physics 10th class notes

NORTH HILLS SCHOOL AND COLLEGESenior Boy’s Section (Branch: 2)

PHYSICS NOTES 10th CLASS

Written by: Zahoor Ahmad M.Sc. Physics Islamia University Peshawar

Reviewed by: Shahid Khan M.Sc. PhysicsUniversity of Peshawar

“Education is not the learning of facts, but the training of mind to think.”

Charsadda Road Near Faqir Abad Police Station PeshawarContact NO: 0300-9176094

Simple Harmonic Motion and Waves

Motion:

A body is said to be in motion if it changes its position with respect to its surrounding.

For example; blowing wind, rotating fan, vibrating pendulum, the motion of a car on a straight road etc.

Types of motion:

There are three types of motion which are as follows.

1. Translatory motion2. Rotatory motion3. Oscillatory or Vibratory motion/ Oscillation

Oscillation /Oscillatory motion :

A body is said to be in oscillation when it performs to and fro motion about its mean position.

For example:

1. The motion of simple pendulum2. The motion of mass-spring system3. The motion of steel strip fixed in a block.

Periodic motion:

Any motion that repeats itself in equal interval of time is called periodic motion.

For example:

1. The second hand of a watch repeats its motion on the dial in every one minute.2. The earth completes its periodic rotation in every twenty four hours.3. The motion of simple pendulum.4. The motion of mass- spring system.

The simplest periodic vibratory motion is called simple harmonic motion.

Simple harmonic motion:

Simple harmonic motion is the type of vibratory motion in which the acceleration produced is directly proportional to the displacement from the equilibrium position and is always directed towards the equilibrium position.

Mathematically:

a ∝ − x

1. Motion of mass attached to spring:

Consider a body of mass ‘m’ attached with one end of the spring placed on a smooth and horizontal frictionless surface while its other end is attached to a fixed support as shown in the figure.

If an external force “F” is applied on the body and it is

displaced from its mean position “O” towards extreme position


“A” by covering certain displacement “x”.

According to Hooke’s law we have

F∝ x⇒ F = k x

Where ‘k’ is constant of proportionality which is known as spring’s constant. After removing the external force, the mass moves back towards the equilibrium position ‘O’ under the action of

restoring force“F res ”, but does not stop at point ‘O’ due to inertia and goes to the new extreme

position ‘ A¿

’. The restoring force is equal in magnitude to the external force but opposite in direction.

F res = − F⇒F res = − k x−−−−−1

Also we know from Newton’s second law

F = m a−−−−−2

Putting equation (2) in equation (1). We get

ma = − k x

⇒ a= −km

x

⇒ a = constant (−x ) ∴ km

= constant

⇒ a ∝ − x−−−−−−3

Equation (3) shows that the mass attached to an elastic spring execute simple harmonic motion. The time period of mass spring system is given by

T = 2 π √ mk

SOME IMPORTANT TERMS:

1. Vibration: One complete round trip covered by a body is known as one vibration. In the given figure it is

the motion of mass ‘m’ from O to A then from A to A’ and then back to O.2. Time period(T): The time required to complete one vibration or oscillation is known as time

period. Time period is measured in seconds.3. Frequency (f): The number of vibrations executed by a vibrating body in one second is known

as frequency.

f = No of vibrations

t In SI unit, the unit of frequency is Hertz (Hz) or Vibs -1. The frequency ‘f’ and time period T are related by the equation.

f = 1

T Frequency is the reciprocal of time period.


4. Displacement: The displacement covered by a vibrating body at any instant is the distance from the equilibrium position at that instant. It is denoted by ‘x’. Its unit is meter (m).

5. Amplitude: The maximum displacement covered by a vibrating body from either side of the equilibrium position is known as amplitude. It is denoted by ‘xm’.

Simple pendulum:

A simple pendulum consists of a small mass (bob) suspended by a weightless and inextensible string (thread) from a fixed support as shown in the figure.

If we displace the bob from its mean position ‘O’ to extreme position ‘A’ through a small displacement ‘x’ and release it. It will move towards ‘O’ under the action of gravity. There are two forces acting on the bob one its weight ‘W’ acting downward and second tension ‘T’ in the string acting upward.

The weight force can be resolved into two rectangular components.

The component mg cosθ balances the tension in the string while the

component mg sinθ is always directed towards the mean position ‘O’,

which produces oscillations of pendulum.

When the bob is released from point ‘A’ the effective component of weight produces acceleration towards the mean position ‘O’. On the way from ‘A’ to ‘O’, the acceleration decreases gradually and becomes equal to zero at ‘O’. The bob does not stop at point ‘O’ due to inertia and moves towards ‘A/’ with decreasing velocity and increasing acceleration. This process is repeated again and again and the bob continues to oscillate between ‘A’ and ‘A/’.

From above discussion it is clear that in simple pendulum the acceleration is directly proportional to the displacement and always directed towards the mean position. Therefore the motion of simple pendulum is simple harmonic motion.

Mathematical form:

Net force acting on the bob.

Fnet = W x sin θFnet = −m g sinθ−−−−−−1 ∴ W x = m g

As we know from Newton’s second law Fnet = m a

m a =− m g sin θ⇒ a = − g sin θ−−−−−2

From the figure we have


sin θ = xℓ−−−−−−−3

Putting equation 3 in equation 2 we get

a = − gxℓ

⇒ a = gℓ

(−x )

⇒ a = constant (−x )⇒ a ∝ −x−−−−−−−−4

Characteristics of simple harmonic motion:

1. In simple harmonic motion, a body oscillates about the mean position O.2. A restoring force Fres acting on the body is always directed towards the mean position O.3. The acceleration produced due to the restoring force Fres is directly proportional to the

displacement and always directed towards the mean position O.4. The velocity of the body is maximum at the mean position O and gradually decreases as the

body moves away from O. At the extreme positions ‘A’ and ‘A/’ the velocity of SHO becomes zero.

5. At the mean position, the K.E of SHO is maximum while its P.E is equal to zero.6. At the extreme positions ‘A’ and ‘A/’, the P.E of SHO is maximum while its K.E is equal to zero.7. In between mean position and extreme position the energy will be partially kinetic and

partially potential. The total energy remains constant during the motion.

Damped vibration/ damped oscillation:

The type of vibration/ oscillation in which the amplitude gradually decreases with time is called damped vibration/ damped oscillation.

For example:

When simple pendulum is disturbed from its mean position and left alone, the amplitude of vibration decreases with time and eventually stops due to the air resistance. These oscillations are called damped oscillations.

Wave:

The disturbance that travels through matter and space and transfer energy from one place to another is known as wave.

Wave motion:

The transmission of energy in a medium due to the oscillatory motion of the partials of the medium about the mean positions is called wave motion.

Propagation of waves:


Whenever a body vibrates, it does work on the particles of the medium and sets them into oscillatory motion. These particles set the neighboring particles into motion by transferring them some of its energy. In this way the disturbance travels through the medium but particles of the medium vibrates locally and does not move with the wave.

Characteristics wave parameters:

The qualitative characteristics such as wavelength, amplitude, velocity and frequency of the waves are discussed below.

1. Wavelength: The distance between two consecutive crests or trough is called wavelength and is denoted by λ. It is measured in meters.

2. Amplitude: The maximum displacement of the particles of the medium from its original position is called amplitude of the wave.

3. Frequency: The number of waves passing through a fixed point in one second is called frequency. It is denoted by ‘f’. Its unit in SI is hertz denoted by Hz.

4. Time period: The time required to complete one cycle by a wave is known as time period. It is denoted by ‘T’. Its unit in SI is second.

5. Velocity of wave: The distance travelled by a wave in unit time is called velocity of wave. It is represented by ‘V’Mathematically:

velocity =distance travelledtime

⇒ v = st

Where‘s’ is the distance travelled by the wave and ‘t’ is the time taken.

Relationship between the velocity, frequency and wavelength of a wave:

As we know from the velocity of the wave

velocity =distance travelledtime

v = st

We also know that a wave travels a distance of ‘λ’ in time period ‘T’. Therefore the above equation becomes.

v = λ

T= 1

T× λ

We know that frequency is the reciprocal of time period. i.e f = 1

T putting in above equation we get

v = f λ


From above equation it is clear the velocity of a wave is the product of frequency and wavelength.

Wave:

The disturbance that travels through matter and space and transfer energy from one place to another is known as wave.

Wave motion:

The transmission of energy in a medium due to the oscillatory motion of the particles of the medium about their mean position is called wave motion.

The waves which require a material medium for their propagation are called mechanical waves.

For example: The waves produced in water, waves produced in stretched string, sound waves etc.

The waves which do not require a material medium for their propagation are called electromagnetic waves.

For example: Radio waves, light waves, heat waves, x- rays, television waves etc.

Modes of transfer of energy:

The energy can be transferred from one place to another by the following three methods.

1. Motion of a macroscopic body 2. Particle transmission 3. Wave motion

1. Motion of a microscopic body: In this method, energy is transferred from one place to another due to the complete motion of macroscopic bodies from one place to another.For example: energy carried by moving truck, energy carried by a projected stone, energy carried by winds, a bullet fired , water fall, ocean tides etc.

2. Particle transmission: In this method, energy is transferred due to collision or motion of microscopic particles. For example: transfer of electric energy, conduction of heat through a metal bar etc.

3. Wave motion: In this method, energy is transferred from one place to another by means of waves.For example: sound energy coming from a bell, light energy and heat energy coming from the sun. The particles of the medium do not move from one place to another but they only vibrate locally.

Mechanical waves:

The waves which require a material medium for their propagation are called mechanical waves.

For example: The waves produced in water, waves produced in stretched string, sound waves etc.

Types of mechanical waves:

There are two types of mechanical waves based on the motion of particle of the medium with the direction of propagation of wave.


