Theyare produced by the oscillating circuits.They are used as carrier waves during thetransmission of Radio-signals, TV-signals,FM-Radios, etc.

(9) Power- frequencies or Power

Waves : They are produced from a.c.circuits. They are used for lightning.

RADIATION IN ATMOSPHERE

The main source of radiations in theatmosphere is the sun. Visible light isweakly absorbed whereas most of the infra-red raditations are absorbed by theatmosphere and is used to heat it. Theozone layer of atmosphere absorbs most of the ultraviolet-rays and thus protects usfrom their harmful effects. The ozone layerconverts the ultraviolet radiations to infra-red which is used to heat the atmosphereand the earth’s surface. It is suspected thatozone layer is slowly being depleted andthis is causing great concern to scientistsand environmentalists.




The earth gets heated by absorbing heat

from the sun & radiated energy mostly inthe infra-red region. Low lying clouds andthe atmosphere prevent these infra-redradiations of earth from passing throughthem and thus serve to keep earth’s surfacewarm at night. This phenomenon is calledas “Green House Effect”.

The upper part of the atmosphere of the earth receives about 1.36 kilo joule of energy per second per unit area from thesun. The earth’s surface receives 47% of this energy emitted from the sun. Theenergy crisis can be overcome by harnessingthis energy. A large number of devices have been developed to harness the solar energydirectly. The commonly used devices are:Solar cookers, solar furnaces, solar waterheaters, solar power plants, solar-cells etc.

All of the above devices are pollution-free and most of them are economical.

Limitations in harnessing solar energyare---

(i) Solar energy reaching the earth is highlydiffused and scattered. Hence, it is notable to perform much useful work.

(ii) It is not available at all places and at alltimes.

ELECTRIC DISCHARGETHROUGH GASES

At normal pressure gases are insulators but at very low pressure they can allow theflow of electric current due to an appliedpotential difference. The electric-dischargethrough a gas is carried out in dischargetube. The discharge tube involves theionisation of gas which gives rise to differentcoloured discharges and finally, the cathoderays are produced depending upon thevalue of pressure inside the tube.

The discharge tube phenomenon is used

in making sodium lamps, mercury vapourlamps, fluorescent tubes, neon signs, floodlight, etc.

Cathode rays consisting of electrons,are produced in the discharge tube at apressure of 0.01 mm of mercury.

ELECTRONIC EMISSION

The phenomenon of emission of electrons from the surface of a metal is

called electronic emission.(a) Thermionic Emission: It involves

emission of electrons due to appropriateheating of the metal surface. The deviceslike diode valve and triode valve are basedon the thermionic emission of electrons.The diode valve is used as rectifier andtriode valve is used as transistor andamplifier in electrical circuits.

(b) Photoelectric Emission: It involvesemission of electrons from the metal surface

due to incidence of light of a suitable frequency.It is also known as photoelectric effect.

PHOTOELECTRIC CELLS ORPHOTO CELLS

They convert light energy into electricalenergy. They are based on the phenomenonof photoelectric-emission. They are alsoknown as Electric-eyes. A photo-cellconverts a change in intensity of illumination into a change in the photocurrent obtained. This current is used tooperate control systems and in lightmeasuring devices.

Applications of Photo-cells:

in TV camera for telecasting scenes andin photo-telegraphy.



Physics [83]

to switch on and off the street lightning

system at dusk and dawn, without anymanual attention.

in photometry to compare the illumi-nating powers of two sources.

in industries for locating small flaws orholes in metallic sheets.

as photoelectric sorters; to sort out thematerials of different shades.

to determine the opacity of solids andliquids.

to control the temperature and chemi-cal reactions.

METALS SHOWING THEPHOTOELECTRIC EMISSION

Alkali Metals: Lithium, sodium,potassium, cesium, etc. show thephotoelectric effect with visible light.

In the photoelectric effect, one photonincident on the metal-surface causes theemission of one electron called photoelectron. Higher the intensity of light, moreis the no. of photons incident and hence,more is the no. of photoelectrons emitted.Higher the frequency of the photon incident,higher is the kinetic energy of the photoelectron emitted. The photoelectric effectshows the particle nature of light i.e. light iscomposed of energy-particles called photons.

The particle nature is given as

E = mc2 where E = energy, m = mass &c= speed of photons.

The wave-nature is given as

E = hv, where E = energy and v=frequency of photon h = Planck’s constant

Photons are electrically neutral andhave zero rest mass.

Dual Nature of Matter: Microscopic

particles like electrons have mass character aswell as wave character. The nature of a particleto behave as a mass as well as a wave is knownas dual nature of matter. Electron-microscopeis the device that exploits the wave-nature of electrons to provide high resolving power.Ithas successfully been employed to investigatestructural details of bacteria, viruses, etc. It hasproved to be a powerful tool of investigationfor research in science, technology, metallurgy,industry, medicine, etc.

X-RAYS

When highly energetic electrons aremade to strike a metal target of high atomicnumber like Tungsten and Platinum, the x-rays are emitted.

The device used to produce x-rays isknown as an x-rays tube or Coolidge tube.

X-rays are electromagnetic waves. Theycan affect a photographic plate more

strongly than visible light. They causefluorescence when incident on certainmaterials as barium platinocyanide. X-rayscan ionise the gas.

They can pass through small thicknessesof aluminium, woods, plastics, human-fleshetc. They are stopped by materials of highdensity and high atomic number. Hence, x-rays machines are used to inspect luggagesat the custom, security gates, airports, etc.The frequent and excess exposure of x-raysmay cause diseases like genetic-disorders;cancer, etc. The screen of Computer, TV,oscilloscopes, etc. are using a cathode-raytube in which a highly accelerated electronsstrike the screen. It results in a small amountof x-rays. Hence, their screens are designedto absorb the x-rays.




Every atom is composed of three kindsof fundamental particles viz. Electrons,Protons and Neutrons. Protons andNeutrons are in the atomic nucleus whileelectrons revolve around the nucleus indefinite circular path called orbits.Electrons are negative and have a definiteenergy in a definite orbit. Protons arepositive while neutrons are neutral.

When an electron changes its orbit, anamount of energy is exchanged (lost orgained) in the form of radiations (infra-red, visible, ultraviolet and even x-rays).Every atom has an equal no. of electronsand protons and hence, an atom is neutral.The no. of protons in an atom is equal to its

atomic number. The sum of the no. of protons and neutrons is equal to the massnumber.

The size of the nucleus is much smallerthan that of the atom. The concept of nucleus, nuclear and atomic size weregiven by Rutherford. The concept of orbitsand energy of an electron in its orbit weregiven by Neil Bohr. The motion of anelectron in its orbit is similar to that of aplanet moving round the sun.

The electron in the orbit closest to thenucleus has least energy and as the orbitnumber increases away from the nucleus,energy of the electrons also increases.

Every nucleus of the atom has neutronsand protons. The volume of the nucleus isdirectly proportional to its mass number.

Hence, the size of the nucleus increaseswith increase in the mass number.

NUCLEAR STABILITY

It depends upon the nuclear size andneutron: proton ratio. As the nuclear radiusincreases, nuclear stability decreases. Higherthe n/p ratio of a nucleus lower is itsstability. From Hydrogen (Atomic No=1)to Bismuth (209

83Bi - At .No. 83 & Mass No.

= 209) all nuclei are quite stable exceptTechnetium (

43Tc) and Promethium (

61 Pm).

However, from atomic number 84(Polonium -Po) to 92 (Uranium -U) arenatural radioactive nuclei. Due to

radioactivity, their nuclei disintegratespontaneously. The nuclei of the elementsNeptunium (

93 Np) onwards are artificial

and do not exist in free state in nature.

The force of attraction responsible for binding the nucleons (protons, neutronsetc.) in the nucleus is nuclear force. As thenuclear size increases the nuclear force of attraction decreases. The nuclear force isthe strongest force in nature.

MASS DEFECT

The total sum of masses of individualnuclear particles (Nucleons) in a nucleus isgreater than the rest of the actual rest massof the nucleus. Their difference is knownas mass defect. This mass defect getsconverted into energy which is calledNuclear-binding energy.

ATOMIC PHYSICS



Physics [85]

Nuclear species called Nuclides are

symbolised as where X= chemical

symbol of the species, Z= Atomic number=Number of proton = Number of electrons,A = Mass number = Sum of number of protons and neutrons, A-Z= Number of neutrons (N).

e.g. . Bi = Bismuth, 83 = Atomic no.= No. of proton

209 = Mass No. = Z+N = A

209 - 83 = 126 = No. of neutrons. Isotopes: They are a group of nuclides

having equal no. of protons.

Isobars: They are a group of nuclideshaving equal mass no.

Isotones: They are a group of nuclideshaving equal no. of neutrons.

RADIOACTIVITY

The property by virtue of which a heavynucleus of element disintegrates itself without being forced by any external agentto do so, is termed as radioactivity. Thephenomenon of radioactivity is quitenatural and can not be stopped. It wasdiscovered by Henry Becquerel in 1896.

The radioactivity results in theconversion of an unstable larger nucleusinto smaller stable nucleus along with theemission of certain radioactive decays. These

radioactive decays are alpha rays, beta rays& gamma rays. During every radioactivedecay an amount of energy is released.

When a nucleus emits an alpha-particle,the daughter nucleus has atomic numberlesser by 2 units and mass number lesser by4- units.

When a nucleus emits a beta-particle,

the mass number of the daughter nucleusremains unchanged but its atomic numberincreases by one unit. There is no effect onthe atomic and mass number due to gama-emission.

Units of Radioactivity:

1 Becquerel (Bq) =1 decay/ second

1 Curie (Ci) = 3.7x 10 10 decays /second

1 Rutherford (Rd) = 10 6 decays/second

HALF- LIFE TIME (T)

The time period during which half of the total initial number of radioactive atomsdecays, is called as half-life time.

e.g. for = T= 5770 years.

for Polonium, T= 3x10-7 sec.

Half-life time (T) is given as:

(where l = decay constant)

Average life time of a radioactivesubstance is given as τ = 1.44 × T, where T= half life.

The full-life time of a radioactivesubstance is infinity i.e. the taken initialsample of a radioactive sample decreases inamount with time but never becomes zero.

Besides α -, β - &, γ - particles, someother subnuclear particles like positrons,neutrinos, antineutrino etc. are also emitted

during the radioactive decays. Positron isan antiparticle of electron i.e. it has massequal to electron but is equally positivelycharged. It is also known as positive β -particle. Symbols used for position,neutrino and anti-neutrino arerespectively. Nuclear changes during theradioactive decays:




ABC OF RADIATION

Properties ααααα-rays βββββ-rays γ γ γ γ γ -rays

1. Similar to Helium nuclei Electrons E.M. Waves like X-rays2. Symbols

3. Atomic No. 2 –1 04. Mass No. 4 0 05. Speed 1.4×107 m/s 33% to 99% of the Equal to the speed of

to 2.1×107m/s light light6. Penetrating power Least and can be stopped Higher and can pass Highest & can pass

by 0.02 mm thick through a few mm through several cmsaluminium sheet. thick alumium sheet thicks iron & lead slabs.

7. Ionising power Maximum due to Intermediate Minium due tomaximum charge (+)2 that of between zero chargeunit & maximum mass a– & g rays

8. Range in air Depends upon the radio Several meters Very longactive source and variesfrom 3 to 8 cm.

9. Effect on photographic Produce smaller effect More effect Maximum effect plate

10. Flourescence effect Produce flouresence in Also produce floure- Produce flouresencein some substance like sence in barium in some substances likebarium-plantinocyanide plantinocyanide and wil limiteand zinc sulphide zinc suplide

11. Effect of electric and Show deflection Show deflection do not show deflection magnetic field

12. Effect on human body Cause burning effect Can cause a shock Can cause canceron longer exposure.

(1) a decay→

Uranium → Thorium + α-particle +Energy

(2) β decay→

Thorium → Protactinium + b-particle +Energy

(3) γ decay →

Sodium → Magnesium + β- particle +gamma-particle.

ARTIFICIAL & INDUCEDRADIOACTIVITY

The phenomenon of disintegration of otherwise stable nuclei by bombardingthem with suitable projectiles is calledartificial radioactivity.

Generally, such disintegration stops as

soon as the bombardment is over. However,if the bombarded nucleus keeps on

disintegrating even after the bombardment

is stopped, the phenomenon is called

induced radioactivity.

e.g. Boron, when bombarded by α -

particle, continue to disintegrate further even

after the bombardment is stopped, as follows :

Boron + a particle → Nitrogen - 14 →Nitrogen -13 + Neutron. Produced is a

radioactive nucleus produced by artificial

radioactivity. Its half-life is 11 minutes.

Nitrogen → Carbon - 13 + positron.



Physics [87]

NUCLEAR REACTIONIt involves the transformation of a stable

nucleus into another nucleus by bombarding the former with suitableparticles. These particles are calledprojectiles e.g. neutron, proton, electron,alpha-particle, γ -ray etc. Neutron due tomass and Zero charge is the best projectile.

Representation of a nuclear reaction.

Target Nucleus +Projectile → DaughterNucleus + Emitted - Particle

e.g.

Nitrogen - 14 + - particle → Oxygen -17 + Proton

Above reaction is the 1st artificialnuclear transmutation discovered byRutherford in 1919.

A nuclear reaction may be of two typesviz. Nuclear Fission and Nuclear Fusion.

I. Nuclear FissionThe process of splitting a heavy nucleus

(usually of atomic mass >230) into twocomparatively small nuclei of middle weightalong with the release of very large amountof energy is called nuclear fission.

In this process, a fraction of massdisappears that appears in the form of energy released.

Otto Hahn & Strassman discovered that

when Uranium -235 isotope ( ) is bombarded with thermal neutron, it splitsup into two smaller nuclei with the emissionof 3 neutrons along with 200.4 Mev of energy per fission.The neutrons producedafter fission are called secondary neutrons.

Such a huge amount (200.4 Mev) of

energy is released only in about 10–9 seconds.

This released energy appears in the form of γ -rays, kinetic energy of nuclei of Barium(Ba), Krypton (Kr) & neutrons (n).

Nuclear Chain Reaction : In a nuclearfission of Uranium three secondaryneutrons are released along with a hugeamount of energy. These three neutronscause the fission of other threeUranium-235 nuclei & produce 9-neutrons which in turn, can bring aboutthe fission of 9-Uranium -235 nuclei &so on. This process multiplies so quickly(i.e. a few micro-seconds) that atremendous energy is released in afraction of time. Such a continuousprocess is known as Chain Reaction. If the chain reaction is controlled theenergy released is used for peacefulpurposes like in Nuclear Reactor orNuclear-pile. If the chain-reaction isuncontrolled, the tremendous energy

released brings about a disastrous effectlike Atom-Bomb.

Controlled Chain Reaction: If only oneneutron, out of 3- neutrons released inevery fission of Uranium, is used to causefurther nuclear fission, the constantamount of energy is released. Such anuclear fission is known as controllednuclear fission or controlled chainreaction. The secondary neutronsreleased are fast moving neutronswhereas to have a controlled chainreaction the slow moving neutronscalled Thermal neutrons are required.Hence to achieve a controlled chainreaction the fast moving neutrons haveto be slowed down and the surplusneutrons have to be absorbed. The fastmoving neutrons are slowed down by




a substance called a Moderator e.g.

graphite, deutrium or heavy water. Thenumber of neutrons released is reduced by Control Rods e.g. Boron - rods orCadmium rods.

Nuclear Reactor : It converts nuclearenergy into heat. It is based oncontrolled nuclear fission.

Fuel: A fissionable material is used as afuel. e.g. (Plutonium), etc.Commonly is used as a fuel.

Coolant:It is used to absorb the heatproduced in the reactor core. e.g. water

and heavy water are used as Coolants atordinary temparature but at hightemperature, generally liquid sodium isused as a coolant.

Shield: The whole reactor is protectedwith a concrete wall; so that radiationsemitted can be stopped.

Moderators & Control-rods:Moderators are around the core of thereactor. Control - rods are inserted insidethe core of the reactor.

Fast Breeder Reactors: Out of the totalUranium available in nature only 2% partis fissionable 235U & 98% part is non-fissionable 238U. In the fast breeder nuclearreactor, 238U is first converted intofissionable 235U which is further brokendown and the nuclear fission reactionproceeds as follows:

Use of Nuclear Reactors:

(a) in electrical power production

(b) for the propulsion of ships, submarinesand air crafts.

(c) to produce radioactive isotopes. These

isotopes are used in medical science,industry and agricultural research.

(d) to produce neutron beam of high inten-sity which is used in the treatment of cancer and nuclear research.

Indian Nuclear Reactors

Our country has adopted a three- stagestrategy of nuclear power generation :

(a) The first stage aims in the use of natural

uranium as a fuel, with heavy water asmoderator.

(b) The second stage involves the develop-ment of the fast breeder reactors. Forthis, the discharged fuel from the reac-tors is reprocessed to obtain plutonium-239, which is further used for the fast breeder reactors.

(c) The third stage involves using fast breeder reactors to produce missile U-233 from Th -232 & to build power

reactors based on them. India is cur-rently, well into the second stage of theprogramme and considerable work hasalso been done on the third stage (i.e.the utilization of Th -232).

Earlier reactors developed in India wereApsara (first) and CIRUS (Canada IndiaReactor). They used natural uranium asfuel and heavy water as moderator. Besidesthese research reactors, EERLINA,PURNIMA (I, II & III), DHRUVA and

KAMINI have been subsequentlycommissioned as research reactors.KAMINI is the country’s first large researchreactor that uses U-233 as fuel.

India gets most of its Uranium from the Jadugura mines in Jharkhand. The Uraniumobtained from these mines is taken to the



Physics [89]

Nuclear Fuel complex situated at

Hyderabad for processing. At this complex,the fuel elements are formed after enrichingthe Uranium. These processed fuel elementsare then supplied to different nuclear plantsas situated, at Kota in Rajasthan, Narora inU.P., Tarapur in Maharastra andKalpakkam in Tamil Nadu.

Nuclear Hazards

The nuclear radiations cause pollutionat three stages:

(a) When nuclear fuel is mined, processedand enriched, it constantly emits nuclearradiations.

(b) Accident can be caused due to leakageof nuclear reactions from reactors. Some-times the nuclear reactor core may getfaults resulting in vast radiations. Suchaccidents have already been occurred,one in USA at Three Mill Island andother at Chernobyl in Ukraine.

(c) Disposal of nuclear waste - It is a bigproblem and it can not be thrown any-where. These waste materials containradioactive substance which radiate ra-dioactive rays. At presently, the nuclearwaste is sealed in the thick stainlesssteel vessels and is dumped deep intothe earth.

Harmful effects of the nuclearradiations:

(a) They can change or damage the struc-ture of cells in the human body.

(b) They cause diseases like cancer, leuke-mia and blindness.

(c) They cause genetic disorder in a hu-man body.

(d) They can cause sterility in the younggeneration.

The Nuclear radiations incident on the

organisms cause the ionisation of atomsand molecules resulting in thedecomposition of the complex organicmolecules in the body of the organisms.Consequently the normal functioning of the biological system becomes disrupted.The damaging effect of the nuclearradiations would depend upon - (i) dose of radiations, (ii) rate of dose given, and (iii)nature of the organism exposed.

The dose of radioactive (nuclear)

radiations is measured in a unit calledRoentgen(R). The quantity of radiationsthat produces 1.61x1012 pairs of ions in 1gm of air is equal to one Roentgen(R). Inpractice, milliroentgen is used as theexposure unit or dosage unit. The radiationabsorbed per second is called dose rate orexposure rate or radiation absorbed dose.

The safe limit of exposure to radiationover the entire body is 250 milliroentgen(mR) per week. This is known as Permissible

dose. Cancer may be caused at a dose rateof 100R. Death of an organism is possible if this dose exceeds 600R.

II. Nuclear Fusion

The phenomenon of combining two ormore lighter nuclei to form a single heavynucleus is called nuclear fusion. It resultsin the release of tremendous amount of energy. It requires very hight temperature(107K) & pressure which are not possibleon earth. Such conditions are available instars & hence, the nuclear fusion is a stellarphenomenon. It is the cause behind thestellar energy. In the biggest star i.e. sun,the nuclei of hydrogen fuse together toproduce helium nucleus and a tremendousamount of energy.




APPLICATION OF RADIOACTIVEISOTOPES

In discovery of new sub-nuclear par-ticles.

In discovery of new radioactive andnon-radioactive isotopes of elements.