1. Transverse waves 2. Longitudinal waves1. Transverse waves:

The waves in which the vibration of the individual particles of the medium is perpendicular to the direction of propagation of waves are called transverse waves.Explanation: In transverse waves, the particles oscillate at right angles to the direction of propagation of wave for example; the waves produced in a stretched string up and down are transverse waves because the different parts of the string vibrate perpendicular to the direction of propagation of waves. The wave pattern of transverse waves is shown in the figure.

Similarly the waves produced in a slinky spring, whose one side is attached to a fixed support and the other side is moved up and down. Transverse waves are produced.

The part of the wave where particles are above the mean position is called crest while the part of the wave where the particles of the medium are below the mean position is called trough.

2. Longitudinal waves: The waves in which the particles of the medium vibrate about their mean position parallel to the direction of propagation of waves are called compressional waves.Explanation: Sound waves are compressional waves, in which the air molecules are displaced forward and backward on the same axis along which the sound travels. Similarly compressional waves are also produced in the spring as shown in the figure.

In compressional waves, the denser part of the wave is called compression while the less dense part is called rare friction.

Difference between transverse waves and longitudinal waves:

Difference between transverse waves and longitudinal waves

Transverse waves Longitudinal waves

1 In transverse waves, the particles of the medium vibrate perpendicular to the direction of propagation of wave.

In longitudinal waves, the particles of the medium vibrate parallel to the direction of propagation of wave.


2 The transverse waves consist of crests and troughs

The longitudinal waves consist of compression and rare friction.

3 The distance between two crests or two troughs is called wavelength.

The distance between two compressions or two rare frictions is called wavelength.

4 Waves produced in water are the examples of transverse waves

Sound waves are the example of compressional waves

Ripple tank:

A ripple tank is a device with the help of which we study various properties of water waves.

Construction:

A ripple tank consists of a rectangular tray containing water fitted with a transparent glass at the bottom.

A light bulb is fixed above the tray while the image of the waves can be seen on a viewing screen placed at the bottom of the tray. A mechanical motor disturbs the water in different ways and thus waves are produced.

Working:

Various kinds of waves can be produced by disturbing water in different manner. When the light shines through the water. The crests in the water appear as bright band while the trough appears as dark band on the viewing screen. The wave properties can be demonstrated as under.

Properties of waves:

1. Reflection of waves:

The bouncing back of a wave from a certain surface is called refection of waves.

In ripple tank, reflection of waves can be demonstrated by placing an up-right surface in water as shown in the figure.

2. Refraction of waves:

The change in path of the waves, when they enter from one medium to another medium is called refraction of waves.


The refraction of waves can be demonstrated in a ripple tank by placing a plastic sheet in the bottom portion of the tray. We shall observe that when the water waves enter from the shallow water into deeper water they slightly change their path as shown in the figure. The speed of the waves is greater in deeper water than in shallow water.

3. Diffraction of waves: The bending of waves around the edge of an obstacle ORSpreading of waves after passing through a small opening is called diffraction of waves.When the waves pass through an opening whose size is smaller than the wavelength of the wave then they spread out this spreading is called diffraction of waves. In ripple tank diffraction of waves can be demonstrated by placing two straight rods with a small opening between them. When the waves pass through the opening they diffract around the corners of the opening.

4. Interference of waves: When the two waves are passing through the same point simultaneously, then the combined effect of the two waves in the overlapping region is called interference of waves.a. Constructive interferenceb. Destructive interference

CONCEPTUAL QUESTIONS

Q.1 the mass attached to a vibrating spring is increased four times. What is the affect on the time period and frequency of oscillation of mass spring system?

Ans. As we know that the time period and frequency of mass spring system is given by;

T = 2 π √ m

k f = 1

2 π √ km

Now when the mass is increased four times, then we replace ‘m’ by ‘4m’ in equation 1 and equation 2 so we get.


⇒T ¿ = 2 π √4mk

⇒ T¿ = 2 (2 π √mk )

⇒T ¿ = 2 T−−−−−1

⇒ f ¿ = 12 π √k

4 m

⇒ f ¿ = 12 ( 1

2 π √km )

⇒ f ¿ = 12

f −−−−−−2

Equations 1 and 2 shows that if the mass becomes four times, then the time period of mass spring system become doubled while its frequency becomes halved.

Q.2. A wire hangs from a dark high tower so that its upper end is not visible. How can be the length of wire be determined?

Ans. We will attach a mass to the lower end of the wire, so that the arrangement becomes like a simple pendulum.

Now we know that the time period of simple pendulum is given by,

T = 2π √ ℓ

g−−−−−−1

Squaring both sides of equation 1 ,we get,

⇒T 2 = (2 π √ℓg )

2

⇒T 2 = 4 π2 (√ℓg )

2

⇒T 2 = 4 π2 ℓg

⇒T 2 g = 4 π2 ℓ

⇒g T 2

4 π2= ℓ

⇒ ℓ = g T 2

4 π2−−−−−−−−2

Now we will set the pendulum into vibration and note the time for one vibration which gives its time period ‘T’. Putting the values of T, g and π in equation 2 we can calculate the length of the rope.

Q. 3. Will the period of a vibrating swing increases, decreases or remains constant by addition of more weight?

Ans.The vibrating swing can be considered as simple pendulum, whose time period is given by equation.

T = 2π √ ℓ

g−−−−−−−−1


From above equation it is clear that the time period of simple pendulum is independent on mass or weight. Therefore the period of vibrating swing remains constant by addition of more weight or mass.

Q.4. Water waves move from the shallow end of a pool to the deeper end. State the changes to the wavelength and speed of the wave?

Ans. The speed of wave ‘v’ , its wavelength ‘ λ’ and its frequency ‘f’ are related by the equation i.e

v = f λ−−−−−−1

We also know that the speed of waves in deeper water is greater than the speed of the waves in shallow water. Then according to equation 1 the wavelength will also increases because its frequency is remains unchanged and is source dependent.

Thus when water waves move from the shallow end of a pool to the deeper end, the speed and the wavelength of the wave increases.

Q.6: What is the K.E of a simple pendulum when the bob is at,(i)- Mean position (ii)- Extreme positionAns: (i)- The simple pendulum is shown in the figure point “O” represent the main position of a simple pendulum while point “A”.”A” represents the extreme position of the simple pendulum.

At main position, the velocity of simple pendulum is maximum due to restoring force during oscillation. As we know that, K.E=1/2 mv2, so due to maximum velocity, the K.E will be also maximum at mean position during oscillation.

(ii)- At extreme position, the simple pendulum comes to rest momentarily and then move back towards mean position due to restoring force. Thus the velocity of simple pendulum is zero at extreme position. So the K.E of simple pendulum will be zero at extreme position.

Q.8: The diagram shown in the figure, a wave moving into shallower water. Why the wave length of the wave is reduced?

Ans: We know that the speed of wave in deep water is greater that the speed of wave in shallow water.

We know that the relation b/w speed of wave length is give by,

v = f λ−−−−−−1

Now when a wave enters into shallow water from deep water, its speed will be decrease.

According to eq (1), if speed of a wave decreases, than its wave length “λ ” will also decrease.

Q.9: A dipper moving up and down makes wave in ripple tank. What will happen when the dipper frequency is increased?

Ans: As we know that,

v = f λ

⇒ f = vλ−−−−−1

If v= constant then equation 1 becomes


f = constant1λ

⇒ f ∝ 1λ

−−−−−−2

Relation 2 shows that frequency of wave is inversely proportional to the wavelength for constant velocity i.e greater the frequency of the waves, then smaller will be its wavelength and vice versa.

Now if the frequency of the dipper in ripple tank increases, then the frequency of the generated waves also increases. As a result the wavelength of the generated waves decreases.

Written and composed by

Zahoor Ahmad

M.Sc Physics


SOUND

Sound:

The sensation felt by our ear is called sound.

OR

Sound is a form of energy that produces the sensation of hearing in ear.

Explanation:

Sound is a form of energy which travels through a medium in the form of longitudinal waves and produce the sensation of hearing when falls upon the ear membrane.

Production of sound:

Three things are necessary for production and hearing of sound.

1. Vibrating body 2. Transmitting medium 3. Hearing device

1. Vibrating body:

The sound waves can be produced by the vibration of a vibrating body in a medium. For example a tuning fork is a device used for the production of sound in laboratory. When we struck the tuning fork on a rubber pad the prongs begin to vibrate and a special sound is produced.

2. Transmitting medium:

When a body vibrates it transfers its energy to the surrounding medium. The transmitting medium transmits this energy from the source to the receiver by means of sound waves. The medium could be solid, liquid or gas. Sound cannot travel through vacuum.

3. Hearing device:

Any device that can detect sound wave can be called as hearing device. Ear is a device that converts sound waves into the sensation of hearing. The energy of sound waves set the membrane of ear or other device into vibrations. The vibration of the source and vibrations of membrane posses same characteristics, so the listener hears the sound.

Sound waves do not travel through vacuum:

Sound waves need a material medium for their propagation. It cannot travels through vacuum. It can be proved with the help of the following experiment.

Experiment:


Consider an electric bell connected with an electric circuit placed in a bell jar as shown in the figure. When the circuit is closed the hammer hits the bell and thus the sound produced propagates through air and finally reaches to our ear.

When air is pumped out gradually from the jar with the help of vacuum pump, then the intensity of sound decreases. Finally no sound can be heard when air is totally pumped out of the jar and vacuum is created.

Thus sound requires a material medium for their propagation and it cannot travels through vacuum.

Characteristics of sound:

There are certain characteristics of sound by which one sound can be distinguished from another sound. These characteristics are given below.

1. Loudness of sound:

Loudness of sound enables us to distinguish between a faint and loud sound. Loudness depends upon the following factors.

i. Area of the vibrating body: The larger the area of vibrating body louder will be the sound produced and vice versa. For example the drum produces a loud sound as compared with that of produced by a dhoolac. Similarly a school bell produces loud sound due to large area as compared to that of house bell.

ii. Amplitude of vibrating body : Larger the amplitude of vibrating body, louder will be the sound produced while the sound will be low if the amplitude will be small. For example a drum produces loud sound when its membrane is struck strongly whereas sound will be faint if the membrane is struck gently.

iii. Distance from vibrating body: The loudness of a sound increases or decreases as the distance between the source and listener decreases or increases respectively.