In the radio-active and nuclear research.

In agriculture- by using the tracer-tech-nique the fertilizer consumption of plants can be measured by the help of

Geiger-Muller Counter (GM Counter).G M Counter can detect the presence of a radioactive isotope and can measureits activity.

In industry- The tracer technique can be applied by using a radioactive iso-tope to study the wear of auto mobile-engines.

In medical science- by using the radio-active isotope of Iodine by the help of tracer technique, the position of goitre

can be detected.

In carbon dating- It involes the deter-mination of age of fossils by measuringthe radioactive isotope of Carbon - 14.

In Radio dating - The age of old rocksis determined by measuring the radio-active materials e.g. Uranium presentin the rocks.

The last two applications given aboveare based on the half-life period of C-14

and Uranium. The half-life of C-14 isapprox. 5770 years.

ATOMIC BOMB

It involves uncontrolled nuclear chainreaction (fission). In an atom bomb twosubcritical masses of U-235 or Pu - 239 are

brought together in less than a

microsecond. It results in a violent explosioncausing destruction because the combinedmass becomes super critical. At the criticalmass of a radioactive sample, a controlledchain reaction occurs.

TRIVIA

In photoelectrical effect, the electron isassumed to be bound. That is, the en-ergy of the incident photon is of theorder of the binding energy of the elec-tron.

In Compton effect, the electron is as-sumed to be free. That is, the energy of the incident photon is much larger thanthe binding energy of the electron.

Einstein was awarded Nobel Prize forexplaining the photoelectric effect.

Photoelectric effect was discovered byHallwach.

The kinetic energy of photelectrons var-ies from zero to h (v – v

0).

Einstein’s photoelectric equation is inaccordance with the law of conserva-tion of energy.

Cesium is the best photosensitive metal.

The hardness of the X-rays depends onthe accelerating potential of the elec-trons incident on the target.

Voltage applied across the X-ray tube isof the order of 10000 V.

The atomic number of the target deter-mines the hardness of X-rays.

Mosley related the frequency of char-acteristic X-rays to the atomic numberof the target.



Physics [91]

Production of X-rays is an atomic phe-

nomenon and the production of γ -raysis a nuclear phenomenon.

Frequency of γ -rays is greater than that of X-rays.

Efficiency of production fo X-rays is lessthan 1%.

The target in the X-ray tube should havehigh atomic weight and high meltingpoint.

Absorption coefficient of X-rays is high-est for lead.

Hydrogen atom cannot emit X-rays.

Compton effect is observed with X-raysand γ -rays.

Electron microscope works on the wavenature of moving electrons.

Wavelength of the matter waves doesnot depend on the charge of the par-ticle.

Bohr’s theory is applicable to both thehydrogen as well as hydrogen like at-oms.

Observation of faint spectral line verynear to blue line of hydrogen spectrumlead to the discovery of deutrium.

As the quantum number increase thedifference of energies between the con-secutive energy levels decreases.

In the Bohr model of the atom, an elec-

tron can revolve around the nucleus ina stable orbit without emitting radia-tions of its orbit has whole number of de Broglie waves.

Iron, nickle, cobalt, etc. has ferromag-netic properties due to partially filledinner subshells.

Total energy of the orbital electron is

negative.

Lead is the heaviest atom which is ra-dioactively stable.

All naturally occuring radioactive sub-stances are finally converted into lead.

For radioactive nuclei : .

where n = number of neutrons and p =number of protons.

The quantisation of the enegy states of an atom was demonstrated by FrankHertz experiment. The experiment dem-onstrated the existence of discrete en-ergy levels in the atom.

The binding energy of the electron inthe ground state of hydrogen is calledrydberg, 1 rydberg = 13.6 eV.

The isotope1H3 of hydrogen is unstable

and decays to2He3.

Radium was isolated by Pierre Curieand Madma Curie.

Out of electron, proton and neutron,only neutron is unstable in free space.It decays into a proton and an electron.

When β-particle is emitted the parentand daughter products are isobar.

Half life period of lead is infinite.

Artificial radioactivity was discovered by Henry Becquerel.

Natural radioactivity was discovered by Henry Becquerel.

In free space, the neutron decays as :

Probability of a radioactive atom to surviven times longer than the half life is : 2–n.




The radioactive decay rate is not af-

fected by temperature or pressure.

The first nuclear transmutation wasachieved by Rutherford.

In the nuclear reactions, mass + energyis conserved.

The decay constant as well as half lifeperiod are independent of the age of the radioactive sample.

The radioactive substances are stored

in lead containers, because lead absorbsthe radioactive radiations.

The moderator of the nuclear reactorshould slow the neutrons without ab-sorbing them.

In the sun and stars, the energy isreleased through natural fusion of hydrogen into helium with carbonserving as nuclear catalyst.

To photograph the brain tumour, Hg197

radio isotope is generally used. The neutrino has no charge and negli-

gible mass.

Both photon and neutrino are chargelessand have negligible mass. But the spinof photon is 1 and that of neutrino is

.

92

U238 can be fissioned by fast neutronsand

92U235 is fissioned by slow neutrons.

Nuclear fission was discovered by Hahnand Strasman.

Positron was discovered by Anderson.

Proton was discovered through artificaldisintegeration of nitrogen by ∝=par-ticles as follows :

Critical mass of fissionable uranium –235 can be reduced by surrounding itwith neutron reflecting substances.

The decay of artificial radioactive iso-topes is accompanied by positron de-cay.

In the fission of92

U235 energy released is200 MeV.

Enriched uranium is better fuel for thereactor because it has greater propor-tion of U-235.

The percentage of mass which changesinto energy during fusion is of the or-der of 0.7%.

In the fission of uranium, the percent-age of mass converted into energy isabout 0.1%.

Protons attract each other when theyare separated by a distance of 10–14m.

A neutron can be added to or taken outof the nucleus of an atom withoutchanging its chemical properties.

Nuclear fusion requires very high tem-perature.

Energy released in nuclear fission mostlyappears as kinetic energy of the fissionfragments.

The mass of the sun is decreasing at therate of 4 × 109 kg per second.

Mass number of nuclei is either equalor more than the atomic number.

First atomic bomb was designed by E.Teller.

Man made element produced in thefirst nuclear reactor was plutonium.



Physics [93]

Plutonium - 239 is the best nuclear fuel.

First atomic reactor was designed byEnrico Fermi.

Artificial transmutation of elements wasdiscovered by Rutherford.

Enriched uranium is that in which thepercentage of U-235 is more than 0.7%.

The fissionable material used in theatom bomb dropped on Nagasaki in1945 was Plutonium.

Thermal neutrons have kinetic energyof the order of other gas molecules intheir surrounding.

The density of nucleus is of the orderof 1017 kg/m3.

The thickness of pn junction is of theorder of 10–6 cm.

Sound waves after being converted into

electrical waves are not transmitted assuch because they are heavily absorbed by the atmosphere.

For full wave rectifier, the minimumnumber of diodes required is two.

Both n as well as p type semiconduc-tors are neutral.

The output of rectifier is DC mixed withAC.

The resistance of intrinsic semiconduc-

tor decreases with the increase in tem-perature.

Diode was discovered by Fleming.

The production of electromagneticwaves by an oscillating charge was pre-dicted by Maxwell.

Thermionic emission was discovered byThomas Alva Edison.




The branch of science which deals withthe study of position, composition, motionand other related facts of the heavenly bodies, is called astronomy. The source of information about the heavenly bodies iselectro magnetic waves radiated fromthem. It includes the two windows of theastronomy viz.

(i) Optical Window of Astronomy: Itinvolves the study of the heavenly bodiesthrough visible radiations by the help of optical telescope.

(ii) Radio Window of Astronomy: Itinvolves the study of the heavenly bodiesthrough radiowaves by the help of radio-

telescope.

ASTRONOMICAL UNITS OFDISTANCE

(i) Light Year (ly):- It is equal to thedistance travelled by light in vacuum inone year. 1ly = 9.46 × 1015m

(ii) Parsec (Parallactic Seconds):-

1 parsec = 3.1 × 1016m

(iii)Astronomical unit (Au):- It is equal tothe distance between the earth and thesun.

1A.U = 1.5 × 1011m

The universe is the limitless expanse of space around us consisting of the solarsystem, stars, galaxies, etc.

SOLAR SYSTEM

The sun has its own family, known asthe Solar System. Solar system consists of the sun, its nine planets and other heavenly

bodies like asteroids, satellites (Moons),comets, meteors, etc.

The sun is at the centre and all othermembers of system move around it inelliptical orbits due to the gravitational forceof attraction between them.

Sun

It is the nearest star to us. It is heaviestand largest member of the solar-system. Itsmass is 1.98x 1030 kg and diameter is 1.392

x 109

m. It is about 109 times the diameterof the earth. It rotates about its axis andcompletes one rotation in 25 days. Our sunis also called Yellow dwarf star. It emitscontinuously the electromagnetic radiations both in visible and in radiowaves regions.

The sun consists of a bright layer at thecentre called Photosphere that emits light.The photosphere is surrounded by a hot-gaseous layers called the chromosphere.

The sun comprises different elements in

gaseous state. Hydrogen - 70%, Helium -28%, other heavier elements Lithium toUranium - 2%. The temperature, density andpressure increase from the surface to thecentre of the sun. Temperature increases from6000K on the surface to 14x106 k at thecentre and density increases from 10–4 kg/m3 at the surface to 104 kg/m2 at the centre.

ASTRONOMY



Physics [95]

Source of solar energy is due to energy

released during the nuclear fusion. It involvesthe conversion of a certain mass into energy.The sun has been continously radiating energyat the rate of 46x1026 Joules per second forthe last several billion years without gettingcooler. The light emitted from the sun reachesthe earth in 8.3 minutes.

Solar Constant(S): The amount of radiant energy received per second per unitarea of a perfectly black body surface atright angles to the direction of the sun-rays

at the mean distance of the earth from thesun is known as the solar constant.

Solar constant = 1388 Watt/m2 = 2 cal/(cm2 - minute).

Sun-Spots: Certain dark spots areobserved in the photographs of the visiblelight disc of the sun. These dark-spots arecalled sun-spots. They have relatively lowertemperature (4500K) at the sun- surface.Their numbers vary with time. Themaximum sun spots activity occur

regularly after every eleven years. Thisperiod is known as the sun-spot cycle. Thesun-spots are also associated with highmagnetic fields (2000 to 3000 gause).

Solar Flares: At the time of themaximum sun-spot activity, some portionsnear the sunspots sometimes suddenly become much brighter than the usual. Suchregions are called Solar Flares. These sun-flares emit protons, electrons and α-particles. It results a lot of magnetic and

radio disturbances all over the earth. Italso affects the plant growth.

Solar Wind: The out flow of materialsas charged particles e.g. electrons andprotons, etc. from the sun is called as solarwind. It results in emission of one milliontonne particles per second from the sun.

Planets

There are eight planets in the solarsystem. These planets are revolving aroundthe sun in elliptical path. They do not emittheir own light but reflect some of theincident light of the sun. These planets inthe order of increasing distance from thesun are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Untilrecently Pluto was recognised as the ninthplanet. However, Neptune and Pluto keepon exchanging their distance in their

elliptical orbits in such a way thatsometimes Neptune is the farthest andsometimes Pluto.

The mass of the earth is about 6x1024

kg. The planets Venus and Uranus rotateabout their axis from the east to the westwhereas all the other planets rotate fromthe west to the east. Venus and Uranus arecalled retrograde.

(1) Mercury: It is the hottest, thesmallest and the nearest to the sun amongst

all planets. It is seen just before the sunrise, and after the sun-set. Its atmospherecontains H, He, Ar and Ne-gases. Thetemperature of its sun-facing side is 427° Cand the opposite dark side is at -270° C. Itdoes not have favourable temperature,oxygen and water for the existence of life.

Because it reflects 76% of the sunlightfalling on it. Its atmosphere contains 95 to97% CO

2 and small quantities of N

2, Ar,

SO2, CO & water vapour. Its temperature

is 480°C and pressure is very high. Hence,life is not possible on it. It is seen 3 hrs before the sun-rise and 3 hrs after thesunset. Hence, it is known as Morning/Evening star.

(3) Earth: It is one of the unique planetshaving life. Its atmosphere contains 78% of




N2, 21% of O

2 and traces of He, Ne, Kr,

CO2 and water vapour. 71% surface of itsis covered with water. Moon is its naturalsatellite.

(4) Mars: Its very thin layer of atmosphere contains 95% CO

2, 2.7% N

2,

1.6% Ar and traces of O2, CO and other

gases. Its surface has craters of differentsizes. It is also known as “Red - Planet” because it appears red owing to the presentof iron- oxide upto 16% of its soil. Its day-temperature ranges from 21°C to 27°C but

at night it becomes (-) 84°C. Phobos(diameter =27 km) and Deimos (diameter= 14 km) are its two satellites.

(5) Jupiter: It is the largest planet. It is aspinning ball of gases and liquids likeammonia, hydrogen (NH

3,H

2) and helium

He with no solid surface. Its temparature isabout (-) 140°C. No life is possible on it dueto ammonia clouds and intense emission of radio wave on it.

(6) Saturn: It is the second largest planet.

It consists of H2 and He with traces of NH3

and methane (CH4). It is recognised by a

system of revolving rings around it. Therings have dust particles and ice. Itstemperature is about (-) 180°C. The largestsatellite is Titan (diameter = 5800 km).

(7) Uranus: It is made up of H2, He,

NH3 and CH

4. It appears green due to the

large amount of CH4 and NH

3 clouds in its

atmosphere. Its temperature is about (-)127°C.

(8) Neptune: It is the 8th planet of thesun. It has six satellites.

Its surface has frozen methane (CH4). Its

orbit crosses the orbit of Neptune. Its singlediscovered satellite is Charon. It was treatedas the coldest and lightest planet of the sun.

Moon: It is the natural satellite of theearth. Its period of revolution around theearth as well as the period of rotation about

Solar System : Profile

S.N. Name of Radius Mean Mass Time Time Period Number

Planets (in thousand) distance compared period around of

km) from the to earth of revolution own axis satellites

(sum in (around (of rotation)

m. km) the sun)

1. Mercury 2.4 57.9 0.055 88 days 59 days 0

(Buddha)

2. Venus (Shukra) 6.1 108.2 0.815 225 days 243 days 0

3. Earth (Prithvi) 6.3 149.6 1 1 year 23 hrs. 56 min 1

4. Mars (Mangal) 3.4 227.9 0.108 1.9 year 24 hrs.27 min 2

5. Jupiter (Brihaspati) 71.4 778.3 317.9 11.8 years 9 hrs. 50 min. 16

6. Saturn (Shani) 60 1427 95.2 29.5 years 10 hrs. 14 min. 22 + Ring

7. Uranus (Indra) 23.4 2870 14.6 85 years 10 hrs. 49 min. 12 + ring

8. Neptune (Varuna) 22.3 4594 17.2 165 years 15 hrs. 6

9. Pluto (Yama)* 3.2 5900 0.002 248 years 6.39 days 1

The mass of the earth is about 6 × 1024 kg.

The planets Venus & Uranus rotate about their axis from the east to the west whereas all other planets rotate from the

west to the east. Venus & Uranus are called retrograde. * Now demoted



Physics [97]

its own axis is 27.33 days. So the same face

is pointed towards the earth. Its diameteris 3476 km and the distance from the earthis 3.54x105 km. Its mass is 0.0123 times themass of the earth. Its surface is rocky andrough. There are deep depressions or holesand high mountains on the surface of themoon. It does not have atmosphere andwater. Its gravitational pull is 1/6th thaton the earth. Latest measurements showedthat the maximum temperature on themoon at day is 117°C and minimumtemperature at the night is - 171°C. Moondoes not emit light by itself. It simply reflectslight of the sun falling on it. The reflectedlight on the Moon reaches the earth in1.28 seconds. Due to its rotation aroundthe earth, it looks different depending onits position with respect to the sun and theearth. The age of the moon is the same asthat of the earth i.e. about 4 billion years.

Asteroids: The groups of small solidobjects made of rocks and minerals whichmove in the gap between the orbits of Marsand Jupiter are called Asteroids. They arealso known as Minor-planets. They aremore than 5x105 in number. They movearound the sun in the form of rings. Theyare about 320 to 490 million km away thesun. Their orbits are highly elliptical whichenable them to approach the Earth, Venusand Mercury after regular intervals of time.Their size varies from 50 m to 350 km inradius. They are believed to be the pieces of a much larger planet which broke up due

to the gravitational pull of Jupiter. Ceres isthe largest known asteroid. Theircomposition is similar to that of Moon.

Comets: Comet is a heavenly bodywhich is made of gases, ice and dustparticles and moves around the Sun in along spindle shaped orbit. When a comet

passes close to the sun it has a bright head

and a long tail. Near the sun, some of thematerials of the comet gets evaporated dueto solar heat. The light of the sun exerts apressure on these vapours and forces themaway from the comet in the form of tail.When a comet is far away from the sun, itshows no tail. A comet appears after regularintervals of time in the sky e.g. Halley’scomet has a time period of 76 years. It waslast seen in 1986 and is expected to be seenagain in 2062.

Meteors: They are smaller pieces of stones and metals which are produced dueto breaking of comets while approachingthe sun. When they enter the earth’satmosphere, they burn due to friction of the atmosphere and burn completely beforethey fall on the earth’s surface. They appear bright due to burning. Hence, they arecalled Fire balls or shooting stars.

Meteorites: They are unburnt meteorsreaching to the earth’s surface. They

produce craters on the earth’s surface.Terrestrial Planets: They are closer to

the Sun e.g. Mercury, Venus, Earth andMars.

Giant or Jovian or Superior Planets:They are far away from the Sun e.g. Jupiter,Saturn, Uranus, Neptune & Pluto.

Albedo: The ratio of the sun’s energyreflected by a planet to that incident on it iscalled as albedo. It is also known as the

reflecting power of a planet. The cloudsand atmosphere are good reflectors of light.Higher the value of albedo of a planet,more is the possibility of atmosphere onthe planet. e.g. For Venus, albedo = 0.85 &for Mercury, albedo =0.06 & for Moon,albedo = 0.07. Hence, Venus has denserclouds and atmosphere but Mercury and




Moon do not have any clouds and

atmosphere.

Existence of Atmosphere on a Planet:The possibility of existence of atmosphereon a planet depends upon the escapevelocity of that planet. Higher the value of escape velocity, more is the possibility of atmosphere on the planet. The escapevelocity depends upon the acceleration dueto gravity (g) and temperature of the planet.If the value of ‘g’ is more, the escape velocityis more. If the temperature of the planet is

high, the average velocity of the gasmolecules may achieve the escape velocityand consequently, the gas molecules escapeout from the surface of the planet.

Thus, to have atmosphere on a planet,the planet must have lower surfacetemperature, higher value of ‘g’ and higherescape velocity.

FORMATION OF SOLAR SYSTEM

Formation of Sun: The Sun was formedabout 5 billion years ago. The clouds of H2,

He and dust formed Nebulae. At the beginning, these clouds were at very lowtemperature. These clouds collapsed slowlydue to their mutual gravitational force of attraction to form a Protostar. As theseclouds collapsed, the atoms of H

2 and He

collided with each other and hence,temperature accelerated. At a very hightemperature, the atoms of H

2 ionised into

ions which further fused together to form

He- nuclei along with the release of tremendous energy. Thus, sun was formed.At present, the sun is a middle aged starand is expected to live for another 5 - billionyears.

Formation of Planets & Satellites :When a protostar was converted into a

star (Sun), then the remaining mass (H2,

He & dust) formed disc shaped cloudsaround the sun. These clouds were at verylow temperature vis-a-vis the sun and hencecontracted due to gravitational pull andcondensed to form a small chunk of matterknown as planetesimal. There were largernumber of planetesimals around the sun,which attracted each other to form bigerchunks of matters. These big chunks of matters continued to grow by attractingsmaller pieces of matters by gravitationalpull. The matter was scattered in space inall directions. They gave rise to planets andsatellites. The matters which could notparticipate in formation of planets andnatural satellites collided with these formedplanets and satellites. It led to form deepcraters on the surface of planets andsatellites. These craters are visible on thesurface of moon and Mars.

Evolution of Earth : The Earth wasformed by the collection of large number of cold planetesimals. These planetesimalscame from two regions (i) from the innerplanets like Mercury and Venus and theywere rich in iron, silicon, magnesium andtraces of other elements (ii) from the outerplanets like Jupiter, Saturn and Uranus andthey were rich in H

2, CO

2, CH

4 and water.