2. Intensity of sound:

The energy carried by sound waves per unit time per unit area placed perpendicular to the direction of propagation of waves is called intensity of sound.

Mathematically:

I = E

A ×t

Where ‘I’ is the intensity of sound when energy ‘E’ is transferred through an area ‘A’ in time interval ‘t’.

The unit of intensity of sound is watt per square meter (watt/m2).

Weber Fechner law: It is a relation between loudness of sound and its intensity.

Statement:


According to Weber Fechner’s law, “the loudness of sound is directly proportional to the logarithm of intensity.”

L ∝ log I⇒ L = k log I−−−−−−−1

Equation 1 represents the mathematical form of Weber Fechner’s law. This equation gives us the relation between loudness of sound and intensity of sound ‘k’ is constant of proportionality and its value depends upon the system of units.

Intensity level or sound level:

The difference between the loudness of two sounds where one sound is faintest audible sound is known as intensity level or sound level.

Let Lo be the loudness of faintest audible sound and L be the loudness of any two levels. Then

L0 = k log I 0−−−−−−1L = k log I−−−−−−−2

Now for finding the intensity level we subtract the equation 1 from equation 2 we get

L − L0 = k log I − k log I 0

⇒ Intensity level = k ( log I − log I0)

⇒ Intensity level = k logII 0

−−−−−−−3

Putting k = 1 in equation 3 we get

⇒ Intensity level = log

II0

−−−−4 bels

bel is the unit of intensity level or sound level. In terms of decibels we will use the following equation

⇒ Intensity level = 10 log

II 0

−−−−5 decibels

From equation 5 we can construct a scale for measuring the intensity level of sound known as “decibel scale”.

3. Pitch of sound:

The characteristic of sound by means of which we can distinguish between shrill sound and grave sound is called pitch of sound


Pitch of sound depends upon the frequency. The greater the frequency, the higher is the pitch and the lower the frequency; the lower will be the pitch. For example the sound produced by the birds and cats are of high pitch whereas the sound by dogs and frogs are of low pitch. Similarly men have grave sound with low pitch and women have shrill sound because of high pitch.

4. Quality of sound:

The property of sound by which two sounds of same frequency and pitch are distinguished is called the quality of sound.

The loudness and the pitch of these two sounds may be the same but their wave forms may be different as shown in the figure. So their quality is different and they can be distinguished from each other.

Wave form of different instruments having same loudness and pitch

Musical sound:

A sound which produces pleasing sensation in ear is called a musical sound. Musical sound is composed of regular and uniform vibrations. There is no sudden change in its wave form as shown in the figure.

The sound produced by sitar, piano, violin, tuning fork etc are the examples of musical sound.

Noise:

A sound which produces displeasing sensation in ear is called a musical noise. Such a sound is composed of irregular and disordered vibrations. There is sudden change in its wave form as shown in the figure. The sound of explosion, horn of vehicles, and sound of donkey are the example of noise.

Noise pollution:

The excessive noises which disrupt the balance or activity of living or non living things in the environment, is called noise pollution.

Explanation:


There are many sources which causes noise pollution. For example the horn of vehicles, the machinery used in construction, the low flying crafts, loud speakers, metal work etc cause noise pollution.

Noise is nuisance because of its harmful effect on human beings, animals, plants and other inert things in environment. Some harmful effects of noise pollution are given below.

1. Noise causes temporary or permanent deafness.2. It reduces the working efficiency and interferes with communication.3. It increases the rate of errors which causes accidents.4. It decreases the ability of understanding, listening, writing and reading of humans.5. It causes dangerous diseases like blood pressure, heart problems and mental illness.6. It damages the nervous system of animals.7. It decreases the productivity of animals and affects the normal growth of plants.8. Loud noise can also affect the high building and bridges etc.

Speed of sound:

The distance covered by sound waves in unit time is called speed of sound.

The speed of sound depends upon the medium in which they travel; it is greater in denser medium than in rare medium.

Determination of speed of sound in air:

The speed of sound can be determined by using resonance tube method which works on the principle of resonance phenomenon.

Construction:

The experimental arrangement consists of a glass tube which is connected to a water reservoir as shown in the figure.

The glass tube contain air column whose length can be increase or decrease by moving the reservoir up and down.

Working:

A vibrating tuning fork of known frequency is brought near the open end of the tube by gradually increasing the length of the column until we hear a loud sound. At this stage the frequency of tuning fork becomes equal to the frequency of air column. The compressional waves send by the tuning fork are reflected from the water surface, producing standing waves in state of resonance.

There will be a node at the water surface and antinodes at the open end of the tube.

As the length ℓ between node and anti node is given by

ℓ =λ4

⇒ 4 ℓ = λ⇒ λ = 4 ℓ−−−−−−1


We know that,

v = f λ−−−−−−2

Putting equation 2 in equation 1, we get;

v = f × 4 ℓ

⇒ v = 4 f ℓ−−−−−3

As the values of “ℓ ”and “f ” are known, so speed of sound can be calculated. In air the speed of sound is 330 m/sec.

Reflection of sound:

The bouncing back of sound when it strikes a hard surface is called reflection of sound.

Explanation:

As we know that waves have the property to reflect. Sound waves also reflect from the hard surfaces like walls, metal sheets, plywood etc. the reflection of sound does not require a smooth and shining surfaces. The reflection of sound is utilized in the working of three devices; megaphone, sound boards and ear trumpet.

Echo:

The repetition of sound by reflection of sound waves from a surface is known as echo.

Explanation:

When a person shouts in a big empty hall, we first hear his original sound. After a little while, we hear the reflected sound of shout called echo. Thus an echo is simply the reflected sound. If we shout at a wall from 344 meters away, the sound takes 1 second to reach the wall. The sound takes 1 second to return. So we hear echo after 2 seconds.

Audible frequency range:

The human ear can hear only those sounds whose frequency is between 20 Hz to 20,000 Hz (20 kHz) called audible frequency range.

Explanation:

The audible frequency range of human ear is 20 Hz to 20,000 Hz. We cannot hear the sound whose frequency is above 20,000 Hz, because the membrane of our ear cannot vibrate with such a high frequency.


Similarly we cannot hear the sound whose frequency is below 20 Hz, because loudness of sound is very low to be heard.

The sounds whose frequency is above 20,000 Hz are known as ultrasonic sounds for example sound produced by bat while the sounds having frequency less than 20 Hz are known as infrasonic sounds.

Infrasonic sounds or Infrasonics:

The sounds of frequencies lower than 20 Hz are known as infrasonic sounds.

For example: The earthquakes, volcanic eruptions, simple pendulum and some animals like whale and elephants also produce infrasonic sounds which cannot be heard by human beings.

Ultrasonic sounds or ultrasonics:

The sounds of frequencies higher than 20,000 Hz are known as ultrasonic sounds or simply called ultrasound.

Explanation:

We cannot hear the sound whose frequency is above 20,000 Hz, because the membrane of our ear cannot vibrate with such a high frequency. The ultrasound is reflected just like ordinary sound and produce echoes which cannot be heard by our ears. They can be detected by special equipments. Due to high frequency, ultrasound has a greater penetrating power than ordinary sound and can be used in medical, scientific and industrial purposes. Some of the applications of ultrasound are as follows.

1. Ultrasound is used in sonar to measure the depth of sea and locate the underwater objects.

2. Ultrasound is used to investigate inside the human body.3. Ultrasound is used in the treatment of muscular pain and in the treatment of disease

called arthritis.4. Ultrasound is used in industry for detecting flaws in metal blocks or sheets without

damaging them.5. Ultrasound is used for finding the level of a liquid in a metal tank without opening it.

Acoustics:

The study of production, propagation and properties of sound as well as various applications of sound is known as acoustics

OR The study of sound is called acoustics.

Acoustic protection:

The elimination of acoustic factors which affect the clear hearing of sound is known as acoustic protection.

Explanation:

The factors which affect the acoustics of rooms and halls are as follows.


1. Echo 2. Reverberation 3. Focusing of sound at certain spots

1. Echo:

The reflection of sound from the hard surface is called echo. Original sound cannot be heard clearly due to echo. The echo can be avoided by making use of absorbing materials in the walls of halls and rooms.

2. Reverberation: The presence of sound after the sounding source has stopped is known as reverberation. It causes general confusion of sound impressions on ear. It can be avoided by using absorbing things like carpet and curtains in halls and rooms.

3. Focusing of sound at certain spots: The curved walls focus the sound waves at a certain spots. Thus sound cannot be heard clearly at some places. It can be avoided by using flat walls instead of curved walls.

Written and composed by:

ZAHOOR AHMAD

M.Sc. PHYSICS

CONCEPTUAL QUESTIONS

1. Why the waves produced by simple pendulum cannot be heard?

Ans. The membrane of human ear can be vibrated by those sounds whose frequency is greater than 20 Hz and below 20,000 Hz. In other words the audible frequency range for human ear is 20 Hz – 20,000 Hz.

The frequency of waves produced by simple pendulum is below 20 Hz. Which cannot vibrate the membrane of ear and thus we cannot hear such sounds.


2. Why does the school bell produce a loud sound?

Ans. The loudness of sound depends upon the surface area of vibrating body. Greater is the surface area of vibrating body, louder will be the sound produced by that body and vice versa.

Since we know that the surface area of a school bell is larger. So it produces louder sound when struck due to its larger surface area.

3. Why the sound produced by a ‘dhool’ is louder than a ‘dhoolac’?

Ans. The loudness of sound depends upon the area of vibrating body i.e Greater the surface of vibrating body, greater will be the loudness of sound produced by that body and vice versa.