A larger amount of heat was produced whenthese cold planetesimals collided with eachother. Heat was also produced due to (i) thecontraction of planetesimals under the forceof their gravitational pull (ii) the

disintegration of some radioactive elementslike uranium and thorium. Hence, due tolarge amount of heat the initial earth was ina molten state. The melted earth then beganto reorganise itself under the gravitationalpull as follows :

The heaviest materials like iron & nickel



Physics [99]

sank towards the centre of the earth

which formed the core of the earth.

The lightest materials began to float onthe surface and formed the crust of theprimitive earth.

The intermediate dense materials ar-ranged itself in between the core andthe crust of the earth and formed themantle of earth.

The process of reorganisation of moltenmatter of the primitive earth into various

layers of different densities is calleddifferentiation. At the time of differentration, the water vapours and thegases in the various minerals were released.Later on, these water vapours werecondensed to give rain. The rain waterfilled the craters on the earth and thus,oceans were formed. The various gasesreleased formed the atmosphere of theearth. The changes occuring on the earthwere fast in the begining now they areslowing down. At present, these changes

are responsible for the occurence of earthquakes, eruption of volcanoes andmaking and raising of mountain ranges.

STARS

A star is a luminous heavenly bodyemitting its own heat and lightcontinuously. Stars twinkle because of refraction of light through different layersof the atmosphere. Stars appear to movefrom east to west because our earth rotatesabout its own axis from the west to east,otherwise stars are stationary with respectto the earth. The pole star (Dhruv Tara)lies above the earths north pole i.e. on theaxis of rotation of the earth. The points onthe axis of rotation of the earth appearstationary to the observer on the earth.

Hence, pole star appears stationary to the

people on the earth.

Evolution of a Star : The vast space between the stars is fi lled with dustparticles and gases like H

2 and He. This

material is called Interstellar matter, fromwhich a star is formed. The interstellarmatters are in the form of clouds. Whenthe clouds of gas-particles are illuminated by nearby stars, the clouds appear as brightnebulae whereas the clouds rich in dustappear as dark nebulae because they absorb

the radiation of the stars. The process of the formation of star begins only if theoriginal clouds have a mass of at least 1000solar mass.

Formation of Protostar (First Stage) :Initially, the interstellar matter is at verylow temperature (–173°C). Due to thegravitational attraction the interstellarmatter contracts and is compressed to suchan extent that a huge ball of gases is formed.This gaseous ball is known as a protostar.

A protostar contains highly condensedgases mainly H

2 & He. A cloud of

intersteller matter takes about 105 years to become a protostar. Protostar does not emitlight and heat.

Formation of a Star from Protostar(Second Stage) : In the protostar the gasmolecules collide with each other.Gradually these collisions become fast andnumerous due to continuous increase inthe contraction inside the protostar. It gives

rise to a temperature of 107

°C that isenough to begin nuclear fusion. Thesenuclear fusions result in huge amounts of heat and radiations. Consequently, aprotostar is converted into a star. Thisprocess is very slow and takes about 105

years. Thus, a huge interstellar matter inthe form of clouds of gases and dust, is first




Theories of Origin and Evolution of the Universe

The Big Bang Theory : This theory was proposed by Le Maitre and Gammow. According to this theory,at the beginning of the universe, the entire matter of the universe was once concentrated in an ex-

tremely dense and hot (1012k) fire ball. Then about 20 billion years ago a vast explosion (big-bang)

occurred. The matter was broken into pieces which were thrown out with high speed in all directionsforming stars and galaxies; which are still moving away from one another. The Hubble’s law agrees

with this theory.Steady State Theory : It was given by Bondi, Gold and Fred Hoyle. According to this theory, the no.

of galaxies is constant and new galaxies are continously created to occupy the position of those gal-axies which have the crossed the boundary of the universe. Thus, the size and mass of the universe

are constant.

Pulsating Theory : According to this theory the expansion and contraction of the universe occur alter-nately i.e. the universe is pulsating. At present, the universe is expanding and after a certain amount of

expansion, it will begin contraction due to the gravitational pull. The alternate expansion and contrac-tion of the universe gives rise to a pulsating universe.

converted into a cluster of protostars and

then into a cluster of stars i.e. a galaxy.

Inspite of a large attractive force of gravitation inside the star, it does notcollapse because there is a high temperaturethat provides a high-pressure to counter balance the attractive force. This balancecontinues for billions of years.

Formation of Red-giant (Third Stage) :Due to continuous nuclear fusion of hydrogen nuclei to form He-nuclei, thenumber of H-atoms decrease. It results inthe decrease in pressure in the core of astar. Hence, the inward force of attractionincreases in the core and the core contractsrapidly but the outer layer expandsoutward. Thus the volume of a star becomesvery large. The outer layer gradually coolsand appears red. This stage of a star is

known as giant, super gaint or red-giant.

This stage lasts for several million yearsand at the end, the rate of production of energy becomes so high that it explodes inthe form of a Nova or Super Nova, throwingout the major outer mass into interstellarspace. This is the death of a star. Dependingupon the mass of left core of a star thefollowing heavenly bodies are formed:

(a) White Dwarf : If the original massof a star is less than two times the mass of the sun, the gravitational compression

leaves the core of the star, composed of protons with electrons flying around in theform of electron clouds. When this electron-cloud with stands the inward gravitationalforce, then a stable equilibrium is achieved.At this stage, the star is called a whitedwarf. It gradually cools finally ceasing to

emit radiations. Then it becomes a black-dwarf. The most famous white dwarf is

Sirius-B close to the brightest star Sirius.

(b) Neutron Stars : If the original massof a star is between 2 to 5 times the mass of the sun, the recoil of supernova explosionis set up in the core. Now, the electrons areforced into the nuclei and the result iscombination between electrons and protons

to form neutrons. Hence, the core containsneutrons only. It gives the birth of a neutron

star.

(c) Black Hole : If the original mass of astar is more than 5 times the mass of thesun, the recoil of supernova is extremelyviolent. Hence, the core collapses to havean object which has a very high densityand gravity. As a result any radiation or



Physics [101]

material particle that enters this object

never comes out. This object is called aBlack Hole. As the black hole is invisible, sowhat is happening inside it and what hadhappened to the matter of the core of thestar is unknown. The recent experimentshave shown that one of the components of Cygnus XI is a black hole.

GALAXY

A Galaxy consists of very large groupof stars, their families (i.e. planets etc.),dust and gases held together by theattractive force of gravitation. A galaxy isa building block of the universe. Totalgalaxies are about 100 billions and eachgalaxy has about 100 billion stars. Thus,there are about 1022 stars in the universe.Our solar system belongs to the galaxyknown as Milky Way galaxy (AkashGanga). Another known galaxy isAndromeda which is about 2.2 million lightyears away from us.

Types of Galaxies :

(i) Normal Galaxies : They emitcomparatively small amounts of radioradiation vis-a-vis the total radiationsemitted. They are bright from the centreand gradually get dim towards the edges.Milky Way and Andromeda are normalgalaxies. Depending upon theirappearance, the normal galaxies can beeliptical, spiral or irregular.

(ii) Radio Galaxies : They emit milliontimes more radio waves than normalgalaxies. They contain stars called Quasarswhich are known as quasi-stellar-radio-sources.

Milky Way Galaxy : It is so called because the light from the various starstogether gives the impression of a stream

of milk flowing across the sky. In the front

view it appears spiral. From the side view,it hasa thick central part (Nucleus) whichgradually becomes thin towards the edge.Its diameter is about 105 light years. Thesun is at 3×104 light years away it centre.The sun completes one revolution aroundthe galactic centre in 220 millions yearswith a speed of 250 km/second.

Pulsars : They are neutron stars whichemit radio signals at extremely regularintervals of time. Their pulsating radio

signals are due to their high rotation and avery large magnetic field.

CONSTELLATION (NAKSHATRAS)

A constellation is a collection or groupof a few stars. It forms a pattern whichresembles the shape of an animal, object ora human being. There are 88 knownconstellations. They are visible in a definitepart of the sky.

Some of the important constellationsare :

(i) Ursa Major or Big Bear (Saptarishi): The word Ursa has been derived from theLatin word Ursus which means a bear.Ursa is the feminine of Ursus. Ursa majorconsists of seven bright stars. The shape of the pattern of these stars resembles the of agreat bear. This constellation is seen clearlyin the summer season from April toSeptember, in the northern part of the sky.

(ii) Ursa Minor (Little Bear) or DhruvaMatysa : It consists of seven stars whichare closer to each other and less brightthan the star in Ursa Major. It’s mostimportant star is at its tail. This star isknown as Polestar (Dhruv Tara) which isnearer to the North Pole. The pole starhelps in determining the direction of sailors




and travellers. The Indian name of this

constellation is Dhruva Matysa or LaghuSaptarishi. This constellation can be seenclearly in July in the northern part of the sky.

(3) Scorpio or Vrischika : It consists of seven stars whose arrangement resemblesa scorpio. It can be seen clearly fromFebruary to August in the Southern part of the sky.

(4) Orion, The Hunter (Vyadha orMirga) : Its shape is like that a hunter. Thetwo stars at the top row makes the right andleft shoulder of the hunter. Three stars in themiddle row form his belt and the two stars inthe bottom row form his feet. The faint stars below the belt are arranged in the shape of asword of the hunter. This constellation can be seen clearly from October to April in thesouthern part of the sky.

(5) Cassiopeia (Sarmistha) : It consistsof five dominated stars. The pattern of thestars is like the letter W. This is clearly seenin October in the Northern part of the sky.

(6) Pleides (Krutika) : It does not havea specific pattern of stars. It is just like acluster of twinkling gems. The cluster of stars in this constellation are called globularclusters. This is visible from October to January in the northern part of the sky.

RED-SHIFT OF GALAXIES

It is explained on the basis of theDoppler-effect. When the source of wavemoves away the observer, the apparentwave-length increases or the apparentfrequency decrease. This is known as Red-shift because the region of spectra has largewavelength and smaller frequency. Thestudy of the spectral pattern of the galaxiesshows a red-shift. It means that most of thegalaxies are moving away from each other

with enormous speed and the universe iscontinuously expanding.

Hubble’s Law : Hubble and Miltonstudied the red-shift of various gallaxiesand put forth the law. The Hubbles lawstates that the recession velocity of a galaxyis proportional to it distance from us i.e.

Where V= recession velocity of a galaxy.

r = the distance of the galaxy from usand H = Hubble’s constant = 16 km persecond per million light years. It meansthat any galaxy which at a given momentis situated, one million light years from us,is receding away at a rate of 16 km/sec.




The digital communication involves the

quantization of the digitial data (0 and 1) by the help of a device called quantizer.Hence, it becomes relatively more error-free and noise-free. It makes the digitialcommunication more reliable than theanalog-communication.

DEMODULATION

It is the reverse processs of modulationthat is carried out in a receiver to recordthe original information signal.

In an electronic communication system,the transmitter modulates the informationsignals and transmits them by an antennavia transmission channels. The receiverreceives the modulated transmission signalsand demodulates them to recover theoriginal information signals. A transmitterperforms data transmission and a receiverperforms data retrieval. In electroniccommunication, data to be transmittedmeans information signals.

MODEM

It connects one computer to anotheracross ordinary telephone lines. It canfunction as modulator as well asDemodulator and hence, its name isModem (Mo ® Modulation and dem ®demodulation). In the transmitting mode,it acts as a modulator while in the receiver-mode, it acts as a demodulator.

When the information data are suppliedfrom computer-I to computer-II, modem-Iacts as a modulator and modem-II acts asa demodulator. When data are suppliedfrom computer II to computer-I, Modem-II

acts as a modulator & modem-I acts as a

demodulator.

Modems that support a voice/dataswitch have an inbuilt microphone andspeaker for voice communication. In voicemode, the modem acts as a regulartelephone while in data mode, the modemacts as a regular modem.

The rate at which a modem can transmitand receive data (message) is measured asBits per second (bps). Higher data transferrate can be achieved by data compression

which is performed by some specific modems.

The digital communications, computer& modem use a binary number system todeal with digital signals (data). Binarysystem involves two volues as 1 and 0 only.0 ® refers, open-circuit or No or space.

1 ® refers to close circuit or yes ormark.

Both 0 and 1 are called Bits. A group of bits is called a Byte or a binary word. A byte

of two bits (0 & 1) can give a four code-combination : 00, 01, 10, & 11. Hence ingeneral, the no. of codes is given as N = 2 n.

When N = total no. of code-combinations

n = the no. of bits in a byte.

FAX(FACSIMILE TRANSMISSION)

Facsimile means exact reproduction.

Fax transmission usually concerns theelectronic reproduction of a document(printed words, graphs or photographs) ata distant place through telephone lines.

At the transmitting end, a fax machinedigitizes the printed matter on paper andtransmits the corresponding data over the



Physics [105]

telephone lines. At the receiving end, a fax

machine receives digital image-information andreconstructs the received image on the paper.This is also known as Facsimile Telegraphy.

COMMUNICATION CHANNELS

The communication channel is thephysical medium between a transmittingand a receiving stations through which thetransmitted signals may propagate.

Types-(i) Space Communicationchannel including satellite communicationand (ii) line communication channels.

Space Communication : It involves thetransmission of the message signals freelyin space by a transmitting antenna andreceiving the signals by intercepting themwith the help of receiving sets or antennasat the other end.

The space communication channelinvolves:

(i) Ground wave communication (ii) Sky

wave communication (iii) Space orTropospheric or surface wavecommunication.

(i) Ground Wave Communication : Itis the transmission of the signals betweenthe transmitting and receiving antennasclose to the earth. It is not suited for verylong range communication becausee itinvolves the loss of energy to the earthduring the propagation of the wave. It isused for between radio broadcasting at

low frequencies ranging from 500 Khz to1500 Khz. The ground of the wave includesthe medium wave (MW) band.

(ii) Sky Wave Communication : In skywave propagation, the wave signals arefirst transmitted to the ionosphere and thenis received by receiving sets on the earth

after being reflected back from the

ionosphere. It is also known as short wave(SW) band in the radio transmission.

(iii) Space or Tropospheric WaveCommunication : It involves the transmissionof FM-Radio-signals, TV signals andMicrowave signals in the range of VHF &UHF. These signals cannot be transmitted byusing ionosphere because the ionsospherecan reflect the frequency upto 40MHz only.Hence, these signals are transmitted fromthe transmitting antenna to the receiving

antenna directly. Its range depends uponthe height of the transmitting antenna. Itsrange can be increased (a) by using a no. of antennas called Repeaters in between thetransmitting antenna and receiving antenna.(b) by increasing the height of the transmittingantenna. This height can be maximised bylocating the transmitter on a satellite.

SATELLITE COMMUNICATIONS

It involves those frequency signals

which can pass through the ionospherewithout being reflected by the ionosphere.The satellite carries an equipment calledRadio-Transponder. The function of thetransponder includes reception, storage,amplification and transmission of theinformation signals.

The communication satellite is usuallya geo-stationary satellite that appears fixedat a position above the earth. Its time-periodof revolution is 24 hrs, height 36000 kms

and direction of revolution from west toeast similar to that of the earth. It isrevolving in a circular orbit called Geo-synchronous orbit that is coplanar withthe equatorial plane of the earth.

Owing to the curvature of the earth asingle satellite cannot cover the entire earth.




For global comunication, three satellites are

parked 120° apart from each other in thegeo-stationary orbit.

Remote-Sensing : It is a techniquewhich is used to observe and measure thecharacteristics of the object or phenomenonat a distance without being in physicalcontact with it. Its working is related withthe measurement of some kind of signalsemitted, transmitted or reflected from anobject in order to determine certain physicalproperties of the object, like its temperature

location, size, colour nature, etc. The remotesensing uses, electromagnetic techniquesthat covers the entire electromagneticspectrum. Any photography is a kind of remote sensing. RADAR, SONAR, Satellitewith sensors, etc. are other examples of remote sensing.

Satellite remote sensing systems providethe data critical to weather-prediction,agriculture forecasting, resourceexploration (archaeology, geological survey,forestry, etc.) and environmental

monitoring.

Uses of Remote Sensing :

To know the area of earth’s surfacecovered by forests, types of plants anddensity of forest at a particular loca-tion.

To prepare waste land maps.

To locate the potential fishing zones.

To locate the debris of a crashed

aeroplane or jet plane. To identify the extent of pollution and

its source.

To estimate the damage being done bythe floods.

To locate the place where undergroundnuclear explosion has been carried out.

For ground water surveys.

In the field of spying, to lcoate theposition and movements of the enemy.

T.V. REMOTE CONTROL

The Remote Control used in TV andmusic system is an electronic device. Itconsists of an integrated circuit (I.C.) andother components like a diode, a transistor,a capacitor, etc. When a key on the remoteis pressed, it translates it into infrared

signals which are received by the electroniccircuit of the TV and required operationsare performed.

Line Communication : In order tointerconnect (i) a transmitter to an antenna,(ii) a transmitter to a receiver (iii) a receiverto an antenna some specific wires arerequired. These wires are known astransmission-lines. Some of them are asfollows:

(i) Parallel Wire Lines : They are

commonly used to connect an antenna withthe T.V.

(ii) Twisted Pair Wire Lines : They areused as telephone lines.

(iii) Coaxial Wire Lines : They are used by T.V. cable operators.

(iv) Optical Fibre Lines : They are usedin optical communication.

The efficiency of the transmission lineincreases from the parallel wire lines to the

optical fibre lines. The optical fibre line isthe best because it has maximum efficiencyand minimum loss in energy.

Coaxial cable system is better thanparallel twisted pair wire-lines because of greater available band-width, lower energylosses and much lower cross-walk.



Physics [107]

In the cable system, the deviation infrequency & phase response of the signalspassing through a cable are set by using adevice called equalizer.

A telephone link can be establishedusing ground waves, sky-waves,microwave, co-axial cables and latest, theoptical fibre cables. The microwave link ismore noisy than the coaxial link. The opticalfibre link are free from noise & energylosses.

LASER

(Light Amplification by stimulatedemission of radiation): Laser is a process by which a coherent, hightly intense monochromatic and almost perfectly parallel beam of light is obtained. The term ‘Laser’is also used for the beam of light obtainedthis way.

Use of Lasers : Lasers are used in :

Microsurgery : For delicate organs e.g.

retina, cervical tumours, tissues, etc.

Engineering : Tiny welds are made withprecision using powerful laser beams.

Industry : To drill holes in diamondsand hard steel.

Communication : In optical communi-cation.

Holography : In three dimensional pho-tography without lenses.

Military : As a powerful war weapon.

Weather Forecasting : A laser basedRADAR called Lidar is used for weatherforecasting.

Reading bar codes, manufacturing andreading compact discs, cutting cloth in gar-ment industry etc. are also some of the usesof lasers.

MICROWAVE DEVICE

(1) Radar (Radio Detetection &

Ranging) : RADAR system detects theobjects like aeroplanes and ships. Itsantenna transmits microwave (frequencygreater than 1000 MH

z) signals which are

reflected from the object. Knowing the timeinterval between the transmission andreception of the signals, the distance of theobject is measured.

(2) Microwave Oven : It bakes foodusing microwaves. These microwaves areproduced in the oven at a high frequency

of about 2450 MHz by means of aMagnetron. Since the metals block themicrowaves, hence, most microwave ovensare made of glass. Microwaves producedare directly absorbed by the food-materialwithout heating the surroundings. Themolecules of food content vibrate onabsorbing the microwaves that causes heat.The energy is used by the food only andhence, it takes a small interval of time tocook the food.




ELECTROSTATICS

Electrostatics deals with the study of electric charges at rest. Electric charges atrest are produced either by friction between

two insulating bodies which are rubbedagainst each other or by electrical induction between a charged body and an eartheduncharged body.

When two insulating bodies are rubbedagainst each other, electrons are transferredfrom one body to the other. Electrons arenegatively charged particles. Hence, the body giving electrons becomes positivelycharged while the body gaining electrons becomes negatively charged. Electric

charges are of two types only viz. positiveand negative. Like charges repel and unlikecharges attract each other.

Bodies gaining two kinds of charges onrubbing

Positive Charge Negative Charge

Glass Rod Silk Cloth

Fur or woollen cloth Rubber rod, Ebonite

and Amber.

Woollen Carpet Rubber Shoe

Nylon or Acetate Cloth

Dry hair Comb

Woollen Coat Plastic Seat.

The minimum amount of charge thatcan be transferred from one body to theother is the charge on an electron (–1.6 ×10–19c). Hence the total charge on a charged body is equal to the sum of products of no.of electrons transferred and charge on an

electron or a proton. Electric charge isalways conserved when some electrons aretransferred from one body to the other, the body giving electrons becomes slightlylighter and the body gaining electrons

becomes slightly heavier.

COULOMB’S LAW

The electric forces between the twopoint charges is directly poportional to theproduct of the charges and inverselyproportional to the square of distance between the charges. The electric force may be attractive or repulsive depending uponthe nature of the two charges. Thecoulombic force (electric-force) is much

stronger than the force of gravitation. Anuncharged body can be charged by placingit near a positively charged body withoutactual physical contact between them (byinduction). The positive charge present onthe charged body attracts electrons presentinside the atoms at the surface of theuncharged body. Hence, a few electronscome out at the surface resulting in anegative charge at the surface and an equalpositive charge appears on its opposite face.Thus it gets polarised because it has equal

and opposite charges at the two oppositesurfaces. If this body is earthed, the positivecharge disappears and the body becomesnegatively charged. The uncharged bodyhas now a net negative charge. Theuncharged body can acquire a net positivecharge too provided the charged bodyplaced near it was negatively charged.

ELECTRO

MAGNETISM



Physics [109]

When a charged glass rod is brought

near a piece of paper, it attracts the paper because of force of attraction between thecharge on rod and induced charge whichhas appeared on the paper.

ELECTROSTATIC SHIELDING ORELECTROSTATIC SCREENING

Protecting a certain region of spacefrom external electric fields is known aselectrostatic shielding.

A hollow conductor stores charge onlyat its surface. Hence, electric field remainsoutside only, and inside the hollow electricfield is zero. Such a space having zeroelectric field is known as Faraday’s Cage.Therefore, to protect delicate instrumentsfrom external electric fields, they are keptinside a hollow conductor. In a thundersto-rm accompanied by lightning, it is safer to be inside a car or a bus than to be in theopen ground or under a tree. The metallic body of a bus or car provides electrostatic

shielding from the lightning.If electric charge is supplied continously

to a conductor having certain pointed ends,the electric charge is sprayed out throughthe pointed ends. A conductor with pointedends is a good receiver of charge also. Alightning conductor placed at the top of a building has a number of sharp spikes atits two ends. Upper end is free in air tocatch electric charge produced due tolightning and the lower end sprays chargeinto the ground. Thus it provides a safety

to the building.

Electroscope : The device is used todetect the presence of electric charge andtype of charge on a body. Its most commontype is gold leaves electroscope. It canmeasure potential-difference as well.

Capacitor or Condenser : A device

used to store electric charge is known as

capacitor.Van de graaff Generator : It produces

highly energised positively chargedparticles or ions by producing very highelectric voltage. It is used in scientific orindustrial works.

Atmospheric Electricity : Earth is agood conductor of electricity. Its electricpotential is ‘0’ Volt everywhere at theearth’s surface. When a positively charged body comes in contact with the earth,

electrons flow from earth to the bodywhereas when a negatively charged bodycomes in contact with earth, electrons flowfrom the body to the earth. Loweratmosphere is a poor conductor of electricity but its conductivity increases with increasein height. Ionosphere is a very goodconductor. This is primarily due toionization of particles of air by cosmic rayparticles having energies to the order of 1014 MeV.

Thunderstorms & Lightning Flashes :In every thunderstorm, the positive andnegative ions produced are separated inthe cloud. The positive ions are at about 6km and the negative ions are at about 2-3km above the earth’s surface. It results in avery high potential difference (107 to 108v) between the lower negatively charged levelof cloud and the earth’s surface. It causesthe electric breakdown of air. Consequentlythe intervening air gets ionised andconducts charge from the cloud to the earth

in the form of a lightning flash. Each flashlasts for 2×10–3 seconds and desposits about20 coulomb charge on earth. All the worldover, there occur about 40000thunderstorms per day.

In recent years, many importantindustrial applications of electrostatics have been developed. Some of them are:




Metallic Conduction and Electrolytic Conduction

Metallic Conduction Electrolytic Conduction

1. The flow of current through a metallic The flow of current through electrolyte is calledconductor is known as metallic conduction. electrolytic conduction.

2. Metallic conduction is due to the movement Electrolytic conduction is due to the movement

of electrons in the metallic conductor. of positive and negative ions in the electrolyte.3. The chemical properties of the metallic conductors The chemical properties of the electrolyte

do not change due to metallic conduction. change due to electrolytic conduction.4. The metallic conductor is not The electrolyte decomposes due to electrolytic

decomposed due to metallic conduction conduction.

but it is only heated up.

Conductor & Insulator

Conductors Semi Conductors Insulators(1) They are good conductors of (1) They are poor conductors (1) They are bad conductorselectricity. of electricity. of electricity.(2) They have higher density of free (2) They have lower density of free (2) They do not have carrierselectrons as carriers of charge. electrons & holes as carriers of of charge.

charge.(3) They have very high (3) They have relatively lower (3) They have very low conductivity. conductivity. conductivity.(4) Conductivity decreases with rise (4) Conductivity increases with rise (4) Conductivity increasesin temperature. in temperature. with temperature.

(3) In electrostatic spraying of paints,powders etc.

(4) In understanding volcanic lightningand common lightning strikes from thecloud base to the ground.

(5) In the design of cathode ray tubes forRADAR and TV.

(6) In ink-jet printing, which is high qualityand delivers high speed printing.

(7) In electrostatic loud speakers andmicrophones.

Electroplating with Silver: Forelectroplating with silver, solution of sodium silver cyanide [Na[Ag (CN)

2] is used

as an electrolyte.

Joule’s heating effect of current is com-mon to both d.c. and a.c. That is whythe instruments or eleectrical appliancessuch as heater, press, geyser, toaster

etc. works both on d.c. and a.c. Jouleheating effect is irreversible. It means if

the direction of current in a resistor isreversed, the cooling of resistor doesnot occur whereas heating of the resis-tor takes place. If the resistances areconnected in series, the current I is samethrough each resistor, then

P µ R and V µ R (as V = lR)

It means, in series connections, the po-tential difference and power consumedwill be more in higher resistance.

If the resistence are connected in paral-

lel, the potential difference V is same

across each resistor. Then and

(as V = lR)

It means in parallel connections, thecurrent and power consumed will bemore in smaller resistances.

(1) In prevention of pollution of atmosphere

by electrostatic precipitation of fly ash.

(2) In designing electrostatic generators like

Van de Graaff generator.



Physics [111]

In parallel grouping of bulbs, the bulb

of higher wattage will give more brightlight and will pass greater currentthrough it. It will have lesser resistanceand same potential difference across it.If one bulb gets fused, other bulbs willwork.

In series grouping of bulbs, the bulb of higher wattage will give less bright lightand will have smaller resistance and po-tential difference across it. If one bulbgets fused the other bulbs will not work.

CURRENT ELECTRICITY

Electric Potential : The amount of workdone in bringing unit positive charge at apoint is called as potential at that point.Electric potential = work done/charge = Joule/coulomb = volt. Unit of electricpotential is volt (v).

Potential difference (p.d.). It is equal tothe work done in moving a unit positivecharge from one point to another point.

Electric potential and potentialdifference are physically similar quantitieshaving common unit of volt. Electricpotential determines the direction of flowof charge from one conductor to anotherconductor or from one point to anotherpoint through a conductor. Electrons flowfrom lower potential to higher potential.Electric potential and potential differencecan be positive and negative dependingupon the nature of electric charge.

Electric Current : The amount of electric charge flowing through any cross-section per unit time is known as electric-current.

Electric Current (I)

Unit of electric current is Ampere (A).

1 Ampere . Conventionally, the

electric current flows from higher to lower

potential against the flow of electrons.

Ohm’s Law

The current flowing through a

conductor is directly proportional to the

potential difference across the ends of the

conductor provided the temperature and

other physical conditions of the conductorremain the same.

Potential difference (V) ∝ Electric

current (I)

or, pot. diff. (V) = constant (R) × Electric

current (I)

or

Where constant for a

conductor known as resistance. Unit of resistance is ohm (W).

Resistance of a conductor is its ability

to oppose the flow of current. For a

conductor, the resistance

, where l = length of conductor.

A = area of cross-section of conductor.

r = resistivity of conductor.

Resistivity (r) of a conductor is aconstant. It depends upon the nature of

material of the conductor.

Conductivity (C)

= ohm–1 = mho.




Conductivity (K)

= ohm–1×m–1

Combination of Resistances

When two or more resistances arearranged in series i.e. in a common path of flow of current, the net resistance is equalto the sum of individual resistance. Hence,in series, the net resistance becomes larger.For two resistances R

1 & R

2 : R

s = R

1 + R

2 >

R1 or R2.When two or more resistances are

arranged in parallel i.e., between twocommon points of two different paths of flow of current, the reciprocal of the netresistance is equal to the sum of reciprocalsof individual resistances. Hence, in parallel,the net resistance becomes smaller. For two

resistances R1 & R

2 in parallel,

or < R1 or R2

EMF (electromotive force) of a cell orbattery: It is equal to the potential difference between the electrodes when current is notdrawn out from the cell.

Heating Effect of Current : When anelectric current flows through a conductor,an amount of heat is produced.

Joule’s Law of Heating Effect :

where H = amount of heat produced.

I = amount of current in conductor.

R = resistance of conductor.

V = Potential difference across

conductor.

t = time interval for which current issupplied.

Since heat is the form of energy, hence,heat produced due to the flow of current iselectrical energy consumed in a givenelectrical circuit. Thus, the net

Electrical energy = I2Rt = VIT =

Electrical power

= I2R =VI

Unit of electrical energy is Joule (J) andthat of power is watt (w). Horse-power isanother unit of power. 1 Horse-power(H.P.) = 746 watt. The practical commercialunit of energy is kilowatt hour (KWH).

1 KWH = 1000 × 60 × 60 = 3.6 × 106

Joule.

Electric Bulb : An electric bulb has atungsten filament enclosed in a glassenvelope filled with inert gases like argon.Tungsten filament has a very a high meltingpoint and resistivity. Its resistance is veryhigh. Hence on passing electric current, itproduces heat and light.

Electric Iron (Electric press), Heater,Geyser, Toaster & Immersion Rod : Theseheating appliances have Nichrome (an alloyof nickel, iron and chromium) wire asheating element. Nichrome has highmelting point, resitivity and malleabilityand it does not oxidise in air easily. Onpassing current, it gets heated up.

Carbon Arc Lamp : It consists of twocarbon rods as the electrodes separated bya smaller air gap. When a high potentialdifference is applied across the rods, a spark



Physics [113]

Power Cell Chemsitry

Cell Electrodes Electrolyte Depolariser EM For Voltage

Voltaic Copper and Zinc Dilute solution of Absent and the cell 1.08 Volt

cell rods H2SO

4 (sulphuric suffers from polarisation

acid)

Daniel Copper-container dilute solution of Copper-sulphate 1.1 Volt

cell and zin rod H2SO

4(CuSO

4)

Leclanche cell Carbon and zinc rods Ammonium chloride Manganese dioxide 1.45 volt

(NH4Cl) (MnO

2)

Dry cell Carbon rod and zinc Ammonium chloride Manganese dioxide 1.5 Volt

pot (NH4Cl) (MnO

2). ZnCl

2 being

hygroscopic absorbs

water produced.

Lead-Acid A set of perforated dilute (20%) H2SO

4lead oxide (PbO

2) 2.2 volt

Accumulator lead plates with lead Solution with specific

(PbO2) in holes as gravity 1.25. when the

positive and a set of specific gravity of

lead rods as negative H2SO

4 solution

decreases to 1.18 the

cell requires recharging

Alkali-accul- Anode is a perforated 20% solution of (Fe(OH)2] & 1.36 Volt

mulator or NiFe cell steel grid filled with potassium hydrooxide (Ni(OH)2]

Cell or Edison cell (Ni(OH)2] and cathode (LiOH)

is a steel grid filledwith [Fe(OH)

2] and

jumps across the air-gap due to ionization

of air & a very bright light is emitted by thegap. This light is called an arc-light. Thecarbon arc lamp is used in electric welding,search lights and cinema projectors.

Electric Fuse Wire : Usually it is analloy of tin (63%) & lead (37%). It has highresistance and low melting point. It isconnected in series with the electricalinstallations. All of a sudden, if strongcurrent flows, the fuse wire melts away,causing the breakage in the circuit, thereby

saving the main installations from beingdamaged. Thus, it is capable of saving costlyappliances.

Chemical Effect of Electric Current

Electrolysis : When an amount of electric current is passed through a solutionor melt of an electrolyte, the electrolytedissociates into positive ions (cations) andnegative ion (anions). The cation getsdeposited at negative electrode (cathode)and the anion gets deposited at the positiveelectrode (anode). This phenomenon isknown as electrolysis. Electrolysis isexplained by the Faraday’s laws of electrolysis. Faraday constant (F) = 96500

coulomb/gram equivalent = charge on onemole of electrons. Electrolysis involves directcurrent (d.c.) only and not a.c.




Applications of Electrolysis :

Electroplating : The process of deposit-ing a layer of precious metal like gold,silver, nickel and chromium over cheapmetals like iron and copper by electroly-sis is called as electroplating. The pre-cious metal is taken as anode and cheapmetal as cathode. The electroplating pre-vents corrosion (rusting) and makes thecheap metals attractive.

Anodising : It is the process of coatingaluminium with its oxide electrochemi-

cally to protect it against corrosion. Indilute sulphuric acid as electrolyte, thealuminium article is made the anode.To give the surface of the article beauti-ful colours, dyes are mixed in the elec-trolyte.

Electrotyping : It involves the prepara-tion of exact copies of metallic typeused in the printing work and the en-graved blocks on the metals by the pro-cess of electrolysis. A sheet of wax is

first pressed against the type set or block.The impression obtained on wax is madeconducting by coating it with graphitepowder. Then it is copper-plated by theprocess of electrolysis. The sheet so ob-tained is a copy of the type of block.This process is also used for the manu-facture of gramophone records.

Extraction of Metals from the Ores :

Metals like sodium, aluminium, magne-sium, calcium, zinc, copper etc. are ex-tracted from their ores by electrolsysis.

Purification of Metals : It is carried out by using impure metal as the anode,pure metal as the cathode and a doublesalt of the pure metal as electrolyteswhen a current is passed through theelectrolyte solution, pure metal getsdeposited at the cathode. This methodeis used to purify blister copper.

Production of Oxygen and Hydrogen :Oxygen and hydrogen are manufac-tured commercially by the electrolysisof acidulated water.

Manufacture of Chemicals : By theelectrolysis of sodium chloride solution,caustic soda is prepared.

Medical Applications : Electrolysis isused for nerve stimulation especially forpolio, for removing unwanted hairs onany part of the body etc.

ELECTROCHEMICAL CELL

It converts chemical energy stored inthe electrolyte into electical energy bychemical reaction.

An electro chemical cell consists of twoelectrodes viz. anode and cathode; anelectrolyte to provide ions and adepolariser to prevent the formation of hydrogen coating on the surface of cathode.



Physics [115]

Button Cells : They are tiny dry cells.

They are : Mercury Cells : EMF or Voltage =1.36

volt

Life : upto 3 years used in a watches,hearing aids, portable scientific instru-ments.

Silver Oxide Cells : EMF = 1.62 volt.

Life and uses similar to that of mercurycells.

Lithium Cells : Life : upto 10 years.

Used in quartz watches, mobiles, etc.

Alkali Cells : They are very cheap andenvironmentally safe. Life shorter. Usesimilar to other button cell EMF = 1.5 V

Battery : An arrangement of a no. of cells in series to provide higher voltage of direct current (d.c.) is called a battery.

Voltameter or Electrolytic Cells : Anarrangement consisting of identicalelectrodes (Anode and Cathode) in aninsulating container with an electrolyte-solution to carry out electrolysis is termedas a voltameter or electrolytic cell. Itconsumes electrical power to causeelectrolysis. An electrochemical cell or acommon cell produces electric power.

Magnetic Effects of Current : Whenan amount of electric current flows througha conductor, it develops an amount of magnetic field around itself.

Electromagnet : It consists of a coil of an insulated wire (usually copper) closelywound around a soft iron core. When an

electric current passes through the coil, thearrangement acts as an electromagnet.

Electromagnets are used in manydevices like electric-bell, electric horn,telephone receiver, electric relay, etc.Electromagnets fitted on cranes can liftheavy iron pieces.

Cyclotron : It is a device used to produce

highly accelerated positively chargedparticles under the effect of a strongmagnetic field produced by anelectromagnet. Cyclotron is used fornuclear works so as to cause nucleardisintegration, etc.

Galvanometer : It is based on themagnetic effect of current. When a coilhaving electric current is suspended in auniform magnetic field, it gets deflected.This forms the basis of galvanometer. A

galvanometer connected in any part of anelectric circuit indicates the flow of current.It is used to construct devices like Ammeterand Voltmeter.

Ammeter : It measures the amount of electric current in any part of circuit inseries.

Voltmeter : It measures potentialdifference across a circuit in parallel.

We can produce high voltage on thehuman body without getting a shock.

For this the person must stand on ahighly insulated platform. Hence withhigh voltage on the body, no chargewill flow to the ground through the body and the person will not get anyshock. There is an impression amongmany people that a person touching ahigh power line gets stuck with theline. This is not a correct notion be-cause there is no special attractive forcethat keeps a person stuck with a highpower line. The actual reason is that acurrent of the order of 0.05 ampere oreven less is enough to bring about adisorder in our nervous system. As aresult of it, the affected person maytemporarily lose his ability to exercisehis nervous control to get himself freefrom the high power line.




For a given electrical installation, when

an electrical appliance of high wattageis switched on it draws more current.Consequently, the voltage across thenearby appliance of lower wattage islowered for a moment until it is estab-lished by the transmission grid. e.g (i)When a car-engine starts, the car lightgets dimmer for a moment (ii) Lightfrom a bathroon bulb gets dimmer for amoment when the geyser is switchedon.

When a metallic wire is carrying anelecteric current, it is not charged be-cause the total no. of electrons and thatof protons in any section of the wireremains the same. Hence net charge onthe wire is zero.

Potentiometer : It is an electrical deviceused to compare emfs and internalresistence of the two cells.

Wheatstone Bridge : It measures anunknown resistance with accuracy.

Meter Bridge or Slide Wire Bridge : Itis the practical form of wheatstone bridge.

Shunt : It is a low resistance connectedin parallel to a device or an electrical circuitto provide a subway for the surplus current.Thus, it provides a safety to the device orthe ciruit.

Electromagnetic Induction : Wheneverthe magnetic flux linked with a closedmetallic-loop changes, an amount of emf isinduced in the loop. The induced emf

causes flow of induced current in the loop.This phenomenon is known aselectromagnetic induction. It can be carriedout by (i) moving a magnet towards oraway from the loop (ii) moving a looptowards or away from the magnet (iii)rotating a loop in front of a magnet. Thiseffect forms the basis of serveral devices

and is used to produce alternating current

by an a.c. generator.

Electric Motor : It converts electricalenergy into mechanical energy. It is basedon the fact when a current carrying coil isplaced in a magnetic field, it experiences atorque that causes rotation of the coils.Electric motors are used to operate manyappliances like fans, coolers, mixers,geysers, refrigerators washing machines etc.

Motor Starter : When a large electricmotor is switched on, its initial current is

very high and that can damage its circuit.Hence, the initial higher current should bereduced. A motor starter has a large variableresistance. When an electric motor starts,the motor starter automatically goes in toreduce high current and comes out afterswitching off the motor. Thus it regulatesan electric motor at start and stop.

Choke-Coil : It regulates alternatingcurrent in a given circuit. Its resistancevalue is zero and hence, it does not consume

power. A capacitor can do a similarfunction.

Generator (Dynamo) : It convertsmechanical energy into electrical energy. Itis of two types:

(i) Direct Current (D.C.) generator : itproduces D.C.

(ii) Alternating current (A.C.) generator :it produce A.C.

Both the generators work on a common

principle. When a metallic coil rotates in amagnetic field, an induced em.f. is producedin the coil. However, their constructionsare a bit different.

Transformer : It converts higheralternating current at lower alternatingvoltage into lower alternating current athigher alternating voltage and vice-versa.