Since the surface area of a ‘dhool’ is greater than the surface of a ‘dhoolac’. So the sound produced by a ‘dhool’ is louder than that of a ‘dhoolac’.

4. In which medium air or water, an echo is heard sooner and why?

Ans, the speed of sound depends upon the elasticity of the medium i.e. greater is the elasticity of the medium, greater will be the speed of sound and vice versa.

As water is more elastic than air, so the speed of sound in water will be greater than water. Thus an echo is heard sooner in water as compared to air.

5. If a ringing bicycle bell is held tightly by hand, it stops producing sound why?

Ans: a ringing bicycle bell produces sound due to vibrations. Now if we held the ring bell tightly by hand, then its vibration s will stop as a result producing no sound.

6. Why sound cannot travel through vacuum?

Ans. The sound waves are longitudinal waves (mechanical waves). Thus needs a material medium for their propagation. As in vacuum, there is no material medium, so sound waves cannot travels through vacuum.

7. Why sound cannot be heard on moon?

Ans. . The sound waves are longitudinal waves (mechanical waves). Thus needs a material medium for their propagation. As there is no material medium in the atmosphere of moon through which sound waves can pass. Thus a sound cannot be heard on the moon.

8. When the wire of sitar is plucked. What of waves are produced in the air?

Ans. When the wire of sitar is plucked, then sound waves are produced in air. These sound waves propagate through air in the form of longitudinal waves which consists of compressions and rarefactions which are produced one after the other in a certain order.

9. How bats are able to fly at night without colliding with other objects?

Ans. While flying, the bat produces ultrasonic sound, which travels faster ahead. When an object appears in their path, the ultrasonic reflects from that object and is received by the bat in the form of echo. In this way, the bat comes to know about the object in their path and thus change their path. Thus the bats remain safe from colliding with objects at night.



ZAHOOR AHMAD

M.Sc. PHYSICS

CHAPTER NO:12 Geometrical Optics


Q1. What is meant by reflection of light? State and explain laws of reflection with diagram?

Ans. Reflection of light:- when rays of light travelling in one medium strikes the boundary of another medium a part of it is sent back in the same medium, it is known as reflection of light.

OR

The bouncing back of rays of light from a medium is called reflection of light.

Light mostly reflects from smooth and polished surfaces like mirrors and metals.

Reflection of light from plane mirror:-

The phenomena reflection of light is explained from the reflection of light from plane mirror in the above given diagram.

A plane mirror MM΄ is shown in the above figure. Incident rays of light AO strikes the mirror at the point of incidence “O” and then reflects as ray OB from the point of incidence “O”. ON is known as the normal to the mirror MMʹ. The angle between the incident ray and the normal is called an angle of incident where as the angle between the normal and the reflected ray is known as angle of reflection.

Laws of reflection of light:- The reflection of light from a plane surface obeys two laws, which are known as the law of reflection of light. These two laws are given below.

1. First the law of reflection states that the incident ray AO, reflected ray OB and the normal ON all lie on the point of incidence “O” in the same plane.

2. Second law of reflection states that the angle of incidence∠ i is always equal to the angle of reflection ∠r i.e

m∠ i = m∠r

Q2. What are spherical mirror? Explain its types with the help of diagram?

Ans. Spherical Mirrors: - it is part of a spherical shell with its inner or outer surface polished to reflect light called spherical mirrors.

Spherical mirrors are of two types’ i.e concave mirror and convex mirror.

1. Concave Mirror:- A spherical mirror whose inner surface polished to reflect light is called concave mirror.


Concave mirror has the ability to converge a parallel beam of light; therefore it is called a converging mirror. The inner shinning surface of spoon is an example of concave mirror.

Nature of image formed by Concave mirror: - Real and inverted images are formed by concave mirror. The size and distance of the image formed by concave mirror depends on the distance of the object from concave mirror.

2. Convex Mirror:- A spherical mirror whose outer surface is polished to reflect light is called convex mirror.

Convex mirror has the ability to diverge a parallel beam of light; therefore it is also called a diverging mirror.

The outer bulging shinning surface of a spoon is an example of convex mirror.

Nature of image formed by convex mirror: - virtual, erect and diminished image is also formed by convex mirror for any distance of an object from convex mirror.

Q3. Write some uses of spherical mirror?

Ans. Some important uses of spherical mirrors are given below.

Uses of Concave Mirror:-

i) As shaving mirror: - concave mirror of large radius of curvature is used as shaving mirror. When face is close to the mirror an enlarged, erect and virtual image is formed which help in making a clean and neat shave.

ii) Used by doctors and dentists: - Doctor Use concave mirror to examine ear, nose and throat. Dentists also use a small concave mirror to have a look of back side of tooth and cavity in it.

iii) As objective of reflecting telescope: - in one kind of telescope, a concave mirror is used as objective. Due to larger aperture great amount of light from the object is incident on this concave mirror. So a clear image of the object is seen by such telescope.

iv) In microscope:- concave mirror are also used to concentrate light on microscope slide.v) In automobiles headlight and search lights: - Concave mirrors are used behind the bulb in

headlights and search lights in a particular direction.

Uses of Convex Mirror:-

i) The Vehicles side mirror: - convex mirror are used as side mirror in vehicles to observe rear view.


ii) At road bend: - Large convex mirrors are often placed at dangerous road bends for safe drive.

iii) At Shopping Malls: - Large convex mirror are used at shopping malls for security to observe every place of a shopping mall.

Q4. What is meant by refraction of light? State and explain laws of refraction with the help of a diagram?

Ans. Refraction of light:- The change in velocity and direction of light when it passes obliquely from one medium to another is called refraction of light.

The surface separating two media is called interface.

Explanation:- when a ray of light enter obliquely from a rare medium to a denser medium, it bends towards the normal. The angle of incidence in rare medium is greater than the angle of refraction in a denser medium. Conversely when a ray of light enters from denser into a rare medium, it bends away from the normal. The angle of incidence in a denser medium is less than the angle of refraction in a rare medium.

Laws of refraction:- Refraction of light obeys the following two laws.

i) The normal incident ray, refracted ray and the normal at the point of incidence, all lie at the same plane.

ii) The ratio of sine of angle of incidence and the sine of angle of refraction is constant for a given pair of media.

Mathematically:-

Second law of refraction is also known as Snell’s law.


η = sin∠ isin∠r

Q5. Define refractive index. How it is related to velocity of light in a pair of media?

Ans. Refractive Index: - The refractive index is the ratio of the velocities of light in the air or vacuum to the velocity of light in a given medium. Thus the refractive index of glass with reference to air is given by:

η =Velocity of light in air

Velocity of light in glass

The refractive index of a substance does not depend on the angle of incidence but depends on the nature of material of the medium and on the wavelength of the light used.

Q6. Define the terms centre of curvature, radius of curvature, pole, principal axis, aperture, principal focus and focal length?

Ans. Centre of curvature: - The centre of the hollow sphere of which mirror is a part is called its centre of curvature. It is represented by C. centre of curvature of a concave mirror is in front of it but that of convex mirror is behind it.

Radius of curvature: - The radius of the hollow sphere of which mirror is a part is called radius of curvature of that mirror. It is denoted by R.

Pole: - the centre point of a mirror is called its pole. It is denoted by P.

Principal axis: - The straight line passing through the pole “P” and the centre of curvature “C” of a mirror is called the principal axis of that mirror.

Aperture: - The area of mirror exposed to the incident light is called aperture.

Principal focus: - . The point at which the reflected rays from concave mirror converge is called principal focus of concave mirror. It is denoted by F. principal focus of concave mirror is in front of a concave mirror.


The point from which the light rays appear to diverge after reflection from concave mirror is called principal focus of convex mirror. It is denoted by F. principal focus of convex mirror is behind the convex mirror.

Focal length: - The distance between the pole “P” and principal focus “F” of the mirror is called focal length. It is denoted by “f”.

Focal length of concave mirror is considered positive while focal length of convex mirror is considered negative.

Q7. Explain the rays of light that are used to locate the image formed by concave mirror?

Ans. The following rays of light are usually used to locate the image formed by concave mirror.

1. The rays of light travelling parallel to principal axis passes through principal focus “F” after reflection from concave mirror

2. The rays of light passing through principal focus “F” become parallel to principal axis after

reflection from concave mirror.


3. The rays of light passing through the centre of curvature “C” of concave mirror is reflected back

along the same path.

4. The rays of light incident at the pole of a concave mirror reflect with the same angle as the

incident ray makes with the principal axis.

Q8. Discuss the nature of the image formed by concave mirror as the object is brought close to concave mirror?

Ans. The type of image formed by a concave mirror depends on the position of an object in front of a concave mirror. If the distance of an object from a concave mirror is changed than, nature, size and location of image is also changed.

The following results are obtained by changing the distance of an object from concave mirror.

1. When the object is beyond the centre of curvature: - if the object is placed beyond the centre of curvature “C” of the concave mirror than its real, inverted and smaller image is formed between centre of curvature “C” and focus “F”.


2. When the object is placed at centre of curvature: - if the object is placed at the centre of curvature “C” of a concave mirror then its real and inverted image of the same size as that of the object is formed at the centre of curvature.

3. When the object is placed between principal focus “F” and the centre of curvature “C”:- if the object is placed between “C” and “F” then it’s real, inverted and magnified image will be formed beyond “C”.

4. When the object is placed at the principal focus “F”:- if object lies at focus “F” of a concave mirror than highly magnified, inverted and real image will be formed at infinity.


5. When object is placed between pole “P” and focus “F”:- if the object is placed between the pole “P” and focus “F” of a concave mirror, than virtual, erect and enlarged image appears behind the mirror.

Q9. Derive the spherical mirror equation for a concave mirror?

Ans. Spherical Mirror Equation:- A formula which gives the relation between image distance object distance and focal length of a mirror is known as the mirror formula or mirror equation.