Physics [117]

It is of two types viz (i) Step-up

transformer : It converts higher a.c. at lowera.c. voltage into lower a.c. at higher a.c.voltage. It is used at the power station (ii)step-down transformer : It converts lowera.c. at higher a.c. voltage supplied by powerstation into higher a.c. at lower a.c. voltageto be used for domestic purposes. It is usedat the colonies. Transformer works on theprinciple of mutual induction, a form of electromagnetic induction.

Inverter : It converts D.C. into A.C. Fordomestic purposes it is specially designed

to convert D.C. from a battery to A.C. & tocharge the battery. When the mains supplyis available, it charges the battery (convertsA.C. into D.C.) whereas in absence of themains supply, it automotically switches onthe A.C., converted from D.C. supplied bythe battery. Thus it fulfils electric supplyduring the power break down.

Loudspeaker : It converts electricalenergy first into mechenical energy andthen into sound energy. It works on thefact that when a current carrying coil is

placed in a magnetic field, the coilexperiences force. This force causesvibrations in a cone. Consequently soundis produced. Its most common type ismoving coil loudspeaker.

Microphone : It converts sound energyinto mechanical energy of a vibratingdiaphragm and then finally into electricalenergy to be supplied to the loudspeaker.Its most common type is the moving coilmicrophone.

GENERATION ANDTRANSMISSION OF POWER

Electric power generation occurs in twoforms viz. Hydroelectricity and thermalelectricity. Both of them involve an electricgenerator whose armature is connected

with a turbine. In case of a hydroelectricity

plant, the potential energy of water storedin a dam is converted into kinetic energy of the falling water which in turn is convertedinto the kinetic energy of the armature of the generator. This kinetic energy finallyappears as electrical energy known ashydro electricity. At a thermo-electric plant,the function of the falling water is done byhot-steam coming from boiling water. Itrequires heat energy produced by burningthe fuel like coal & petroleum or heatproduced by nuclear reactor.

DOMESTIC ELECTRIC CIRCUITS

The electric power is supplied to thehouses or factories through undergoundcables or overhead wires on poles. A singlephase electric line system comprises threewires, namely phase wire or line wire,neutralwire and earth-wire. These wirescoming from power substation areconnected to an electric meter in a houseor a factory. An electric fuse is placed inthe path of the phase wire before it isconnected to the electric-meter. This fuse isknown as pole-fuse and has a definitecurrent rating. If current exceeds this ratingvalue, the fuse wire will melt.

Systems of power distribution in ahouse: The output electric power of themain switch can be distributed in a house by two systems. They are :

(i) Tree System : This system of powerdistribution is like the distribution of the branches of a tree. The power supply of different circuits originates from differentfuses. So if a particular fuse melts, then theother circuit remain unaffected. Differentappliances in a given circuit are connectedin parallel so that they can be switched onor switched off independently. The parallel




connection gives equal potential difference

across each appliance in a circuit. Thissystem is complex and expensive.

(ii) Ring System: In this system thethree wires namely live, neutral & earth,start from the main fuse and return to themain fuse ofter connecting variousappliances. Each appliance has a fuse. Soif a fuse melts due to short circuiting, thenonly that appliances is switched off. Otherappliance remain unaffected and continueto work. This system is easy to install and

cheaper.Earthing: It is usually a copper wire

whose one end is connected with metalcasing of an electric appliance. The otherend of the wire is connected to the copperplate which is buried deep inside the earth.When the live wire touches the metal casingof an electric-appliance, it can cause electricshock on touching the appliance. But inthe presence of earthing current flows fromthe metallic casing to the earth through theearthing-wire. It produces heat in the circuit

that in turn causes the melting of fuse inthe circuit. The circuit is switched off automatically. It saves the appliance from burning and a person touching theappliances does not get an electric shock.

Whenever there is lightning in the sky,the electric circuits should be switched off to save the electric appliances connected inthese circuits from burning.

Flexible Cables: An electric-applianceis connected with three-core flexible cables.The insulations on the three wires arecoloured (i) Red or brown for live wire (ii) black or light blue for neutralwire and (iii)green or yellow for earth wire.

Plugs: A three-pin plug consists of onelonger and thicker pin and two identicalthin pins. The thick pin is for earthing and

hence it is connected with the green or

yellow wire. The remaining two pins areconnected with red or brown (live) wireand black or blue (neutral) wire. The earthpin being thicker cannot be inserted in thelive of the socket even by mistake.

Sockets: A socket has three holes. Thetop bigger hole is for the earth, the lowerright hole is for the live wire and the lefthole is for the neutral wire.

Switches: An electric switch, is alwaysconnected across the live wire. If a switch

is connected in the neutral wire, the socketremains live even when the switch is in theoff position. It can cause a shock from theelement of a heater or other appliance evenwhen the appliance is not in use.

The insulation on the wire should be of high strength so that it may not melt easilywhen wires are heated due to a largecurrent flowing through them. Wirescarrying current electricity should not betouched bare footed. The live wire is at

higher potential and the earth is at zeropotential. If we touch the live wire barefooted, a large current will pass throughour body. So we will receive a severe shock.This shock affects our nervous system andmay cause even death. Hence, while usingelectricity, we must wear gloves made of insulated material and shoes of rubber soleso that current may not flow through our body.

MAGNETISM

An iron ore piece having the structuralformula Fe

3O

4(ferrous-ferric oxide) can

attract small pieces of iron. Such ore pieceswere first of all found in the district of Magnesia in Asia Minor in Greece. Theseore pieces were called Natural Magnets. AGreek Philosopher, Thales of Miletus had



Physics [119]

observed these ore pieces in 600 B.C. This

iron ore is called Magnetite. Magnetite wasalso called ‘lode-stone’ in the sense of leading stone to show geographic directions(North-South).

A magnet is made of iron or its alloys.Each magnet has two equal and oppositemagnetic poles viz. North pole and South-pole lying at its ends.

It we cut a magnet, we get smallermagnets again, instead of an isolated northor south pole of the magnet. Hence themagnetic poles always exist in pairs andnot as a single magnetic monopole. Likepoles repel each other and unlike polesattract each other. Each magnetic pole canattract small pieces of iron, nickel, cobalt.When a magnet is suspended at mid-point by a thread, its north pole points thegeographic north and south-pole pointsgeographic south. It is its directionalproperty. On heating, the magnets lose theirmagnetism.

Magnetism in a magnet is due to theproper alignment of electron spins.

MAGNETIC SCREENING ORSHIELDING

A space can be shielded from magneticfield by surrounding the space with a softiron ring. As magnetic field lines will be

drawn into the ring, the enclosed region

will become free of magnetic field. Superconductor also provides perfect magneticshielding as no magnetic lines pass throughthe super conductor.

Magnetic shielding is used to protectcostly wrist watches from externalmagnetic fields by enclosing them in a softiron core.

When an iron bar is magnetised, thespins are aligned parallel to the field. Hence,

the length of the bar in the direction of magnetisation increases. This effect isknown as mangetostriction effect. This effectis used for producing ultrasonic waves.

CLASSIFICATION OFMAGNETIC MATERIALS

(1) Diamagnetic Substances : They areweakly repelled by an external magneticfield, e.g. Bismuth, antimony, copper,gold, quartz, mercury, water, alcohol,hydrogen.

(2) Paramagnetic Substances : They areweakly attracted by an externalmagnetic field e.g. aluminium,platinum, chromium, manganese,copper sulphate, crown glass, oxygenetc.

(3) Ferro Magnetic Substances : They arestrongly attracted by an externalmagnetic field e.g. iron, cobalt, nickel

and their alloys like Al Ni, alnico,tinconol vicalloy, steel, perm alloys, mu-metal, radio-metal, etc.

Ferromagnetic substances are used formaking permanent magnets andelectromagnets.




The ferromagnetic materials used for

coating magnetic tapes in a cassette playeror for building memory stores in acomputer are ferrites. Most commonferrites used are MFe

2O

4 where M= Mn,

Fe, Co and Ni.

EARTH’S MAGNETISM

Earth acts as a huge magnet. Itsmagnetic south pole is at its geographicnorth and is located in North-Canada ,

longitude 96° west and latitude 70.5° North.Its magnetic north pole is at the geographicsouth and is located in Antarctica,longitude 84° east and latitude 70.5° south.

Geographic Meridian : An imaginaryvertical plane passing along the axis of rotation of the earth at a place is known asgeographic meridian at that place.

Magnetic Meridian : An imaginaryvertical plane passing along the axis of afreely suspeneded magnet at a place is

known as magnetic meridian at the place.Magnetic Declination (q) : The angle

between the geographic and magneticmeridians at a place is known as magneticdeclination at that place. It is in the orderof 20°.

Magnetic Inclination or Magnetic Dip(d) : The angle between the direction of theearth magnetic field and the horizontaldirection in the magnetic meridian isknown as dip-angle or magnetic dip. Its

value is maximum 90° at the poles andminimum 0° at the equator.

Horizontal Component : It is thecomponent of total magnetic field of theearth horizontally in the magnetic meridian.In the given diagram : Plane PQRS =Magnetic meridian. Plane = PQUT =

Geographic Meridian q = Magnetic

declination.

d = Magnetic dip.

H = Horizontal component of the earth’smagnetic field (B

E).

V = Vertical component of BE.

When a compass is used by marinersand others at a place, the magnetic needleof the compass sets itself along thehorizontal component of the earth’smagnetic field in the magnetic meridian.By knowing the angle of dip and magneticdeclination, the location of the place can

be determined.If d = 0° the place is at the equator,

if d = 90° the place is at the poles.

At the geographic north pole, themagnetic north pole of the magnetic needlewill be vertically down while at thegeographic south pole, the south pole of the needle will be vertically down.

If d is more than 0° and less than 90°,the place is between the pole and the

equator.Evidences behind the earth’s

magnetism :

(i) A fresh suspended bar magnet alwaysaligns itself along the geographic north-south. It means the north pole of themagnet is attracted by the magnetic



Physics [121]

south pole of the earth that lies in the

geographic north and vice-versa.

(ii) When a soft iron piece is buried belowthe earths surface along the north-southdirection, it acquires magneticproperties after some time.

(iii)When we draw field lines of a magnet,we get neutral points. At these pointsthe net magnetic field due to the magnetis completely neutralized by thehorizontal component of the earth’smagnetic field.

TRIVIA

In the gases, the charge carriers areelectrons and + VE ions.

Filament of 100 W lamp is thinner thanthat of 200 W lamp.

When two lamps of different wattageare connected in series, the lamp of lower wattage glows more brightly.

The power of a heater when connected

across a power supply of negligible in-ternal resistance is P. If the length of the coil is halved, the power of the re-maining wire is less than 2P.

Substances which lose their resistivityat low temperature are called super-conductors.

The material of the heating element of an electric heater should have high re-sistivity and high melting point.

The heat generated in a wire is doubled

when both the radius and length of thewire are doubled.

If a heater boils 1 kg water in time T1

and another heater boils the same wa-ter in time T

2, then both connected to-

gether in series will boil the same watertime T

1 + T

2.

The quantity in electricity that is ana-

logues to temperature in heat is poten-tial.

The current capacity increases, whenthe cells are connected in series.

The internal resistance of an ideal cell iszero.

The resistance of an ideal voltmetershould be infinity.

The resistance of an ideal ammetershould be zero.

Energy is dissipated inside a cell due tointernal resistance.

The potential across a cell in closedcircuit is less than the emf due to itsinternal resistance.

Larger the current drawn from a cell,smaller is the potential difference across it.

Energy dissipated by the cell = Elt,where E = emf, I = current, t = time.

Energy dissipated in the external circuitof a cell = VIt, where V = potential dropacross the cell.

The emf of voltaic cell is 1.08 V, that of Daniel cell is 1.12 V and that of Leclanchecell is 1.45 V.

Acid accumulator is most likely to bedamaged due to short circuiting.

Fast charging does not cause sulphationof acid accumulator, but fast discharg-ing does it.

Thermocouple should not be used tomeasure temperature above the neu-tral temperature.

And if they are connected in parallel,the time taken to boil the same amountof water will be T

1T

2/(T

1 + T

2).




The fuse wire is made of an alloy of lead

and tin.

The resistance in electricity is analo-gous to friction in mechanics.

The change in the dimensions of a con-ductor has no effect on its conductivity,although conductance may change.

The quantity in electricity that is ana-logues to temperature in heat is poten-tial.

The current capacity increases, when

the cells are connected in series.

The internal resistance of an ideal cell iszero.

The resistance of an ideal voltmetershould be infinity.

The resistance of an ideal ammetershould be zero.

Energy is dissipated inside a cell due tointernal resistance.

The potential across a cell in closed

circuit is less than the emf due to itsinternal resistance.

Larger the current drawn from a cell,smaller is the potential difference across it.

Energy dissipated by the cell = Elt,where E = emf, I = current, t = time.

Energy dissipated in the external circuitof a cell = VIt, where V = potential dropacross the cell.

The emf of voilaic cell is 1.08 V, that of

Daniel cell is 1.12 V and that of Leclanchecell is 1.45 V.

Acid accumulator is most likely to bedamaged due to short circuiting.

Fast charging does not cause sulphationof acid accumulator, but fast discharg-ing does it.

Thermocouple should not be used tomeasure temperature above the neu-tral temperature.



Physics [123]

LAWS OF MOTION

Newton’s three laws of motion formthe basis of mechanics: According to1st law, a body continues to be in its

state of rest or of uniform motion alonga straight line, unless it is acted upon by some external force to change thestate. The law defines force and is alsocalled law of inertia.

According to 2nd law, the rate of changeof linear momentum of a body is directlyproportional to the external force appliedon the body, and this change takes placein the direction of the applied force. Thislaw gives us a measure of force.

According to third law, to every action,there is always an equal and oppositereaction. This law gives us the natureof force.

Inertia is inability of a body to change by itself, its state of rest or its state of uniform motion along a straight line.Inertia is obviously of three types :

(i) Inertia of rest (ii) Inertia of motion,(iii) Inertia of direction.

From Newton’s 2nd law, we obtain i.e., external force is the productof mass and acceleration of the body.

The absolute unit of force on SI is new-ton (N) and on cgs system, is dyne. Thegravitational unit of force on SI is kilo-gram weight or kilogram force. The

gravitational unit of force on cgs sys-tem is gram weight or gram force.

1N = 105 dyne,

1g wt. = 980 dyne.

According to the principle of conserva-tion of linear moment, the vector sumof linear momentum of all the bodies inan isolated system is conserved, and isnot affected due to their mutual actionand reaction. An isolated system is thaton which no external force is acting.Flight of rockets, jet planes, recoiling of a gun etc, are explained on the basis of this principle. Newton’s 3rd law of mo-tion can also be derived from this prin-

ciple and vice-versa. Apparent weight of man in an eleva-

tor is given by :

W’ = m (g + a), where mg is real weightof the man. Acceleration is (+a), whenthe lift is accelerating upwards and (-a), when the lift is accelerating down-wards. When lift is moving uniformly(upwards/downwards), a = 0, W’ =mg = real weight.

In free fall, a = g

∴ W’ = m (g – g) = 0

ie., apparent weight becomes zero.

The forces which are acting at a pointare called concurrent forces. They aresaid to be in equilibrium, when theirresultant is zero.

PHYSICS

AT A GLANCE




A frame of reference in which Newton’s

laws of motion hold good is called an

inertial frame of references. Such a frame

is unaccelerated frame of reference.Energy Equivalence

S.No. Particle Mass Energy equivalent1. Electron or positron 9.1 × 10-31kg 0.53 MeV2. Neutron or proton 1 amu = 1.67 × 10-37kg 931 MeV3. Alpha particle 6.68 × 10 –27kg 3724 MeV

GRAVITATION

Gravitation is the name given to theforce of attraction acting between anytwo bodies of the universle.

Newton’s Law of Gravitation: It statesthat the gravitational force of attractionacting between two bodies of the uni-verse is directly proportional to the prod-uct of their masses and is inversely pro-portional to the square of the distance between them i.e, = Gm

1m

2/r

2; where

G is the universal gravitational constant.

Gravitational Constant (G): It is equalto the force of attraction acting betweentwo bodies each of unit mass, whosecentres are palced unit distance apart.The value of G is constant throughoutthe universe. It is a scalar quantity. Thedimensional formula of G.

= [M–1L3T–2]. The value of G = 6.67 × 10–11 Nm2kg–2

Gravity: It is the force of attraction ex-erted by earth towards its centre on a body lying on or near the surface of earth. Gravity is the measure of weightof the body. The weight of the body of mass m = mass × acceleration due to

gravity. The unit of weight of a bodywill be the same as those of force.

Acceleration due to Gravity (g): It isdefined as the acceleration set up in a body while falling freely under the ef-fect of gravity alone. It is vector quan-tity. The value of g changes with height,depth, rotation of earth. The value of g

is zero at the centre of the earth. Thevalue of g on the surface of earth is 9.8ms–2.

The acceleration due to gravity (g) isrelated with gravitational constant (G)

by the relation g where M and R

are the mass and radius of the earth.

becomes zero at the centre of earth.

Gravitational Field: It is the space arounda material body in which its gravitationalpull can be experienced by other bodies.The strength of gravitational field at apoint is the measure of gravitational in-tensity at that point. The intensity of gravi-tational field of a body at a point in the

field is defined as the force experienced by a body of unit mass placed at thatpoint provided the presence of unit massdoes not disturb the original gravitationalfield. The intensity of gravitational fieldat a point distance r from the centre of the body of mass M is given by

E = GM/r2 = acceleration due to gravity.

Gravitational Potential: The gravita-tional potential at a point in a gravita-tional field is defined as the amount of

work done in bringing a body of unitmass from infinity to that point with-out acceleration. Gravitational poten-tial at a point,



Physics [125]

Gravitational potential energy of a body,at a point in the gravitational field of another body is defined as the amountof work done in bringing the given bodyfrom infinity to that point without ac-

celeration.Gravitational potential energy = gravi-tational potential × mass of body.

Inertial mass of a body is defined as theforce required to produce unit accel-eration in the body.

Gravitational mass of a body is definedas the gravitational pull experienced bythe body in a gravitational field of unitintensity.

Inertial mass of a body is identical to thegravitational mass of that body. The maindifference is that the gravitational massof a body is affected by the presence of other bodies near it. Whereas the inertialmass of a body remains unaffected by thepresence of other bodies near it.

Satellite : A satellite is a body which isrevolving continously in an orbit arounda comparatively much large body.

(i) Orbital velocity of a satellite is the

velocity required to put the satelliteinto given orbit around earth. Or- bital velocity of satellite, when it isrevolving around earth at height his given by

When the satellite is orbiting close tothe surface of earth i.e., H < < R, then

(ii) Time period of Satellite (T) : It isthe time taken by satellite to com-plete one revolution around theearth.

Geostationary Satellite: A satellitewhich revolves around the earth withthe same angular speed in the samedirection as is done by the earth aroundits axis is called geostationary or geo-synchronous satellite.

The height of geostationary satellite is =

36000 km and its orbital velocity =3.1kms–1.

Polar Satellite: It is that satellite whichrevolves in polar orbit around earth i.e.,polar satellite passes through geo-graphical north and south poles of earthone per orbit.

Escape Velocity: The escape velocityon earth is defined as the minimumvelocity with which a body has to beprojected vertically upwards from the

surface of earth (or any other planet) sothat it just crosses the gravitational fieldof earth (or of that planet) and neverreturns on its own. Escape velocity v

e is

given by.

States of Equilibrium

Stable Unstable Neutral

1. When displaced from 1. When displaced from 1. Particle always remains in the

equilibrium position, equilibrium position, particle state of equilibriumtends to come back tends to move away from irrespective of any displacement.

equilibrium position.2. Potential energy = Minimum 2. Potential energy = Maximum 2. Potential energy = Constant




Conservative and Non-conservative Force

S. Conservative Forces Non Conservative Forces

1. Work done by/against such forces in 1. Work done by/against such forces indisplacing a particle does not depend upon displacing a particle depends upon the path

the path along which particle is displaced. along which particle is displaced.2. Work done by/against such forces in 2. Work done by/against such forces in

displacing a particle around a closed path is displacing a particle around a closed path is

zero. NOT zero.3. K.E. of particle remains constant 3. K.E. of particle changes.

For earth, the value of escape velocity is

11.2 kms–1.

For a point close to the earth’s surface,the escape velocity are related as

Weightlessness: It is a situation inwhich the effective weight of the body becomes zero.

Kepler’s Law of Planetary Motion:

(i) Kepler’s First Law (Law of Orbit):

Every planet revolves around thesun in an elliptical orbit. The sun issituated at one focus of the ellipse.

SOLID & LIQUID

Interatomic Forces : These are thoseforces which are acting between theatoms due to electrostatic interaction between the charges of the atoms. Theseforces are electrical in nature and areactive between the atoms if the distance between them is of the order of atomicsize (= 10–10m.)

Intermolecular Forces : These are thoseforces which are acting between themolecules due to electrostatic interaction between the charges of the molecules.These forces are also of electrical originand are active between the moleculeswhose separation is of the order of molecular size (= 10–9m).

Molecular forces are weaker thaninteratomic forces.

Crystalline Solids : These are solidswhich have a definite externalgeometrical form and whoseconstitutent atoms/ions/molecules arearranged in a definite pattern in threedimensions within the solid.

Examples, are quartz, calcite, rocksalt,sugar, mica, diamond, etc.

Crystalline solids have sharp meltingpoint and they are anisotropic in behaviour.

Amorphous Solids : These are thosesolids which have no definite externalgeometrical form and whoseconstitutent atoms/ions/molecules are

(ii) Kepler’s Second Law (Law of

Area): The radius vector drawnfrom the sun to a planet sweeps outequal areas in equal intervals of timei.e, the areal velocity of the planetaround the sun is constant.

(iii) Kepler’s Third Law (Law of Pe-riod) : The square of the time pe-riod of revolution of a planet aroundthe sun is directly proportional tothe cube of semi-major axis of theelliptical orbit i. e., T2 µ R3.

where R is the semi major axis of theelliptical orbit of the planet around thesun.



Physics [127]

Elastic and Inelastic Collisions

Perfectly elastic collision Perfectly Inelastic collisions1. Particles do not stick after collision. 1. Particles stick after collision.2. Rel. vel. of separation after collison = rel. 2. Rel vel. of separation after collision is zero.

vel. of approach before collision.

3. Coeff. of restitution, e = 1 3. Coeff. of restitution e = 04. Linear momentum is conserved 4. Linear momentum is conserved5. K.E. is conserved 5. K.E. is not conserved

not arranged in a defintie pattern in

three dimensions within the solid.

Examples. Rubber, glass, plastic,cement, paraffin, etc. Crystalline solidsdo not have sharp melting point andthey are isotropic in behaviour.

Deforming Force : It is that force whichwhen applied, changes theconfiguration of the body.

Elasticity : It is the property of the body by virtue of which the body regains its

original configuration (length, volumeor shape) when the deforming forces areremoved.

Stress : The internal restoring forceacting per unit area of a deformed bodyis called stress i.e.,

stress = restoring force/area.

According to Newton’s third law of motion, the restoring force is equal andopposite to the external deforming forceapplied, when there is no permanent

change in the configuration of the body.Thus,

Stress is a Tensor Quantity: Its S.I. unitis Nm–2 and c.g.s unit is dyne/cm2. Its

dimensional formula is [M1L–1T–2].

Stress can be of two types :

(i) Normal stress

(ii) Tangential stress.

Strain : It is defined as the ratio of change in configuration of the deformed body due to a deforming force on it tothe original configuration of the bodyi.e.,

Strain can be of three types :

(i) Longitudinal strain

(ii) Volumetric strain

(iii) Shearing stream

Hooke’s Law : It states that the exten-sion produced in the wire is directlyproportional to the load applied withinthe elastic limit. Hook’s law also statesthat the stress is directly proportionalto strain within the elastic limit.

Modulus of Elasticity or Coefficient of elasticity of a body is defined as the ratioof the stress to the corresponding strainproduced, within the elastic limit.

Modulus of elasticity is of three types :

(i) Young’s Modulus of Elasticity (Y):It is defined as the ratio of normalstress to the longitudinal strainwithin the elastic limit i.e., (ii) Bulk Modulus of Elasticity (K) : It




is defined as the ratio of normal

stress to the volumetric strain withinthe elastic limit i.e.,

(iii) Modulus of Rigidity (ηηηηη) : It is de-fined as the ratio of tangential stress tothe shearing strain, within the elasticlimit i.e.,

Where Shearing strain is defined as theangle through which a line perpendicu-lar to the fixed face gets rotated underthe effect of a tangential force acting onthe opposite face of the body.

The S.I. units of modulus of elasticity of Y or K or η is Nm–2 or Pa and c.g.s. unitis dyne/cm2. The dimensional formula= [M1L–1T–2].

Ductile Materials : These are thosematerials which show large plastic

range beyond the elastic limit. Examplesare copper, silver, iron, aluminium, etc.

Brittle Materials : These are those ma-terial which show very small range be-yond elastic limit. For such materials,the breaking point lies close to the elas-tic limit. Examples are cast iron, glassetc.

Elastomers : These are those materialsfor which stress and strain variation isnot straight line within elastic limit andstrain produced is much larger thanthe stress applied. Such materials haveno plastic range. Example is rubber.

Elastic after Effect : The temporarydelay in regaining the original configu-ration by the elastic body after the re-moval of a deforming force is called

elastic after effect. It is least for quartz

or phosphor bronze. Elastic-Fatigue : It is the property of

the elastic body by virtue of which its behviour becomes less elastic under theaction of repeated alternating deform-ing forces.

Breaking force = breaking stress × areaof cross section of the wire.

Breaking stress is fixed for a material but breaking force will vary, depend-ing on area of cross-section of the wire.

Elastic Potential Energy in a StretchedWire: Elastic potential energy per unitvolume of the stretched wire is,

Thrust: The total normal force exerted by liquid at rest on a given surface incontact with it is called thrust of liquidon that surface.

Pressure of Liquid or Hydrostatic Pres-sure (P): It is defined as the thrust act-ing per unit area of the surfaceincontact with liquid i.e.,

S.I. unit of P is Nm–2 or pascal (denoted by Pa). Pressure is a scalar quantity.

Pascal’s Law : It states that if gravityeffect is neglected, the pressure at everypoint of liquid in equilibrium of rest issame. Pascal’s law also states that theincrease in pressure at one point of theenclosed liquid in equilibrium of rest istransmitted equally to all other pointsof liquid provided the gravity effect isneglected.



Physics [129]

Atmospheric Pressure : The pressure

exerted by atmosphere is calledatmospheric pressure. At S.T.P. thevalue of atmospheric pressure is1.01×105 Nm–2 or 1.01 × 106 dyne/cm2.

Archimede’s Principle : It states thatwhen a body is immersed partly onwholly in a liquid at rest it loses some of its weight, which is equal to the weightof the liquid displaced by the immersedpart of the body. Observed weight of the body in a liquid = true weighh - weight

of liquid displaced. Law of Floatation: It states that a body

will float in a liquid if weight of theliquid displaced by the immersed partof the body is atleast equal to or greaterthan the weight of the body.

Molecular Range : It is the maximumdistance upto which a molecule canexert some measurable attraction onother molecules. The order of molecularrange is = 10–9m in solids and liquids.

Sphere of Influence : It is a spheredrawn with molecule as centre andmolecular range as radius.

Surface Film : It is the top most layer of liquid at rest with thickness equal to themolecular range.

Surface Tension : It is the property of the liquid by virtue of which the freesurface of liquid at rest tends to haveminimum surface area and as such it behaves as a stretched membrane.

Quantitatively surface tension.

where F is the force acting on the imagi-nary line of length l, drawn tangen-tially to the liquid surface at rest.

S.I. unit of surface tension is Nm–1 and

c.g.s unit is dyne/cm. Surface tensionis a scalar quantity as it has no specificdirection for a given liquid.

Surface energy = surface tension × areaof the liquid surface.

(i) Excess of pressure inside a liquiddrop.

p = 2 S/R

(ii) Excess of pressure inside a soap bubble,

p = 4 S/R

where S is the surface tension and R isthe radius of the drop or bubble.

Angle of Contact : The angle of contact between a liquid and a solid is definedas the angle enclosed between thetangents to the liquid surface and thesolid surface inside the liquid, both thetangents being drawn at the point of contact of the liquid with the solid. Thevalue of angle of contact is less than 90°for a liquid which wets the solid surface

and is greater than 90° if a liquid doesnot wet the solid surface.

Capillarity : The phenomenon of riseor fall of liquid in a capillary tube iscalled capillarity. The height (h) throughwhich a liquid will rise in a capillarytube of radius r which wets the sides of the tube will be given by

where S is the surface tension of liquid,θ is the angle of contact, ρ is the densityof liquid and g is the acceleration dueto gravity.

Surface tension of a liquid/waterdecreases with rise of temperature orwhen a detergent is dissolved in it.




Viscosity : Viscosity is the property of

a fluid (liquid or gas) by virtue of whichan internal frictional force or viscousdrag comes into play when the fluid isin motion and opposes the relative mo-tion of its different layers. The coeffi-cient of viscosity of liquid is defined asthe tangential force (= backward vis-cous drag) required to maintain a unitvelocity gradient between two layers of fluid each of unit area.

Viscous drag F acting between two lay-

ers of liquid each of area A, movingwith velocity gradient dv/dx is given by

F = – ηΑdv/dx

where η is the coefficient of viscosity, andits c.g.s unit is poise or dyne cm–2 sec.

Stoke’s Law : It states that the back-ward dragging force F acting on a smallspherical body of radius r, movingthrough a medium of viscosity η, withvelocity v is given by

F = 6 πηrv. The Viscosity of Liquid decreases with

increase in Temperature : The viscos-ity of gases increases with the increasin temperature. With the increase inpressure, the viscosity of liquid increases but of water decreases where as of gasesremains unchanged.

Stream line flow of a liquid is that flowin which every particle of the liquidfollows exactly the path of its preced-

ing particle and has the same velocityin magnitude and direction as that of its preceding particle while crossingthrough that point. A stream line flowis accompanied by stream lines of liq-uid.

Laminar Flow : That steady flow in whichthe liquid moves in the form of layers is

called a laminar flow. In this flow, one

layer slides over the other layer of liquid.The velocity of liquid flow is always lessthan the critical velocity of the liquid.

Turbulent Flow : It is that flow of liquid,in which a liquid moves with a velocitygreater than its critical velocity. The mo-tion of the particles of liquid becomesdisorderly or irregular.

Critical Velocity : It is that velocity of liquid flow, upto which the flow of liq-uid is a streamlined and above which

its flow becomes turbulent. Reynold’s Number : It is pure num-

ber which determines the nature of flow of liquid through a pipe. Quanti-tatively, Reynold’s number N

R is given

by

where η is the coefficient of viscosity of liquid, g is the density of liquid. D is thediameter of the tube and v

e

is the criti-cal velocity of liquid flowing throughthe tube. For streamline or laminar flow,the value of N

R is less than 2000 and for

turbulent flow the value of NR is more

than 3000. For NR in between 2000 to

3000, the flow is uncertain.

Equation of continuity, a v = a constantof v α 1/a.

i.e., deep water runs slow.

Bernoulli Theorem : It states that forthe stream line flow of an ideal liquid,

the total energy (the sum of pressureenergy, the potential energy and kineticenergy) per unit volume remains con-stant at every cross-section throughoutthe tube i.e.,



Physics [131]

or, = another constant

Here, = pressure head

h = potential head

and = velocity head.

If the liquid is flowing through a hori-zontal tube, then h is constant.Bernoulli’s Theorem states that

= a constant

The Bernoulli’s Theorem, also states thatin a stream line flow of an ideal liquidthrough a horizontal tube, the velocityincreases where pressure decreases andvice-versa.

Torricelli’s Theorem : It states that thevelocity of efflux i.e., the velocity withwhich the liquid flows out of an orifice(i.e., a narrow hole) is equal to thatwhich a freely falling body would ac-quire in falling through a vertical dis-tance equal to the depth of orifice be-low the free surface of liquid. Quantita

tively velocity of efflux, , where

h is the depth of orifice below the freesurface of liquid.

HEAT

The ratio of work done (W) to the

amount of heat produced (Q) is always

a constant, represented by J. i.e.,

where J is called Joule’s mechanicalequivalent of heat. The value of

J = 4.186 joule/calorie.

When a body of mass m falls under

gavity through a height h, then W =mgh.

When a body of mass m, moving with a

vel. v is made to stop,

If temperature of a body of mass mrises by ∆T, then Q = m L, where L islatent heat of the body.

Temperature on Celcius Scale: Fahr-enheit scale, Reumer scale and Kelvin

scale are related as:

Various Thermometers used for mea-suring unknown temperatures are : Jolly’s constant volume air thermom-eter; Platinum resistance therometer;Thermoelectric thermometer; radiationPyrometer.

All Solids expand on Heating: Thecoefficient of linear expansion, coeffi-cient of superficial expansion and coef-ficient of cubical expansion are related

as α

Thermal capacity of a body is theamount of heat required to raise its tem-perature through one degree ∆Q = mc.From here, we may define specific heatof a body as the heat capacity per unit

mass of the body.

Water Equivalent of a body is the massof water which absorbs or emits thesame amount of heat as is done by the body for the same rise or fall in tem-perature. It is represented by w = mc.




When two substances at different tem-

perature are mixed together they ex-change heat. If we assume that no heatis lost to the surroundings, then ac-cording to principle of calorimetry,

Heat lost = Heat gained.

Specific Heat of Gases : Specific heatof a gas is the amount of heat requiredto raise the temperature of one gram of gas through 1°C. As the gas can beheated under different conditions, anddifferent amounts of heat is required in

each case, therefore, a gas does not pos-sess one single value of specific heat.Out of many values of specific heat of agas, two are important. These are calledtwo principal specific heats of a gas :

1. Specific heat of a constant volume (cv)

2. Specific heat of a constant pressure (cp)

Boyle’s Law: It states that the volumeV of the given mass of a gas is inverselyproportional to its pressure P, whentemperature is kept contant i.e. V = 1/

P or V = K/PPV = K a constant.

Charle’s Law: It states that the pres-sure remaining constant, the volume of the given mass of a gas is directly pro-portional to its kelvin temperature i.e.,V α T if P is constant.

Standard gas equation, PV = nRT

where n is the number of moles con-tained in the given ideal gas of volumeV, pressure P and temperature TK.

Gas Constant: R is a universal gas con-stant and r is a gas constant for 1 gramof a gas. The universal gas constant isdefined as the work done by (or on) agas per mole per kelvin i.e.,

The value of gas constant r for different

gases is different.

Ideal Gas or Perfect Gas: It is that gaswhich strictly obeys gas laws. for sucha gas, the size of the moelcules of a gasis zero and there is no force of attrac-tion or repulsion amongst its molecules.

Assumptions of Kinetic Theory ofGases:

(i) A gas consists of a very large numberof molecules which are perfectly elas-

tic spheres and are identical in allrespects for a given gas and are dif-ferent for different gases.

(ii) The molecules of a gas are in a state of continous, rapid and random motion.

(iii) The volume occupied by the moleculesis negligible in comparison to the vol-ume of the gas.

(iv) The molecules do not exert any forceof attraction or repulsion on each other,except during collision. on each other,

except during collision.(v) The collision of the molecules with

themselves and with the walls of thevessel are perfectly elastic.

(vi) Molecular density is uniform through-out the gas.

(vii) A molecule moves along a straightline between two successive collisions.

(viii) the collisions are almost instantaneous.

Pressure exerted by a Gas : It is due tocontinous bombardment of gas mol-ecules against the walls of the containerand is given by the relation.

where C is the r.m.s. velocity of gasmolecules.



Physics [133]

Average K.E. per molecule of a gas

. It is independent of the

mass of the gas but depends upon thetemperature of the gas.

Absolute Zero : It is that temperatureat which the root mean square velocityof the gas molecules reduces to zero.

Thermodynamical System : An assembly of extremely large number of par-ticles (such as gas molecules in a con-tainer) is called a thermodynamical sys-tem. Such a system has a certain valueof pressure P, volume V and tempera-ture T and heat content Q. These arecalled Thermodynamical parameters.

The equation of state of gaseous phaseof a substance is PV = n RT where n isnumber of moles of the gas.

The other two phases are solid phaseand liquid phase.

Internal Energy of a Gas is the sum of kinetic energy and the intermolecularpotential energy of the molecules of thegas.

Intermolecular potential energy of a realgas is a function of its volume. K.E. of gas molecules is a function of its veloc-ity and hence the temperature T.

K.E./molecules where k is

Boltzmann’s constant.

Cell Profile

Daniel Cell Leclanche Cell Dry Cell

Positive Copper Carbon rod Carbon rodelectrode vessel with brass capNegative Zinc rod Zinc rod Zinc vesselelectrodeElectrolyte dil H

2SO

4NH

4Cl solut ion Paste of NH

4Cl and

saw dustDepolariser CuSO

4Manganese Manganese

solution dioxide dioxide

In an ideal gas, intermolecular forces

do not exist. Therefore, it possesses nointermolecular potential energy. Thusinternal energy of an ideal gas is whollykinetic.

First Law of Thermodynamics is restate-ment of the principle of conservation of energy as applied to heat energy. Ac-cording to this law

dQ = dU + dW

where dQ is the small amount of heat

energy given to a system, dU is smallincrease in internal energy of the sys-tem and dW is the small external workdone by the system.

Thermodynamical Operations : Iso-thermal Changes are the changes inpressure and volume of the systemwhen temperature is kept constant. Freeexchange of heat between the systemand the surroundings is allowed. Thecontainer must be perfectly conductingand the changes must be brought about

slowly.

Equation of isothermal changes is PV =constant or P

2V

2 = P

1V

1

Adiabatic Changes are the changes inpressure and volume of the system,when heat content of the system is keptconstant. No exchange of heat is al-lowed between the system and the sur-roundings. The container must there-fore be perfectly insulating and thechanges must be sudden.

Second Law of Thermodynamics statesthat it is impossible for self-acting ma-chine, unaided by an external agencyto convey heat from the body at lowertemperature to another at higher tem-perature. This statement of the law wasmade by Calusius.




According to Kelvin, it is impossible to

derive a continuous supply of work bycooling a body to a temperature lowerthan that of the coldest of its surround-ings.

Heat Engines : A heat engine is a de-vice which converts heat energy intomechanical energy. Efficiency of a heatengine is the ratio of useful work done(W) by the engine per cycle to the heatenergy absorbed from the source (Q

1)

per cycle.

where Q2 is the heat rejected to the sink

Power of a steam engine = 2 PLAN(watt)

where P is the pres-

sure of steam measured in Nm–2, L ishalf the length of the stroke in metre,A is area of cross section of the pistonin m2 and N is number of strokes persecond.

Steam engine is an external combus-tion engine. There are internal combus-tion engines like petrol engine and die-sel engine. These are four stroke en-gines.

Carnot engine is an ideal heat enginewhich is based on Carnot’s reversiblecycle. Its working consists of four stepsviz. isothermal expansion, adiabaticexpansion, isothermal compressionand aidabatic compression. The efffi-ciency of Carnot engine is given by

where Q1 is heat energy absorbed from

the source maintained at a constant

high temperature T1°K and Q

2 is

amount of heat energy rejected to thesink at constant low temperature T

2°K.

A Refrigerator or a Heat pump absorbsheat Q

2 from a sink at lower tempera-

ture T2°K. The sink is the substance to

be cooled. Heat absorbed from the sinkis rejected to the source (i.e., surround-ing air) at high temperature T

1°K. Elec-

tric energy W has to be supplied for thispurpose Q

1 = Q

2 + W.

Coefficient of performance (C.O.P.) of a refrigerator is the ratio of the heatabsorbed from the sink (Q

2) to the elec-

tric energy supplied (W) for this pur-pose per cycle i.e.,

i.e.,

Transfer of Heat: Three modes of trans-fer of heat are : conduction; convec-tion; and radiation. Coefficient of ther-

mal conductivity (K) of a solid conduc-tor is calculated from the relation.

,

where A is area of hot face, ∆x is dis-tance between the hot and cold faces,

Cell Chemistry

In charged Lead Accumulator Alkali AccumulatorPositive Perforated lead Perforated steelelectrode plates coated with plate coated with

PbO2

Ni(OH)4

Negative Perforated lead Perforated steele lectrode plates coated with plate coated with Fe

pure leadElectrolyte Dil. H

2SO

420% solution of

KOH + 1% LiOH



Physics [135]

roundings provided the temperature dif-

ference is small (= 30°C). i.e., E α (T– T0)

ASTRONOMY

It is a branch of science that deals withthe study of position, composition, mo-tion and other related facts of heavenly bodies.