In order to derive the mirror formula,

We will be a ray diagram

Consider two similar triangles APB and AʹPʹB΄. as two triangles are similar therefore

AB

A ' B'= PB

P B '−−−−−1

Triangle ABF and FPE are also similar therefore

ABEP

= BFFP

EP=AʹB΄ therefore

AB

A ' B'= BF

FP−−−−−−2


Comparing equation (i) and (ii) we get,

PB

P B'= BF

FP

As BF= PB-FP, therefore

PB

P B'= PB−FP

FP

In the given figure P= PB, q= PB΄ and f= FP therefore,

pq

= p−ff

By cross multiplication

p f =q ( p−f )p f =q p −q f

Dividing both sides by pqf

p fp q f

=q p−q fp q f

p fp q f

= q pp q f

−q fp q f

1q

=1f

−1p

1f

= 1p

+ 1q

This is known as mirror formula. It gives relation between the object distance, image distance and focal length of a spherical mirror.

Q10. Define linear magnification. Write the sign convention for spherical mirror?

Ans. Linear Magnification: - The ratio of the size of the image to the size of the object is called linear magnification.

The ratio of the image distance to the object distance is also called linear magnification.

Mathematically:-

M =Size of image Size of object

=Image distance Object distance

M =hi

ho

=qp

Sign Convention: - The following sign convention is adopted to obtain the correct result in a given problem of image formed by a spherical mirror.

(i) Distance of real object and real images are taken positive.


(ii) Distance of virtual objects and virtual images are taken as negative.(iii) The focal length of a concave mirror is taken positive and that of convex mirror is taken as

negative.

Q11. Explain total internal reflection of light and critical angle. Also write applications of total internal reflection of light?

Ans. Critical angle: - The angle of incidence in the denser medium for which the angle of refraction in rare medium becomes 90° is called critical angle.

Total internal reflection of Light:- when the angle of incidence in the denser medium becomes greater than the critical angle ˂c, there is no refracted beam. All the rays are internally reflected back in the optically denser medium. Such a reflection is called internal reflection of light.

Conditions for total internal reflection:- the following conditions must be satisfied for the effect of total internal reflection.

i) The incident light must pass from a denser medium into a rare medium.ii) The angle of incidence in denser medium must be greater than the critical angle of the

medium.

Application of total internal reflection:-

i) The Periscope : - it is an optical instrument used to see the objects above the level of viewer or when the objects cannot be seen directly. The prison periscopes are used in tanks and submarines.

ii) The binoculars : - The binoculars also make use of prism to reduce the length of the instrument and produce an erect image.

iii) Optical Fibers : - an important application of total internal reflection is in the operation of optical fibers. Optical fibers are made of a core of high refractive index i-e glass or plastic. The thickness of the fibers is about the thickness of the human hair. Optical fibres are now widely used in telecommunication industries. They can carry much more information than copper wires. They are also lighter and are cheaper as compared with copper cables.


iv) Endoscope :- A very important application of total internal reflection is found in endoscope. An endoscope is an optical instrument which is used for viewing and photographing the inside of a hollow larger of the body such as the bladder womb etc.Gastroscope is an endoscope to view the stomach.

Q12. Explain refraction of a light through a prism?

Ans. Refraction of light through a prism: - A prism is a transparent refracting body having three rectangular and two triangular surfaces.

Angle of a prism:- the angle between the two refracting rectangular surfaces opposite to the base is called the angle of prism ˂A.

Refraction of light through a prism is explained from the given diagram.

EF is the incident ray to the face AB of a prism. The incident ray EF on entering the glass prism is refracted towards the base BC of the prism. When this refracted ray FG emerges out of the prism, it bends away from the normal.

The emergent ray GH has been deviated from its original paths EFI. If we produce GH backward it will meet EFI at O making an angle IOH.

Angle of deviation (∠ IOH):- The angle through which the emergent ray has been deviated is called the angle of deviation D (˂IOH).

Angle of minimum deviation Dm (˂IOH):- When the refracted ray becomes parallel with base BC of the prism then the angle of deviation becomes minimum and is called the angle of minimum deviation DM. the value of angle of deviation depends upon the following factors.

i) Angle A of the prism.ii) The refractive index of the material of the prism. iii) The angle of incidence.

The index of refraction of the material of the prism can be determined by the relation.

Refractive index = η =sin ( A+

Dm

2)

sinA2

Q13. Define lens, its working and uses. Also write the types and subtypes of lenses?


Ans. Lens:- A lens is a piece of transparent refracting material such as glass or plastic which refracts light in a regular way. Lens is bounded by one or two spherical surfaces.

Working: - The working of the lens is based on the refraction of light rays when they are passing through it.

Uses of lens: - lenses are widely used in spectacles, cameras, projectors, optical instruments. Human eyes have two special crystalline lenses which enables it to form images.

Types of lenses: - There are two types of lenses. Convex lens and concave lens.

1) Convex Lens :- A lens that is thicker at the centre than at the edges is called convex lens. Convex lens converges parallel beam of light at a single point and hence is also called converging lens.Real and inverted images are formed from convex lens.

Sub types of convex lens: - convex lens has three sub types.

(i) Double convex lens:- A lens having both the surfaces convex or curved outward is called biconvex lens or double convex lens.

(ii) Plano convex lens: - A lens having one surface Plano and other convex is called Plano

convex lens.

(iii) Concave Convex: - A lens having one surface concave and the other is convex is called concave convex lens.


2) Concave Lens : - A lens that is thinner at the centre than at the edges is called concave lens.

Concave lens diverges a parallel beam of light and is also called diverging lens.

Virtual, erect and diminished images are forms from concave lens.

Sub types of concave lens: - Concave lens has three sub types.

(i) Double Concave Lens: - A lens having both the surfaces concave or curved inward is called biconcave lens or double concave lens.

(ii) Plano concave lens: - A lens having one surface plane and the other is concave is called

Plano concave.

(iii) Convex Concave lens: - A lens having one surface convex and other concave is called Convex Concave lens.


Q14. Define the special terms related to lens?

Ans. Optical Centre: - The centre point of a lens is called optical centre. Usually it is denoted by O. optical centre “O” of a lens lies on the principal axis. A ray of light passing through optical centre “O” suffer no deviation and goes on straight.

Principal Axis: - A line passing through the optical centre of the lens and perpendicular to the both faces of the lens is called principal axis.

Principal Focus: - The point where the refracted rays from convex lens converge or appear to diverge in case of concave lens is called principal focus of a lens.

It is denoted by F.

A lens has two foci that are at equal distance from optical centre on either side of a lens. A convex lens has a real focus and a concave lens has a virtual focus.

Focal Length: - The distance between the principal focus “F” and optical centre “O” of a lens is called the focal length of a lens.

Q15. Write the important rays that enable us to find the position of the image?

Ans. Following are the most important rays that enable us to find the position of the image.

1) Ray 1 :- An incident ray through optical centre “O” passes without bending.

2) Ray 2 :- An incident ray travels parallel to the principal axis after refraction passes through

principal focus “F” of the lens or appears to diverge from principal focus “F”.

3) Ray 3 :- An incident ray passing through principal focus “F” is refracted parallel to the principal axis.


Q16. Discuss the nature and the size of the image formed by convex lens when object is brought closer to lens?

Ans. The type of image formed by a convex lens depends on the distance of the object from the lens. When the object is brought closer to the convex lens, image of different nature is formed.

The following cases illustrate the graphical ray diagram method of constructing the images of an object.

(i) When the object is beyond 2F :- when an object AB is placed beyond 2F in a front of a convex lens. Its image A'B' will be formed between F and 2F. The image will be diminished, real and inverted.

(ii) When the object is at 2F :- when an object is placed at 2F in front of a convex lens. The image will be formed at 2F. The image will be of same size as that of object, real and inverted.

(iii) When an object is between F and 2F :- When an object is placed in front of a convex lens between F and 2F. Then the image will be formed at a distance beyond 2F from the lens. The image will be magnified, real and inverted.


(iv) When the object is placed at the focus of a convex lens : - When the object is at F in front of the convex lens. Its image will be at infinity, much enlarged, real and inverted.

(v) When the object is placed between optical centre and focus : - When the object is placed between optical centre and principal focus. Then the image will be found on the same side behind the object. The image will be magnified, erect and virtual.

Q17. Derive lens formula?

Ans. Lens Formula: - A lens formula gives the relationship between image distance “q” object distance “p” and focal length “f”. It is obvious from the given figure that the right angle triangle ABC and A'B'C' are similar therefore we can write:-


AB

A' B' =CB

C B'

ABA' B' = p

q−−−−−1

Triangle ECF and A’B’F are also similar therefore

EC

A ' B'= CF

F B'

EC = AB therefore

AB

A' B' =CF

F B'

ABA' B' = CF

C B'−CF∴ F B' = C B'−CF

Since from the figureCF = f , C B ' =q and CF = f putting in above equation we get.

AB

A ' B'= f

q−f−−−−−−2

Comparing equation (i) and (ii) we get,

pq

= fq−f

By cross multiplication

q f = p (q−f )q f =p q −p f

Dividing both sides by pqf

q fp q f

=p q−p fp q f

q fp q f

= p qp q f

−p fp q f

1p

=1f

−1q

1f

= 1p

+ 1q

This equation is called lens formula.


Q18. Define power of a lens and its unit?

Ans. Power of lens:- The power of the lens is the measure of degree of convergence or divergence of right rays falling on it.

The power of lens depends upon its focal length. The smaller the focal length the greater is the convergence or divergence power of the lens.

Mathematically:- Power of lens is defined as the reciprocal of its focal length in meters. It is denoted by D.

D =1 Focal length of a lens in meters

D= 1f (m)

Unit of power of a lens (ɸ):- The unit of power of a lens dioptre which is defined as “One dioptre is the power of a lens whose focal length is one meter”.

Q19. Write a note on simple microscope?