Windows of Astronomy : The visiblelight and radio waves coming from theheavenly bodies are two windows to

study about the sun, stars i.e., aboutthe universe, hence called windows of the astronomy.

Universe : The limitless expanse of space around us consisting of solar sys-tem, stars, galaxies etc. is called uni-verse.

Main constitutents of universe are: (i)Solar system (ii) Stars (iii) Galaxies.

Solar System : It is the name given tothe family of sun. Our solar system con-

sists of sun, nine planets (Mercury, Ve-nus, Earth, Mars, Jupiter, Saturn, Ura-nus, Neptune, Pluto) orbiting aroundthe sun, satellites (or moons) orbitingaround the planets, asteroids, cometsand meteroids.

Direction and time period of revolu-tion of a planet about its axis: Thesense of rotation of all the planets (exceptfor Venus and Uranus) is normally fromwest to east. For Venus and Uranus, the

sense of rotation is east to west and iscalled retrograde. Our earth rotates aboutits north axis to south axis from west toeast in 23 hrs. and 58.1 minutes.

Surface Temperature of a Planet : Itcan be estimated by two methods.

(i) Using Stefan’s law, i.e, E = s T4

∆Q is the small amount of heat con-

ducted in a small time (∆x), ∆T is differ-ence in temperature of hot and coldfaces.

Here (∆T/∆x) temperature gradient i.e.,rate of fall of temperature with dis-tance in the direction of flow of heat.

All liquids and gases are heated by con-vection. Heat comes to us from the sun by radiation.

Wien’s Displacement Law : It statesthat the wavelength (λ

m) correspond-

ing to which energy emitted by a per-fectly black body is maximum is in-versely proportional to the temperature

(T) of the black body i.e.,

where b is a constant of proportionalityand is called Wien’s constant. Its valueis b = 2.898 × 10–3 mK.

Stefan’s Law : It states that the totalamount of energy radiated per secondper unit area of a perfectly black bodyis directly proportional to the fourth

power of the absolute temperature of the body i.e., E ∝ T4 or E = σ T4 where σ

is called Stefan’s constant. Its value is σ= 5.67 × 10–8 watt m–2K–4

In case a perfectly black body at tem-perature T is placed in an enclosure attemperature T

0, then net amount of en-

ergy radiated per second per unit area bythe black body is given by E = σ (T4 - 4

0T )

If the body and enclosure are not per-fectly black and have relative emissiv-

ity ∈ we, may rewrite.

Newton’s Law of Cooling : It statesthat the rate of loss of heat of a liquid isdirectly proportional to difference intemperature of the liquid and the sur-




where E is the thermal energy emit-

ted per second per unit area of theplanet and T is its surface tempera-ture, s is the Stefan’s constant.

(ii) Using Wien’s Displacement law.

λ m

T = b.

where λ m is the wavelength correspond-

ing to maximum intensity of radiationemitted from the black body at tem-perature T and b is the Wien’s con-stant.

Existence of Atmosphere on a Planet :The existence of atmosphere on a planetcan be decided by two factors :

(i) Acceleration due to gravity; and

(ii) Surface temperature of the planet.

The planets like Mercury, Mars andPluto have large value of accelerationdue to gravity but they have high tem-perature also. With the increase in tem-perature, the average velocity of thegas molecules increases and becomes

more than their escape velocities. Dueto this, the gas molecules have escapedone by one and there is no atmosphere.

Chemical Composition of Atmosphere:The spectrum of the radiations emitted by a planet helps us to decide the na-ture of the gases present in the atmo-sphere of a planet. In the spectrum,dark lines correspond to particular ele-ments present in the atmosphere of thatplanet and intensity of the line tells

about the relative abundance of the ele-ment.

Atmosphere of earth consists of 88%nitrogen, 21% oxygen with somecarbondioxide and water vapour.

Basic conditions for the existence of lifeon a planet are :

(i) There should be suitable tempera-

ture range as required for the life toexist on a planet.

(ii) There should be proper atmospherefree from poisonous gases.

(iii) There should be plenty of water.

These conditions donot exist on anyother planet except our earth.Hence, the life is not possible onany other planet except earth.

Sun : It is the nearest star that we can

see. It is the heaviest body of our solarsystem. It is also known as yellow dwarf star.

(i) The composition of the material of the sun is 78% hydrogen, about 21%helium and remaining 2% all otherheavier elements from lithium touranium. The temperature at thecentre of the sun is about 14 × 106Kand on the surface is 6000 0K. Thedensity of the material of the sun atthe surface is about 10–4kg m–3 andat the centre is 104 kg m–3.

(ii) Surface energy of the sun is due tofusion process taking place insidethe sun, in which certain mass dis-appears and equivalent amount of energy is produced. The fusion re-action can be represented as

41H1 →

2He4 +

1e0 + 26.7 MeV

Stars : A star is a fiery luminous bodywhich emits light of its own. After sun

the next nearest star to earth is Alphacentauri. Its distance is 4.3 light years.

Brightness of a Star : The brightness of a star is expressed through the systemof magnitude. Whereas the observedmagnitude of a star is the measure of its brightness when observed from the



Physics [137]

earth and absolute magnitude of a star

is the measure of its brightness at 10per second distance.

Steller Spectra : The different stars areof different colours and the spectrumof a star is related to its colour. Thereare seven classes of stellar spectra de-noted by letters O,B, A, F, G, K and M.

Our sun belongs to G class star.

Birth of a Star : When the interstellardust and gas particles come closer toform a cloud due to gravitational forceof attraction between them, the birth of a star begins if the original cloud has amass of at least one thousand solarmasses. As the particles of cloud comecloser, the gravitational attraction in-creases. As the contraction of the cloudcontinues, the pressure and tempera-ture of the cloud continues, the pres-sure and temperature inside the cloudrise. At a certain stage, the cloud breaksup into large number of fragments of smaller size and each fragment contin-

ues to shrink and its temperature con-tinues to rise. When the temperature of a fragment reaches millions of degrees,nuclear fusion begins to take place dueto which the fragment begins to radiatelight energy from its surface and it be-comes a star.

Death of a Star : In each born star,during the fusion process, the hydrogenis converted into helium. When the con-tent of hydrogen of a star decreases to acertain proportion, the core begins tocontract while the outer regions expand.

As a result, the surface temperature of star drops and the star becomes giant orsupergiant star. At the end, this starproduces energy at a very high rate tosuch an extent that it explodes in theform of nova or supernova, throwing

out the major portion of its mass into

interstaller space. This is the death of astar. The core left behind may end inthree ways:

(i) If the original mass of a star is lessthan two times the mass of the sun,a white dwarf is produced. When itceases to emit light, it becomes blackdwarf.

(ii) If the original mass of a star is be-tween 2 to 5 times the mass of thesun, neutron star is produced.

(iii) If the original mass of a star is morethan 5 times the solar mass, the black hole is produced.

Milky Way or Akash Ganga : It is thename of the galaxy to which our earth belongs. The milky way is the glowing belt of the sky formed by the combinedlight of a very large number of stars. Itis called milky way or Akash Ganga because the light from the various starstogether gives the impression of a stream

of milk flowing across the sky.Milky way is a spiral galaxy. Its mass is150 solar masses.

Quasars : These are quasi-stellar Radiosources. Quasars are situated at verylarge distance from our solar systemand are further receding away with aspeed of 0.9 times the speed of light.

Pulsars : These are radio sources whichemit radio signals at extremely regularintervals of time. Their origin is due toneutron star.

Hubble’s Law : It states that the redshift (z) is directly proportional to thedistance (r) of the galaxy from us i.e., zα r but




where H is called Hubble’s constant.The quantity (l/H) has the dimensionsof time. Hubble’s law helps us to esti-mate the age and evolution of the uni-verse. The value of Hubble’s constant is16 Km-s-1 per million light year.

INFORMATION TECHNOLOGY

All modes of long distance communi-

cation have been superceded by com-munication through electrical signals,as they can be transmitted at extremelyhigh speeds (–3 × 108 m/s).

A basic communication system consistsof information source, transmitter, re-ceiver and the link between transmitterand receiver.

Modulation is the process of super-im-posing an audio frequency signal overa radio frequency carrier wave. Three

types of modulation are : amplitudemodulation (AM), frequency modula-tion (FM) and pulse modulation (PM).

In AM, amptitude of high frequencycarrier wave is made proportional tothe instanteneous amplitude of the au-dio frequency modulating voltage.

In FM, frequency of carrier wave isvaried in accordance with the in-stantaneous value of the modulat-ing voltage. However, the amplitudeof the carrier wave is kept constant.

Modulation index (m1) of FM wave is

defined as the ratio of maximum fre-quency deviation to the modulating fre-quency.

Frequency modulation is more advan-tageous compared to the amplitudemodulation.

Pulse modulation is a system in which

continous wave forms are sampled atregular intervals. Information regard-ing the signal is transmitted only at thesampling times together with any syn-chronizing pulses that may be required.

The analog pulse modulation is of twotypes : pulse amplitude modulation(PAM) and pulse time modulation(PTM).

The digital pulse modulation is also of two types : pulse code modulation(PCM) and pulse delta modulation

(PDM). Demodulation is the reverse process of

modulation which is performed in areceiver to recover the original modu-lating signal. Tuned frequency receiver(TRF) and superheterodyne receives arecommonly used for demodulation.

Data transmission is through a modu-lator and data retrieval is through ademodulator.

A modem is a modulating and demodu-

lating device that connects one com-puter to another across ordinary tele-phone line.

Fax or facsimile telegraphy is the elec-tronic reproduction of a document at adistance.

The term space communication refersto the sending, receiving and process-ing of information through space. Theinformation in the term of sound wavescan be sent from one place to another

place by superimposing it on undampedelectromagnetic waves of high fre-quency called carrier waves. The re-sulting wave so produced is calledmodulated wave which can travelthrough space; with the velocity of light.Our earth’s atmosphere helps in thepropagation of electromagnetic waves.



Physics [139]

The electromagnetic waves of frequency

ranging from a few kilo hertz to abouta few hundred mega hertz (i.e., wave-length 0.3 m or above) are called radiowaves.

The radiowaves emitted from a trans-mitter antenna can reach the receiverantenna by following any of the fol-lowing modes of propagation :

(i) Ground wave or surface wavepropagation.

(ii) Sky wave propagation.(iii) Space wave propagation.

The radiowaves which travel throughatmosphere following the surface of theearth are known as ground waves orsurface waves. The maximum range of ground or surface wave propagationdepends on (i) the frequency of the ra-dio waves; and (ii) power of the trans-mitter. The ground wave propagationis suitable for low and medium fre-

quency (i.e. upto 2MHz only). The radiowaves of frequency ranging

from 2 MHz to 30 MHz are called skywaves. They can propagate through at-mosphere and are reflected back by theionosphere layer of earth’s atmosphere.Since these waves travel through sky,their propagation is known as sky wavepropagation.

Criticial Frequency (CF) : It is thathighest frequency of radiowave which

when set straight (i.e., normally) to-wards the layer of ionosphere gets re-flected from ionosphere and returns tothe earth. If the frequency of radiowavesis more than the critical frequency, itwill not be reflected by ionosphere.

Maximum Usable Frequency (MUF) :

It is that highest frequency of radiowaves

which when sent at some angle towardsthe ionosphere gets reflected from thatand returns to the earth.

Skip Distance : It is the smallest dis-tance from a transmitter along theearth’s surface, at which a sky wave of a fixed frequency, but more than criti-cal frequency is sent back to the earth.

Fading : It is the variation in the strengthof a signal at a receiver due to interfer-ence of waves. Fading is more at high

frequencies.

Space Wave Propagation : The spacewaves are the radio waves of very highfrequency (i.e., between 30 MHz to 300MHz or more). The space waves cantravel through atmosphere from trans-mitter antenna to receiver antenna ei-ther directly or after reflection fromground in the earth’s troposphereregion. The space wave propagation isalso known as line of sight propagation.

The range of space wave propagationcan be increased by using repeaters atsuitable distances from each other on thesurface of the earth. The space wavepropagation is utilizing radio waves of very high frequency (between 30 MHzto 300 MHz), ultra high frequency andmicrowaves. They are used in televisionsignal propagation and microwavepropagation.

Satellite Communication : It is a mode

of communication of a signal betweentransmitter and receiver through satel-lite. The satellite communication is likethe line-of-sight microwave communi-cation. A geostationary satellite is usedfor satellite communication. The presentIndian communication satellites areINSAT 2B and INSAT 2C.




Remote Sensing : It is a technique which

is used to observe and measure the char-acteristics of the object with respect toits location, size, colour, nature, etc. Itis related with measuring some kind of energy which is emitted, transmitted orreflected from the object. Remote sens-ing systems provide us the data criticalto weather prediction, agriculture fore-casting, resource exploration and envi-ronmental monitoring. The Indian re-mote sensing satellites are IRS-1A, IRS1B and IRS-1C.

Most commonly used wire communicationlines are :

(i) Parallel wire line, (balanced lines)

(ii) Twisted pair wire lines; and

(iii) Co-axial wire lines (unbalancedlines).

Energy is dissipated in a transmissionline in three ways : Radiation losses,conductor heating & the dielectric heat-

ing. A co-axial cable system consists of a

tube carrying a number of co-axialcables together with repeaters andother ancilliary equipment.

A telephone link can be establishedusing ground waves, sky waves, mi-cro waves, co-axial cables and the lat-est optical fibre cables.

Optical Communication : It is a systemof communication by which we trans-fer the information over any distancethrough optical range of frequencies.The light frequencies used in fibre opti-cal system are in between 1014 Hz to 4 ×1014 Hz. (i.e. 100,000 to 400,000 GHz.).

Optical Fibre: It consists of a centralcore made of glass or plastic which is

surrounded by a cladding of material

of refractive index slightly less than thatof core and a protective jacket of insu-lating material. There are three types of optical fibre configurations:

(i) Single mode step-index fibre.

(ii) Multi-mode step-index fibre.

(iii) Multi-mode graded-index fibre.

Laser: Laser stands for light amplifica-tion by stimulated emission of radiation.The working of laser is based on stimu-

lated emission of radiation. The follow-ing types of laser are used:

(i) Gas lasers (ii) Liquid lasers (iii) Solidlasers (iv) Semiconductor lasers.

Light Modulation: It is achieved by twoways :

(i) Direct modulation (ii) Indirect or ex-ternal modulation. Direct modulationsuffers from system degradations butindirect or external modulation doesnot.

LIGHT

Light is a form of energy which pro-duces the sensation of sight.

Speed of light in vacuum/air = 3 × 108

ms–1

Luminous Body : A body which emitslight of its own. For example, the sun.

Non-luminous Body : A body which

does not emit light of its own but reflectthe light falling on it. For example, themoon.

Medium : Any substance through whichlight may or may not pass is called me-dium.

Transparent Medium : A medium



Physics [141]

through which light can pass is called

transparent medium.

For example : Air, glass, clean water.

Translucent Medium : A mediumthrough which only some parts of lightcan pass is called translucent medium.

For example : Greased paper, butter pa-per, ground glass, paraffin wax, etc.

Opaque Medium : A medium throughwhich light cannot pass .

For Examples : Wood, stone, brick, iron,etc.

Ray of Light : A line drawn in the direc-tion of propagation of light is called rayof light.

Reflection of Light : The phenomenonof returning of light in the same mediumafter striking a surface is called reflec-tion of light.

Types of Reflection :

(i) Regular reflection (ii) Irregular reflec-tion.

Reflector : A surface which reflects thelight falling on it is called a reflector or areflecting surface.

Laws of Reflection : The reflection of light from a surface obeys certain lawscalled laws of reflection.

(i) Incident angle is equal to the reflectedangle i.e. ∠i = ∠r.

(ii) Incident ray, reflected ray and nor-mal to the reflecting surface lie in thesame plane.

Lateral Inversion : The exchange of theright and left side of an object and itsimage is known as lateral inversion.

Number of images formed by two plane

mirrors inclined to each other at an angle

Periscope : A periscope is a device usedto see the objects from a lower level.

Concave Mirror : Concave mirror is apart of a hollow sphere whose outerpart is silvered and the inner part isreflecting surface.

Convex Mirror : Convex mirror is a partof a hollow sphere whose outer part is

reflecting surface and inner part is sil-vered.

Centre of Curvature : The centre of hol-low sphere of which the spherical mir-ror forms a part is called centre of curva-ture. It is denoted by C.

Radius of Curvature : The radius of hollow sphere of which the sphericalmirror forms a part is called radius of curvature. It is denoted by R.

Pole : The mid point of a spherical mir-ror is called its pole. It is denoted by P.

Aperture : The part of the spherical mir-ror exposed to the incident light is calledthe aperture of the mirror.

Principal Axis : A line joining the cen-tre of curvature (C) and pole (P) of aspherical mirror and extended on eitherside is called principal axis.

Principal Focus : A point on the princi-pal axis of a spherical mirror where the

rays of light parallel to the principal axismeet after reflection from the mirror iscalled principal focus. It is denoted by F.

Focal Plane: A plane normal or perpen-dicular to the principal axis and passingthrough the principal focus (F) of thespherical mirror is called focal plane.




Focal Length: The distance between the

pole (P) and principal focus (F) of thespherical mirror is called the focal lengthof the mirror.

Sign Conventions:

(1) All types of distance are measured fromthe pole of a spherical mirror.

(2) Distance measured in the direction of incident light are taken positive. Dis-tance measured in the direction oppo-site to that of the incident light are

taken as negative.(3) The upward distances perpendicular

to the principal axis are taken as posi-tive, while the downward distancesperpendicular to the principal axis aretaken as negative.

Mirror Formula: The relation betweenu, v, and focal length (f) of the sphericalmirror is known as mirror formula.

Linear Magnification: Linear magnifi-cation produced by a mirror is defined

as the ratio of the size (or height) of theimage to the size of the object. It is de-noted by m.

Refraction of Light : The change in thedirection of propagation of light when itgoes from one medium to another me-dium is called refraction of light.

Laws of Refraction :

(1) The incident ray, the refracted rayand the normal to the surface sepa-

rating the two media all lie in thesame plane.

(2) The ratio of the sine of the incidentangle to the sine of the refracted angleis constant for a pair of two media.

Refractive Index (µ) of a medium isdefined as the ratio of the velocity of

light in vacuum or air to the velocity of

light in the medium.

Principle of Reversibility of Light :

‘If the path of a ray of light is reversedafter suffering a number of refractionsand reflections, then it retraces its path’.This is known as principle of reversibilityof light.

Total Internal Reflection : The phe-nomenon of reflection of total light whenlight travelling in a denser medium fallson the surface separating the rarer me-dium and the denser medium at an anglegreater than the critical angle is calledtotal internal reflection.

Condition for Total Internal Reflec-tion :

Total internal reflection of light takesplace if

(1) the light travels from denser mediumto rarer medium.

(2) the angle of incidence is greaterthan the critical angle for the givenpair of denser and rarer media.

Lens : Lens is a transparent medium bounded by two refracting surfaces.

Convex Lens or Converging Lens : Alens having both spherical surfaces orone spherical and the other plane such

that it is thick in the middle and thin atthe edges is known as convex lens orconverging lens.

Concave Lens or Diverging Lens : Alens having both spherical surfaces orone spherical and other plane such thatit is thin in the middle and thick at the



Physics [143]

edges is known as concave lens or di-

verging lens.

Principal Axis : A line joining the cen-tre of curvature to two spherical sur-faces forming a lens is called principalaxis.

Optical Centre : A point in a lens throughwhich rays of light passes undeviated. It isdenoted by C.

Principal Focus : A point in the principalaxis where all rays of light parallel to the

principal axis meet or appear to meetafter passing through the lens is calledprincipal focus of the lens.

Focal Length : The distance between theprincipal focus and optical centre of alens is known as focal length of the lens.It is denoted by f.

Principal Foci : A beam of light may fallon either side of a lens. Therefore, a lenshas two principal foci. They are de-noted by F

1 and F

2. Both principal foci

are equidistant from the optical centre. First Principal Focus (F

1) : The position

of a point on the principal axis of a lensso that the rays of light starting from thispoint after passing through the lenstravel parallel to the principal axis iscalled first principal focus (F

1).

First Focal Plane : A vertical plane pass-ing through the first principal focus iscalled first focal plane.

First Principal Focal Length : The dis-tance between the optical centre andthe first principal focus is called firstprincipal focal length. It is denoted by f

1.

Second Principal Focus (F2) : The posi-

tion of a point on the principal axis of alens where a beam of light parallel to the

principal axis meets or appears to meet

after passing through the lens is calledsecond principal focus (F

2).

Second Focal Plane : A vertical planepassing through the second principal fo-cus is called second focal plane.

Second Principal Focal Length : Thedistance between the optical centre andthe second principal focus is called sec-ond principal focal length. It is denoted by f

2.

Sign Conventions :(1) All distances are measured from the

optical centre of a lens.

(2) Distance measured in the direction of the propagation of incident ray of lightare taken as positive, while distancemeasured in a direction opposite to thedirection of incident ray of light aretaken as negative.

(3) Upward height with respect to the prin-cipal axis are taken as positive, while

downward heights with respect to theprincipal axis are taken as negative.

Lens Formula : The relation betweenobject distance (u), image distance (v)and focal length (f) of a lens is called lensformula.

Power of a Lens is defined as the recip-rocal of the focal length of the lens. It isdenoted by P.

Unit of power of a lens is dioptre (D).

Dispersion : The phenomenon of split-ting white light into seven colours whenit passes through a glass prism is calleddispersion of white light.

Monochromatic Light : A light consist-ing of only one colour or only one wave-length is known as monochromatic light.




Polychromatic Light : A light consistingof more than two colours or more thantwo wavelengths is known as polychro-matic light.

For example, White light.

Spectrum : A band of seven colours of white light is known as spectrum.

Pure Spectrum : A spectrum in whichvarious colours of white light occupy dis-tinct positions without overlapping eachother on the screen is called pure spectrum.

Persistance of vision is about second.

Pigments : A substance which absorbsmost of the colours of white light andreflects one or more colours is called pig-ment.

Primary Colours : The colours whichare never obtained by mixing two ormore colours are called primary colours.

Example : Red, Blue and Green.

Secondary Colours : When any twoprimary colours are mixed, then a newcolour is formed. This new colour isknown as secondary colour. Thus, thecolour obtained by mixing two primarycolours are known as secondary colour.

Example : Yellow, Magenta and Cyan. Complimentary Colours : Any two

colours are said to be a pair of compli-mentary colours if these produce whitelight when mixed together.

Primary Pigments : Yellow, Cyan andMagenta are the examples of primarypigments.

Secondary Pigments : When a pair of primary pigments are mixed together,we get a new pigment known as sec-ondary pigment.

ELECTRICITY

Electric Potential : Work done per unitcharge is called electric potential.

Unit of Potential : S.I. unit of potentialis volt.

Potential is a scalar quantity.

Potential Difference : Potential differ-

ence between two points is defined asthe work done in moving a unit positivecharge from one point to another point.

Potential difference is a scalar quantity.

Equipotential Surface : A surface whoseevery point has the same potential isknown as equipotential surface.

Microscope & Telescope

Compound Microscope Astronomical Telescope1. It is used to observe tiny objects. It is used to observe large and distant

objects.2. The focal length of the objective The focal length of objective lens is

lens is small. large.

3. The focal length of the eyepiece is The focal length of eyepiece is small.large.

4. The second focal point of objective The second focal point of objectivelens and first focal point of eye- lens coincides with the first focal

piece are separated by the point of the eye-piece.

distance equal to tube lengthof the microscope.



Physics [145]

Electric Current : An electric current is

defined as the amount of charge flowingthrough any cross-section of a conduc-tor in unit time.

Units : S.I. unit of electric current isampere (A).

Ampere (A) : Electric current through aconductor is said to be 1 ampere if onecoulomb charge flows through any cross-section of the conductor in one second.

Electric current is a scalar quantity.

Ohm’s Law : This law states that, ‘theelectric current flowing in a conductor isdirectly proportional to the potential dif-ference across the ends of the conduc-tor, provided the temperature and otherphysical conditions of the conductor re-mains the same’.

Resistance (R) : Resistance of a conduc-tor is the ability of the conductor to op-pose the flow of charge through it.

Unit of resistance is ohm. Ohm : Resistance of a conductor is said

to be 1 Ohm if a potential difference of 1volt across the ends of the conductor pro-duces a current of 1 ampere through it.

Laws of Resistance :

(1) Resistance of a conductor dependsupon the nature of the material of the conductor.

(2) Resistance of a conductor is directly

proportional to the length of theconductor.

(3) Resistance of a conductor is inverselyproportional to each of crosssectionof the conductor.

(4) Resistivity of metallic conductor in-creases with the increase of tem-

perature and decreases with the de-

crease of the temperature.

Resistivity or Specific Resistance : Re-sistivity is defined as the resistance of the conductor of unit length and unitarea of cross-section.

Unit of Resistivity :

In CGS system, unit of resistivity is ohm-cm.

In SI system, unit of resistivity is ohm-metre.

Conductance : Conductance of a con-ductor is the ability to pass charge orcurrent through it.

Joule’s Law of Heating :

The amount of heat produced in a con-ductor is

(i) directly proportional to the squareof the electric current flowingthrough it.

(ii) directly proportional to the resis-tance of the conductor.

(iii) directly proportional to the time forwhich the electric current flowsthrough the conductor.

Electric Fuse : Electric fuse is a safetydevice used in an electric circuit or elec-tric appliance.

Electric Energy : The work done by asource of electricity to maintain a cur-rent in an electric circuit is known as

electric energy.

Electric Power : Electric power is de-fined as the amount of electric work donein one second.

Unit of Power: S.I. unit of power iswatt.




Practical unit of power is horse power

(h.p.)

1 h.p. = 746 w

Commercial Unit of Energy : kilowatt-hour (kWH)

1 kWh = 3.6 × 106 J

Electrolysis : The process by which asubstance is decomposed by the passageof electric current through it is calledelectrolysis.

Electrolytes : Substances which in mol-ten state or in aqueous solution conductelectricity by decomposing into negativeand positive ions are called electrolytes.

Strong Electrolytes : Substances whichin molten state or in aqueous solutioncompletely decompose into negative andpositive ions are called strong electro-lytes.

Weak Electrolytes : Substances whichin molten state or aqueous solution par-

tially decompose into negative and posi-tive ions are called weak electrolytes.

Mercury is a liquid which conducts elec-tricity without decomposing into ions.

Electrodes : The conducting rods orplates through which an electric currententers or leaves the electrolyte are calledelectrodes.

Anode : The positive electrode is calledanode.

Cathode : The negative electrode is calledcathode.

Ion : Atom which has either lost orgained electrons is called ion.

Anion : When an atom gains electron orelectrons, it becomes negatively chargedion known as anion.

Cation : When an atom loses electron or

electrons, it becomes positively charged ionknown as cation.

Electrolytic Cell or Voltameter : A ves-sel containing electrolyte and electrodesin which electrolysis takes place is calledvoltameter.

When an electrolyte in a solid state isdissolved in water, it decomposes intoions as the force of attraction between

ions decrease by a factor of in water.

Faraday’s laws of Electrolysis :

1st Law : The mass of any substanceliberated at an electrode is directly pro-portional to the charge flowing throughthe electrolyte.

i.e., m ∝ q or m = Zq or m = Zlt

2nd Law : If same amount of chargeflows through different electrolytes, themasses of substances liberated or depos-ited at electrodes are directly propor-

tional to their chemical equivalents.

Electro-chemical Equivalent (Z) is de-fined as the mass of the substance liber-ated at an electrode per unit charge flow-ing through the electrolyte.

Unit of Z is kg/coulomb or g/coulomb.

Chemical equivalent

Electroplating : The process of deposit-ing a layer of superior metal over an-other cheaper metal or article by elec-trolysis is called electroplating.

Electro-chemical Cell : A device inwhich a constant potential difference ismaintained between two electrodes by a



Physics [147]

chemical reaction is called electro chemi-

cal cell.

E.M.F. of a Cell is the potential differ-ence between two electrodes when nocurrent is drawn from the cell.

Terminal Voltage of a Cell is the po-tential difference between two electrodeswhen current is drawn from the cell.

Internal Resistance of a Cell is the re-sistance offered by a cell to the flow of current through it.

Types of Electrochemical Cells: (i) Pri-mary cells (ii) Secondary cells.

Primary Cell converts chemical energyinto electrical energy by irreversiblechemical reactions.

Secondary Cell : A cell in which electri-cal energy is stored in the form of chemi-cal energy and then chemical energy isobtained in the form of electrical energy by reversible chemical reactions is calleda secondary cell.

Internal resistance of a primary cell ishigher than that of a secondary cell.

Voltaic Cell is primary cell in whichdilute H

2SO

4 is used as electrolyte. Zinc

and copper plates are used as cathodeand anode.

Voltaic cell suffers from two defects: (i)local action (ii) polarization

The value of e.m.f. of voltaic cell = 1.08V.

Polarization is the process of formationof hydrogen bubbles on the electrode of a cell, causing the e.m.f. of the cell todrop and internal resistance of the cellto increase.

Polarization can be removed by a sub-stance known as depolarizer, MnO

2 and

CuSO4 are the examples of depolarisers.

Daniel Cell : Electrolyte used is dilutesulphuric acid. Copper sulphate (CuSO

4)

acts as depolarizer E.M.F. of Daniel cell= 1.5V.

Leclanche Cell : Electrolyte used is am-monium chloride solution, MnO

2 acts as

depolarizer, E.M.F. of Leclanche cell =1.5V.

Dry cell is the modified and portableform of a Leclanche cell.

Primary cells cannot be re-charged whilesecondary cells can be re-charged.

Magnetic Field : The space around acurrent carrying conductor in which itsmagnetic effect can be experienced iscalled magnetic field.

Maxwell’s Cork Screw Rule : If a corkscrew is driven in the direction of cur-rent passing through the wire, then themotion of the thumb shows the directionof the magnetic field.

Right Hand Thumb Rule : If a currentcarrying conductor is imagined to be heldin the right hand such that thumb pointsin the direction of the current, then thefingers of the hand indicate the directionof magnetic field.

Solenoid: A solenoid is a coil of manyturns of an insulated wire closely woundin the shape of a cylinder.

Uniform Magnetic Field : Magnetic field

is said to be uniform if its magnitude isequal and direction is same at every pointin the space.

Electromagnets : When a soft iron bar isplaced inside a solenoid carrying cur-rent, it becomes a magnet as long ascurrent passes through the solenoid.




Such magnets are known as electromag-

nets.

Electromagnetic Induction : Whenevermagnetic flux linked with a circuitchanges, an induced e.m.f. is set up acrossthe circuit. This effect is known as elec-tromagnetic induction.

Faraday’s Law of Electromagnetic In-duction :

First Law : Whenever magnetic fluxlinked with a circuit changes, induced

e.m.f. is produced across it. This inducede.m.f. last so long as the change in mag-netic flux continues.

Second Law : The magnitude of theinduced e.m.f. produced in the circuit isdirectly proportional to the rate of changeof magnetic flux linked with it.

Lenz’s law of Electromagnetic Induc-tion : According to this law, the direc-tion of induced current is always suchthat it opposes the cause producing it.

Lenz’s law is in accordance with thelaw of conservation of energy.

Electric Motor : Electric motor convertselectric energy into mechanical energy.

D.C. Generator or Dynamo : A devicewhich converts mechanical energy intodirect current energy is called D.C. gen-erator or Dynamo.

Direct Current : An electric currentwhose magnitude is either constant or

variable but the direction of flow in aconductor remains the same is calleddirect current. It is donated by D.C.

Alternating Current : An electric cur-rent whose magnitude changes with timeand direction reverses periodically iscalled alternating current. It is denoted by A.C.

Joule’s Law of Heating : It states that

the amount of heat produced in a con-ductor is directly proportional to the-

(i) square of the current flowingthrough the conductor.

(ii) resistance of the conductor and

(iii) time for which the current is passed.

Electric Power: It is defined as the rateat which work is done in maintainingthe current in electric circuit.

Electric power, P = VI = I2R = V2/Rwatt or joule/second-

Electric Energy : The electric energyconsumed in a circuit is defined as thetotal work done in maintaining the cur-rent in an electric circuit for a giventime.

Electric energy = Vlt = Pt = I2 Rt = V2t/R

S.I. unit of electric energy is joule (de-noted by J) where 1 joule = 1 watt × 1

second = 1 volt × 1 ampere × 1 second

Commercial unit of electric energy iskilowatt hour (kWh)

where 1kWH = 1000 Wh = 3.6 × 106 J

Electrolysis : The process of decompo-sition of a solution into ions on passingthe current through it is called elec-trolysis.

Electrolyte : It is a substance whichallows the current to pass through it

and also decomposes into positive andnegative ions. For example, acids, bases,salts dissolved in water, alochol are com-mon electroytes. Agl is a solid stateelectroyte and KCl, NaCl are electro-lytes in their molten state.

Electrodes : These are the two metal



Physics [149]

plates which are partially dipped in

the solution for passing the currentthrough the electroyte.

Anode : The electrode connected to thepositive terminal of the battery i.e., theelectrode at higher potential, is calledanode.

Cathode : The electrode connected tothe negative terminal of the battery i.e.,the electrode at lower potential, is calledcathode.

Ions : The charged constitutents of theelectrolyte which are liberated on pass-ing current are called ions.

Anions : The ions which carry negativecharge and move towards the anodeduring electrolysis are called anions.

Cations : The ions which carry positivecharge and move towards the cathodeduring electrolysis are called cations.

Voltameter : The vessel, containing elec-trodes and electrolyte, in which the elec-

trolysis is carried out is called a voltame-ter. In copper voltameter the electrodesare of copper plates and electrolyte is anaqueous solution of CuSO4. During elec-trolysis copper is removed from the an-ode and is deposited at the cathode. Inwater voltameter the electrodes are of pure platinum. The electrolyte is acidu-lated water i.e., water mixed with a smallquantity of sulphuric acid. During elec-trolysis hydrogen is liberated at cathodeand oxygen at anode. The hydrogen col-lected at cathode is double the amountthan the oxygen collected at anode.

Faraday’s Laws of Electrolysis.

First Law : The mass of the substanceliberated or deposited at an electrodeduring electrolysis is directly propor-

tional to the quantity of charge passed

through the electrolyte i.e.,

m ∝ q

or, m = zq

where z is the E.C.E. of the substance.

Second Law : When same amount of charge is made to pass through any number of electrolytes, the masses of the sub-stances liberated or deposited at the elec-trodes are proportional to their chemical

equivalents i.e., where m1, m2 arethe masses of the substances liberated ordeposited on electrodes during electroly-sis and E

1 and E

2 are their chemical

equivalents.

Faraday’s second law of electrolysis alsostates that E.C.E. of a substance is di-rectly proportional to the chemicalequivalent i.e., z ∝ E.

Faraday constant

Thus Faraday constant is equal to theamount of charge required to liberatethe mass of a substance at an electrodeduring electrolysis, equal to its chemi-cal equivalent in grams.

One Faraday = 1F = 96500 C/gmequivalent.

Electrical Cell : It is a device by whichelectric current is generated due tochemical action taking place in it. Elec-

tric cells are of two types: (i) Primarycells (ii) Secondary cells.

Primary cell is that cell in which electri-cal energy is produced due to chemicalreaction which is irreversible. For ex-ample : Voltaic cell, Daniel cell,Leclanche cell, dry cell, etc.




Secondary cell is that cell in which the

electrical energy is first stored up as chemi-cal energy and when current is drawnfrom the cell, the stored chemical energyis converted intro electrical energy. Thechemical reaction is reversible in this cell.These cells are also known as storagecells or accumulators. Examples are Acidor Lead accumulator and Alkali or Edisoncell.

Seebeck Effect : It is the phenomenonof generation of an electric current in a

thermocouple by keeping its two junc-tions at different temperatures.

Seebeck found that the magnitude anddirection of thermo e.m.f. developed ina thermocouple depends upon: (i) thenature of metals forming a thermo-couple; and (ii) difference intemeprature of the two junctions.Seeback effect is a reversible effect. Itmeans, if hot and cold junctions areinter-changed, the direction of thermo-electric current is reversed.

Thermocouple : The assembly of twodifferent metals joined at their ends tohave two junctions in a circuit is calleda thermocouple.

Seeback Series : Seeback, from his ex-perimental investigations, arranged anumber of metals in a series known asSeeback series. Some of the metals, inthis series, in the order Seeback arrangedthem are bismuth, nickel, platinum, sil-

ver, gold, copper, lead, zinc, iron andantimony.

Direction of thermo-electric current incopper-iron thermocouple is from cop-per to iron through hot junction. In Sb-Bi, thermocouple, the direction of thermo-electric current is from Sb to Bithrough cold junction.

Neutral Temperature : It is that tem-

perature of hot junction for which thethermo e.m.f produced in a thermo-couple is maximum. Neutral tempera-ture depends upon the nature of thematerial of a thermocouple but is inde-pendent of the temperature of the cold junction.

Temperature of Inversion : It is thattemperature of hot junction at whichthe thermo e.m.f. produced in a thermocouple becomes zero and just beyond, it

reverses its direction. The value of tem-perature of inversion depends upon: (i)the temperature of the cold junction;and (ii) the nature of materials forminga thermocouple.

Sources of energy : Renewable sourcesand Non-renewable sources.

Solar energy is the energy emitted bythe sun in the form of heat and light.

Visible light consists of seven coloursi.e., Red, Orange, Yellow, Green, Blue,Indigo, Violet.

Wavelength range of visible light is from0.4 micron to 0.7 micron.

Red light has the longest wavelengthand violet light has the shortest wave-length.

Infra-red rays are emitted by all hot bod-ies.

Infra-red rays are absorbed by CO2 and

water vapours in the atmosphere of theearth.

Ultra-violet rays are harmful radiationsand cause diseases like cancer and leu-kemia.

Ultra-violet rays are absorbed by ozonelayer of atmosphere.



Physics [151]

Light is a stream of packets of energy

called photons.

Energy of each photon, E = hv

Solar energy is harnessed by solar de-vices like solar cooker, solar furnance,solar cell, etc.

Nuclear fusion reactions are the sourceof energy of the Sun.

Nuclear fusion reactions occur at about107°C.

German physicist Hans Bethe in 1939,predicted that nuclear fusion of hydro-gen nuclei in the Sun is the source of energy of the Sun.

Atom consists of three particles namelyproton (

1H1), neutron (

on1) and electron

(–1

e0).

Nucleus of an atom contains protonsand neutrons and hence is positivelycharged.

Electrons move around the nucleus in cir-cular orbits.

Number of protons in the nucleus of anatom is equal to the number of electronsrevolving around it, so atom is electri-cally neutral.

Mass of electron = 9.1 × 10–31 kg. Mass of proton = 1.672 × 10–27 kg.

Mass of neutron = 1.674 × 10–27 kg.

Charge on an electron = –1.6 × 10–19C.

Charge on a proton = + 1.6 × 10–19C.

Protons and Neutrons inside the nucleusare collectively known as nucleons.

Atomic Number (Z) of an atom is equalto the number of protons in the nucleusof the atom.

Mass Number (A) of an atom is the sum

of the number of protons and neutronsin the nucleus of the atom i.e. A = Z + N.

Isotopes : The atoms of an element hav-ing same atomic number (Z) but differ-ent mass numbers (A) are known as iso-topes of the element.

All isotopes of an element have the samechemical properties.

Radioactivity is the phenomenon of spon-taneous emission of invisible radiations

by heavy elements. Radioactivity was discovered by Henri

Becquerel.

The radiations emitted by radioactiveelements are alpha particles (

2He4), beta

particles (–1

e0) and gamma rays (γ ). Theseradiations are very harmful to the livingorganisms.

Gamma rays have the highest penetrat-ing power than the other two radia-

tions. Radio Isotopes are the isotopes of radio-

active elements.

Nuclear forces keep the nucleons insidethe nucleus. Small nuclei are stable and big nuclei are unstable.

Nuclear fission is the process of splittinga heavy nucleus into two small nuclei of middle weight with the release of largeamount of energy.

Uranium-235, isotope of Uranium is usedas a fuel in nuclear fission.

Energy released in a single nuclear fis-sion of U-235 is about 200 MeV.

Uncontrolled chain reaction is the prin-ciple of atom bomb.



Nuclear reactor is a device to carry out

controlled chain reaction.

Nuclear fission reaction starts when U-235 absorbs a slow neutron.

Moderator is a substance used to slowdown fast moving neutrons. Heavy wa-

ter and graphite are the examples of

moderator.

Cadmium rods are used to increase ordecrease the nuclear fission reactions inthe nuclear reactor.

physics capsule

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