Ans. Simple Microscope: - A simple microscope is a convex lens of short focal. It is also called magnifying glass.

Working: - A simple microscope works on the principal that when an object is placed within its focal length, a magnified, virtual and erect image is formed. Since the image formed is larger than the object it because easier to see the details of the small object.

Uses: - A simple microscope is used by watch makers to see the small parts of a watch clearly. It is also used by jewelers to see the fine parts of jewellery etc.

Explanation: - In the given ray diagram a small object AB to the magnified is placed between the optical centres “C” and focus “F” of the convex lens. So the erect, virtual and magnified image A’B’ is formed.

When we look into the lens keeping the eyes close to the lens, we see the magnified image A’B’ in which all the parts of the object appear to be much larger than they actually are. We adjust the position of the lens in such a way that the image is formed at a distance of about 25cm from the eye. Which is the least distance of the distinct vision? This is done because when the image is at least distance of distinct vision, then the eye can see it conveniently without undue strain.

Magnification produced by a simple microscope: - Magnification of a simple microscope is given by


M = 1 + D

f

Where “D” is the least distance of distinct vision, which is 25cm “f” is the focal length of a convex. In the given magnification formula, the value of D is constant i-e 25cm. it is clear from the magnification formula that the smaller the focal length of a convex lens, greater will be its magnification.

Q20. Write defects of vision. Its causes and correctness?

Ans. The defects of vision arise due to the inability of the eye lens to produce sharp images at retina. The two main defects of vision are:-

a) Short sightedness (Myopia).b) Long sightedness (Hypermetropia).a) Short sightedness :- Short sightedness is that defect of an eye due to which an eye cannot see

the distant object clearly through it can see the nearly objects clearly.Causes of short sightedness:- This defect of eye is caused due to the high converging power of the eye lens or due to the eye ball being too long.So due to the greater converging power of the eye lens in myopic eye, the image of distant object is formed in front of the retina and hence the eye cannot see it clearly.Correction of short sightedness:- Short sightedness is corrected by using spectacles containing concave lenses, when a concave lens of suitable power is placed in front of the eye, then the parallel rays coming from distant object are first diverged by concave lens and then converged on retina by the action of concave lens. Thus the image of distinct object is formed on the retina of the eye and hence the eye can see it clearly.

b) Long sightedness :- Long sightedness is that defect of the eye due to which the eye cannot see the nearly objects clearly but it can see the distant objects clearly.Causes of long sightedness:- Long sightedness is caused due to low converging power of the eye lens or due to the eye ball being too short. Due to the law converging power of eye lens, the image of nearly objects is formed behind the retina and hence the eye cannot see it clearly.Correction of long sightedness:- long sightedness is corrected by using spectacles containing convex lens. When a convex lens of suitable converging power is placed in front of the eye, then the rays coming from the nearby object are first converged by the convex lens and then by the eye lens. Due to combined converging action, the image “I” of the nearby object is formed on the retina and hence the eye can see the distinct object clearly.


SHORT QUESTIONS

Q1. Draw a label diagram to show the a) pole b) centre of curvature c) principal axis d) principal focus e) radius of curvature?

Ans.

Q2. Define focal length and radius of curvature. What is the relation between the focal length and the radius of curvature in case of a concave mirror?

Ans. Focal length: - The distance between the pole and the principal focus of a concave mirror is known as focal length. Focal length is denoted by f.

Radius of Curvature:- The radius of a hollow sphere of which a mirror is a part is called radius of curvature of concave mirror. Radius of curvature is denoted by R.

Relation between focal length and radius of curvature:- It has been found that the focal length of a concave mirror is equal to half of its radius of curvature.

i-e f= R/2

Q3. Name the spherical mirror which has

a) Virtual principal focus.b) Real principal focus?


Ans.

a) Convex mirror has virtual principal focus because in convex mirror the reflected rays appear to diverge from a focus behind the mirror. Convex mirror form virtual image which is not obtained on the screen in front of true mirror.

b) Concave mirror has real principal focus because in convex mirror the reflected rays converge in front of the mirror on the screen. Thus concave mirror has real principal focus and form real image which is obtained on the screen in front of concave mirror.

Q4. If radius of curvature of a concave mirror is 1m what is its focal length?

Ans. Given:-

Radius = R = 1m

Required:-

Focal length = f?

Solution:-

As we know that

F = R/2

Putting the value

f = ½

f = 0.5 m

Q5. For what position of an object a concave mirror forms and image which is real and equal in size to the object?

Ans. If an object is placed at centre of curvature of a concave mirror then real, inverted image is formed which is equal in size to the object.

Q6. For what position of an object a real and diminished image is formed by a concave mirror?

Ans. when an object is placed beyond centre of curvature of a concave mirror then real, inverted and diminished image is formed between centre of curvature “C” and principal focus “F”.


Q7. Give at least three uses of concave mirror?

Ans. Uses of concave mirror:-

(i) A simple concave mirror is used by doctors to examine nose, throat and ear.(ii) Concave mirror is used in microscope to concentrate light on microscope slide.(iii) Concave mirror of large aperture is used in telescope to observe distant objects on earth

and heavenly bodies.

Q8. Define refractive index. How it is related to velocity of light in a pair of media.

Ans. Refractive Index:- The refractive index is the ratio of the velocity of light in the air and vacuum to the velocity of light in given medium. Thus the refractive index of glass with reference to air is given by

η =Velocity of light in airVelocity of light in glass

⇒ η = cv s

The refractive index of a substance does not depend on the angle of incidence but depends on the nature of the material of the medium. Refractive index is a ratio between two similar quantities therefore it has no unit.

Q9. How would you make the rays from a luminous bulb parallel by means of a concave mirror?

Ans. The rays of light passing through the focus point of a concave mirror become parallel to the principal axis after reflection. Therefore the rays of light can be made parallel to the principal axis of a concave mirror by placing the bulb at principal focus of a concave.

Q10. Which type of spherical mirror has a widely field of view?

Ans. The convex mirror has a wider field of view because it produce virtual, erect and diminished image of an object placed in front of it.

Q11. Why does a driver prefer to use a convex mirror as a back view mirror in an automobile?

Ans. A driver prefer to use a convex mirror as a back view mirror in an automobile because it provides a wider field of view behind the vehicle and helps the driver to drive safely.

Q12. A ray of light travelling in water emerges into air. Draw a ray diagram indicating the change in its path?

Ans. We know that a ray of light entering from denser to rare medium, it bends away from the normal. Thus in the given figure the incident ray AO entering from water to air change its path a ray OC bending away from the normal. The angle of incidence is formed in water is less than angle of refraction∠ r in the air. i-e ∠ i˂∠ r

Q13. What is unit of refractive index?


Ans. As refractive index is given by

η = sin∠ isin∠r Or

n = C/V

Which shows the refractive index is a ratio between two similar quantities therefore it is a unit less quantity having no unit.

Q14. Which one has higher refractive index, water or glass?

Ans. Refractive index of water is 1.33 i-e nw = 1.33 while the refractive index of a glass is 1.96 i-e ng= 1.96 therefore refractive index of a glass is greater than that of water.

Q15. Define Snell’s law of refraction?

Ans. in 1620, Snell presented the law of refraction which is given “ratio of sinθ of angle of incidence to the sinθ of angle of refraction is constant for given pair of media”.

Mathematically :

η = sin∠ isin∠r

Where “n” represents the refractive index of the second medium with respect to the first medium.

Q16.Why a pencil half immersed in water and held obliquely, appears to be bent at the water surface?

Ans. The illusion is caused because of refraction of light. The light rays coming from the lower end of the pencil i-e from point “B” and passes from water into air. AB the ray enters into rare medium from a denser medium, so the ray bends away from the normal. On entering the eye the rays appear to be coming from point “C’ above point B as shown in figure.

Thus point “C” represent the image at point “B” due to refraction. Thus the observer sees the image apparently in the position “AC” and as a result the pencil appears to be bent at the water surface.

Q17. For what positions of an object a real, diminished image is formed by a concave lens?

Ans. when the object “AB” is placed beyond 2F in front of convex lens then a real, inverted and a diminished image is formed between F and 2F.


Q18. What is SI unit of a power of a lens?

Ans. The SI unit of power of a lens is dioptre.

The power of lens is defined as “The reciprocal of focal length in meters”. The power of a lens is denoted

By “D” and is given by

D = 1/ f (meter)

The power of lens is said to be one dioptre if the focal length of a lens I 1m.

Q19. What will be the value of angle of refraction when the angle of incidence in denser medium is equal to the critical for a given pair of media?

Ans. as we know that critical angle is the angle of incidence in denser medium for which the angle of refraction in rare medium becomes 90°. Therefore when the value of angle of incidence in denser becomes equal to the critical angle then the angle of refraction in rare medium will be at 90°.

Q20. Why do stars twinkle on a clear night?

Ans. the light from the stars travel through different layers of the atmosphere of varying densities. Therefore the light rays deviate from its original path. Further these layers are not stationary, but keep on moving. These reasons lead to the twinkling effect of the stars.

Q21. You are given two convex lenses of focal length 7cm and 20 cm. which one will you choose as objective lens for making a compound microscope?

Ans. we know that the magnifying power of compound microscope is given by

M =qo

po(1+ d

f e)

In equation (i), fo= focal length of objective

fe= the length of microscope table


Equation (i) shows that the magnifying power of compound microscope will greater if the focal length of objective lens is smaller and vice versa.

Thus if we use a concave lens, then the magnifying power of compound microscope will increase.

Q22. What type of lens is used to make the eye piece of compound microscope?

Ans. The eye piece of compound microscope is a convex lens having comparatively larger aperture and focal length as compare to objective lens.

Q23. In a telescope, which one has a shorter focal length, the objective or the eye piece?

Ans. we know that the magnifying power of telescope is given by

M = fo/fe --------------------- (i)

Equation (i) shows that the magnifying power of telescope can be increased if we use an eye piece of short focal length and small aperture.

Q24. Writing on the black board?

a) Name the type of defect from which he is suffering?b) With the help of ray diagram, show how this defect can be remedied?

Ans.

a) The defect from which the child is suffering is called short sightedness or Myopia.b) For the correction of these defects, a concave lens is placed in front of the eye. This

combination of two lenses enables the image to be retina.

Electrostatics

Electricity:

The branch of physics which deals with the study of charges at rest or in motion is called electricity.


There are two branches of electricity which are as follows;

1. Electrostatics or static electricity2. Electrodynamics or current electricity

1. Electrostatics:( Electro means charge and static means rest)

The branch of physics which deals with the study of properties of charges at rest is known as electrostatics or static electricity.

2. Electrodynamics:( Electro means charge and dynamics means motion)

The branch of physics which deals with the study of properties of charges in motion is called electrodynamics or current electricity.

Electric charge:

Electric charge is the physical property of matter that causes it to experience a force when close to other electrically charged matter.

OR

Charge is the intrinsic property associated with matter due to which it produces and experiences electrical and magnetic effects.

Explanation:

The existence and nature of charge can be understood by performing the following experiments;

Experiment 1: When an uncharged glass rod is brought near a suspended pith ball as shown in the figure. The pith ball remains in its rest position showing that the gravitational force of attraction between the pith ball and the uncharged glass rod is too weak to cause any movement in the pith ball.

Experiment 2: When a glass rod is rubbed with silk cloth and brought near the suspended pith ball. The ball is seen to move towards the glass rod showing that glass rod rubbed with silk cloth has the ability to attract small objects. Thus we say that glass rod is charged.

Experiment 3: When an ebonite rod is rubbed with fur is brought near a suspended pith ball. The ball is seen to move towards the ebonite rod showing that the ebonite rod is able to attract light objects when rubbed with fur. Thus the ebonite rod is charged.

Experiment 4: when a charged glass rod is brought near a suspended charged glass rod. The two glass rods repel each other showing that “like charges repel each other”.


Experiment 5: when a charged glass rod is brought near a suspended charged plastic rod. The two rods attract each other showing that “unlike charges attract each other”.

Electric nature of matter:

In general, the matter is electrically neutral. The amount of positive charges and negative charges are equal in a neutral matter. The electric nature of matter can be explained on the basis of structure of atom.

Structure of atom:

The smallest particle of matter which can or cannot exist free in nature is known as atom. An atom consists of two parts; the central massive part is called nucleus which has positive charge

consisting of protons and neutron. The proton has mass of1 .67×10−27 kg while the mass of neutron is less than that of proton. Protons and neutron are collectively known as nucleons. The charge on each

proton is equal to +1 .6×10−19 C .

Electrons are the third particles which are revolving around the nucleus in circular orbits. The

mass of electron is 9 .11×10−31kg and the charge on electron is −1 .6×10−19 C . In a normal condition the number of electrons and number of protons are equal. Thus an atom is electrically neutral under normal conditions.

Methods of charging an object:

There are two methods due to which an object can be charged.

a. Charging by rubbing b. Electrostatic Inductiona. Charging by rubbing:

The process in which two neutral objects can be charged by rubbing it with one another is known as charging by rubbing.

Explanation:

When glass rod is rubbed with silk cloth, some electrons from the surface atoms of glass rod are transferred to silk cloth. This makes glass rod positively charged as it is now short of electrons. The silk cloth becomes negatively charged as it gains excess of electrons.


Electrostatic induction:

The process, in which a charge body changes the charge distribution of a nearby neutral body, is known as electrostatic induction.

Explanation:

When a charged body is brought near by a neutral body then the negative and positive charges of neutral body rearranges themselves on the surface as well as in the interior of the body. This effect is known as electrostatic induction.

This process occurs in insulator as well as in conductor. The neutral body regains its original state when the charged body is removed.

Charging an insulator by induction:

When a charged comb is brought near a neutral insulator, the atoms of the insulator induce itself in such a way that partial positive charge is produced on one end of the atom and partial negative on the other end forms a dipole as shown in the figure. One dipole induces another dipole. This phenomenon is known as polarization. In this way the comb attracts the insulator towards itself.

Charging by conductor by induction:

Consider a metallic sphere placed on insulating wooden stand as shown in the figure. When a positively charged rod is brought near the sphere, then the charge distribution of the sphere changes and the free electrons of the sphere will arrange themselves closer to the rod. The quantity of the negative charge on one end of the sphere is equal to the positive charge on the other end.

In this way the forces of attraction takes place between the rod and the sphere. Such distribution of the charge on the sphere due to rod is known as charging a conductor by induction.

Elecroscope:

It is an instrument used for detecting and testing the nature of charge on a body.


Construction:

A simple electroscope consists of a metal rod, which has metallic disc at its upper end while its lower end is attached to gold leaves as shown in the figure. The rod is fixed in a glass jar.

Charge Detection:

The presence of charge on a body can be detected by bringing it near the disc of an uncharged electroscope as shown in the figure. If the body is charged then the leaves of the electroscope will diverge to a definite extent, otherwise they would remain in their normal position. Thus the presence of charge can be detected from the divergence of leaves of electroscope due to electrostatic induction.

Testing the nature of charge:

For testing the nature of charge on a body we first need to charge the electroscope either positively or negatively by touching its disc with positively or negatively charged bodies. Due to charging the leaves of the electroscope diverge to certain extent. If a charged body is brought near the disc of the charged electroscope and the divergence of leaves increases it has same charge as that on the electroscope. If the divergence of the leaves decreases then it has opposite charge to that of the electroscope.

Coulomb’s law:

In 1785, Charles coulomb a French physicist performed various experiments and found that the force between two charge bodies depends on distance between charges and magnitude of charges. He stated it in the form of coulomb’s law.

Statement:

“The force of attraction or repulsion between any two bodies is directly proportional to the product of charges and inversely proportional to the square of distance between them.”

Explanation:

Consider two point charges q1 and q2 placed at a distance ‘r’ from one another as shown in the figure. According to coulomb’s law, the force F of attraction or repulsion between them is given by the relations

F ∝ q1 q2 −−−−−−−−1

F ∝1

r2−−−−−−−−−−−2

Combining equation 1 and equation 2 we get

F ∝q1 q2

r2

F = kq1 q2

r2−−−−−−−−−3


Where “k ” is constant of proportionality known as coulomb’s constant. The value of “k ”depends upon the nature of medium and system of units in which F, q and r are measured. If the medium is free

space and system of units is SI, then the value of k is given by.

k = 1

4 π εo

= 9 × 109 N .m

C2

Where “ε o ” is permittivity of free space and its value is8 .85 × 10−12 C2

N . m . By substituting the value

of k we can also write equation 3 as;

F = 1

4 π ε o

q1 q2

r2

Coulomb’s law is valid for point charges in which the size of charge bodies is small as compared to the distance between them.

Coulomb’s law in dielectric:

An insulating material (other than air or vacuum) known as dielectric placed in between the

point charges the force between the charges is reduced by factor ε r known as dielectric constant or

relative permittivity. The value of dielectric constant is different for different dielectrics. The force F¿

in presence of dielectric constant is given by.

F ¿ = 1

4 πε o εr

q1 q2

r2

Unit of charge:

The SI unit of charge is coulomb.

Coulomb:

One coulomb is the amount of charge which repels an equal and similar charge with a force of

9 ×109 N placed in air at a distance of 1 m from each other.

Electric field:

The space or region around a charge where it experiences electric force on test charges is known as electric field.

Explanation:

The region or space around the charge where it possesses special properties and experience electric force on other charges is known as electric field or force field. It is a vector field.

Electric field intensity:

The electric field intensity at a point is defined as;

“The force experienced on a unit positive test charge placed at a point with in the electric field.”


Mathematically:

Consider a force F is acting on a test charge +q at a point with in the electric field; then

the electric field intensity E at that point is given by.

E = F

+q

Electric field intensity is a vector quantity and its direction is same in which the test charge

will move. The unit of electric field intensity isN /C .

Electric lines of force:

The arbitrary lines used to represent the direction and magnitude of electric field is known as electric lines of force.

Properties of electric lines of force:

1. The lines of force are directed outward for positive charge and inward for negative charge.

2. The field pattern of set up by the two equal negative and positive charges is shown in the figure

3. The field pattern of set up by the two equal positive charges is shown in the figure.

4. The electric field between the two oppositely charged parallel plates is uniform at the central region as shown in the figure.

5. No two lines intersect each other.


Electrostatic potential:

The work done in bringing a unit positive charge from a point of infinity to a point with in the electric field is known as electrostatic potential at that point.

Mathematically:

If W is the amount of work done in bringing a test charge +q from infinity to a certain point in the field, the potential V at that point would be given by.

V = W

+q

Potential difference:

Potential difference between two points is defined as the work done in moving a unit positive charge from one point to another against the electric field.

Mathematically:

V B − V A =

W AB

+q

Electric potential is a scalar quantity. Its unit is volt

Volt:

When one joule work is done in bringing a unit positive charge from a point to another point with in the electric field then the potential at that point will be one volt.

Capacitor:

The device used for the storage of electric charge is called capacitor.

Construction:

A simple capacitor consists of two parallel metallic plates separated by a small distance as shown in the figure. Air or an insulating medium called dielectric is placed in between the plates.

Charging a capacitor:

The capacitor can be charged by placing one of the plates with the positive terminal of the battery while the other is connected to the negative terminal of the battery. Charge +Q and –Q appears on the plates. These charges attract each other due to mutual attraction and thus remain on the surface of the plates after the removal of the battery.


Capacitance of a capacitor:

The ability of the capacitor to store charge is known as capacitance of a capacitor.

Explanation:

It is experimentally found that the charge Q on the plate of a capacitor is directly proportional to the electric potential difference V between them.

Q ∝ V Q = CVOR

⇒ C =QV

Where ‘C’ is constant of proportionality known as capacitance of the capacitor. The value of ‘C’ depends upon the area of the plates, distance between them and the medium between them.

Unit of capacitance:

The SI unit of capacitance is farad. Which is defined as “the capacity of that capacitor which stores a charge of 1 coulomb by applying 1 volt potential difference between the plates”

Combination of capacitors:

The connection of capacitors in electrical circuit is known as combination of capacitors. The capacitor can be connected either in series or in parallel.

Series combination of capacitors:

In series combination of capacitors, the capacitors are connected plate to plate as shown in the figure. The right plate of one capacitor is connected to the left plate of the next capacitor so on. In series combination all the capacitors acquires the same amount of charge, the potential across each capacitor is different.

Characteristic features:

Consider three capacitors are attached to one another in series as shown in the figure.


1. The battery supplies +Q to the left plate of the capacitor C1 Due to electrostatic induction –Q charge is induced on its right plate and +Q charge on the left plate of capacitor C 2 as shown in the figure. Thus the charge on each capacitor is same i-e Q.

2. The sum of potential difference across each capacitor V1, V2, V3 is equal to the total potential difference supplied by the battery. i-e

V = V1 + V 2 V 3−−−−−−−1As Q = CV

⇒V = QC

V 1 =QC1

V 2 = QC2

V 3 = QC3

Putting all the values in equation 1

QC

=QC1

+QC2

+QC3

QC

= Q (1C1

+ 1C2

+ 1C3 )

1C

= 1C1

+ 1C2

+ 1C3

Let “Ce” be the equivalent capacitance of all the three capacitors joined in series.

1Ce

= 1C1

+ 1C2

+ 1C3

3. For ‘n’ capacitors the above equation becomes;

1Ce

= 1C1

+ 1C2

+ 1C3

.. . .. .. . .. .. ..+ 1Cn

4. The equivalent capacitance in series combination is less than any individual capacitance in the combination.

Parallel combination of capacitors:

In parallel combination of capacitors, all the left plates of all the capacitors are connected to one point while the right plates are connected to the other as shown in the figure. A battery of ‘V’ volts is applied to the both points.

In parallel combination of capacitors, the potential difference across each capacitor is the same while the charge stored in each capacitor is different.


Characteristic features:

1. In this type of capacitor the potential difference across each capacitor is same. i.e

V = V 1 = V 2 = V 3−−−−−−−1

2. The charge Q given at point A is distributed over all the capacitors. i.e

Q = Q1 + Q2 + Q3−−−−−−−1

As Q = C V

Therefore

Q1 = C1 V 1

Q2 = C2 V 2

Q3 = C3 V 3

Putting in above equation we get;

C V = C1 V + C2 V + C3 V

C V = (C1 + C2 + C3 ) V

C = C1 + C2 + C3−−−−−−−2

3. For ‘n’ capacitors we can write equation 2 as

C = C1 + C2 + C3 . .. . .. .. . .. ..+Cn

4. The equivalent capacitance in parallel combination is greater than any individual capacitance in the combination.

Different types of capacitors:

Capacitors are classified into various types depending upon their construction and nature of dielectric used in them. Here we will discuss the type of capacitors based on capacity.

1. Fixed capacitor: Those capacitors whose plates are immovable are known as fixed capacitors. The capacitance of such capacitors remains constant and cannot be varied. For example paper capacitor, oil, mica or electrolyte.

2. Variable capacitor: The capacitors in which the area of the plates facing each other can be changed are known as variable capacitors. The capacitance of such capacitors does not remain constant and can be varied. For example; the capacitors used in the tuning of radio.

Practical applications of electrostatics:

The practical applications of electrostatics are as follows;

1. Electro painting: Electrostatic is used in applying paints on the surfaces of different articles such as refrigerators, metallic furniture, cars etc. it has become a standard technology.


For this purpose the body is earthed electrically. The particles of the paint emerging out of the nozzle of the spraying, they acquire positive charge due to friction. Negative charge is induced on the surface of the body. Thus a firm coating of paint is formed on the surface of the body due to force of attraction.

2. Dust Extraction: Electrostatic phenomenon is used for the separation of smoke and dust particles from air coming out of various industries. Highly positively charged Wire gauze is fixed between two metallic plates which are grounded. Due to electrostatic induction negative charges are induced on the inner surface of the metallic plates. The gas containing dust and smoke particles become ionized after passing through the gauze which is trapped by the negatively charged plates. Thus the dust and smoke particles are separated from the air.

CONCEPTUAL QUESTIONS:1. A balloon is negatively charged by rubbing and then clings to the wall. Does this mean that the

wall is positively charged? Why does the balloon eventually fall?

Ans. A negatively charged balloon first cling to the wall, because due to its electric field the point of contact on the wall polarized positively due to electrostatic induction and attract the balloon due to electrostatic force of attraction and thus clings.

Later on the balloon is neutralized because the wall is grounded and due to gravity the balloon eventually falls.

2. Why is it not a good idea to seek shelter under a tree during a lightning storm?

Ans: When charged clouds pass over a tall tree, then an opposite charge is induced on the tall tree due to phenomenon of electrostatic induction. As a result electrostatic force of attraction exist between the tree and the clouds due to which there is a greater possibility of for lightning discharge to take place between the tree and the clouds. Therefore it is not a good idea to seek shelter under a tree during a lightning storm.

3. A charged comb often attracts small bits of dry paper that then fly away when they touch the comb. Explain?

Ans: When comb is brought near the bits, due to polarization, bits gets oppositely charged, hence they are attracted by the comb. When they touch the comb, at point of contact the charge is neutralized but the further points on the comb and bits have similar static charges so due to repulsion fly away.

4. Define the term volt?


Ans: Volt is the SI unit of potential difference. It is defined as;

“If one joule of work is done in carrying one coulomb of charge from one point to another against the electric field, then the potential difference between these two points will be one volt.

Mathematically:

1 v =1j

1C

5. What is meant by charging of an object electrically?

Ans: When positive and negative charge appears on a body, it means that the object is charged electrically. When the number of protons and electrons in an object are equal then the object is electrically neutral. Now when electrons are added to the body or removed from the body then the neutrality of the body is disturbed and is said to be charged.

Negative charge appears on the body if the number of protons exceeds the number of protons while positive charge appears if the number of protons exceeds the number of electrons.

6. Two isolated copper spheres A, B are initially uncharged. They are placed near each other as shown in the figure. What is the charge distribution on A when B is given a negative charge?

Ans: When the negative charged sphere ‘B’ is brought near neutral sphere ‘A’, then charge is induced on sphere A due to electrostatic induction. The positive charges on sphere A are arranged on the side which is closer to A while the negative charges are arranged to the left side of the sphere as shown in the figure.

7. Why electric charge is produced on bodies by friction?

Ans: When two bodies are rubbed together, then electrons are transferred from one body to another due to friction. The body which loses electrons becomes positively charged while the body which gain electron becomes negatively charged.

8. Two capacitors with capacitance 1μ F and 0 .01 μ F are charged to the same potential? Which will give more intense shocked if touched?

Ans: Given data:


C1 = 1 μ F

= 1 × 10−6 F= 10−6 F

C2 = 0 . 01μF

= 0 . 01 × 10−6 F= 10−8 F

V 1 = V 2 = V

Also we know that;

Q1 = C1 V and Q2 = C2 V

Putting the values we get;

Q1 = 10−6 V and Q2 = 10−8V

From above equations it is clear that Q1> Q2 for the same potential. Therefore the capacitor of

capacitance 1μ F will give more intense shock when touched.

9. As shown in the figure a chain hanging from the rear side of the petrol supply tanker. What is its purpose?

Ans: Petrol is highly inflammable fluid. The chain should be touching the ground to keep tanker’s body at zero potential and avoid a spark discharge due to which petrol can catch fire.

10. The tyres of an aircraft are made from special rubber that conducts electricity. Explain why?

Ans: Aircraft’s when moving in air with high speed become charged due to friction due to friction with air molecules and with various cloud. When the aircraft is landing and touches the ground, the charges flows to the ground through tyres, the aircraft body becomes neutral and thus avoids the sparking. In this way the aircraft remains in safe and sound condition.

Written by: Zahoor Ahmad

M.Sc PHYSICS


CHAPTER NO: 14 CURRENT ELECTRICITY

Current electricity or Electrodynamics:

The branch of electricity which deals with the study of electric charges in motion is known as Current electricity.

The study of current electricity is more important than electrostatics, because it has more applications in our daily life.

Electric current:

The rate of flow of current through any cross sectional area of a conductor is called electric current.

Explanation:

If charges ‘Q’ passes through any cross sectional area ‘A’ in time ‘t’ seconds. Then the current ‘I’ flowing through it is given by equation.

I = Q

t

Electric charges are of two kinds, positive and negative. So electric current is produced due to the flow of negative or positive charges or both. In conductors it is due to the flow of free electrons i.e negative charges. In nuclear accelerators, current flow due to the positive charges while in electrolytes, it is due the flow of both positive and negative charges.

Unit of electric current:

The SI unit of electric current is ampere.

Ampere:

When 1 coulomb charge flows in cross sectional area of a conductor in unit time the current passing through it is 1 ampere.

1 A = 1 C1 s


Conventional current:

The electric current produced due to flow of positive charges is known as conventional current. The direction of conventional current is from positive towards negative terminal of a battery.

Explanation:

Early scientists thought that current is due to flow of positive charges but today it is well known that in metals or vacuum tubes the current is due to flow of free electrons i.e negative charges.

In electrolytes current is produced due to the flow of both positive and negative charges and in nuclear reactor it is due to the flow of positive charges (protons). The positive charge moving in one direction is equivalent to a negative charge moving in opposite direction.

Ohm’s law:

In 1826, German scientist George Simon Ohm after performing several experiments find a relation between current flowing in a conductor and the voltage applied across the ends of the conductor. He stated it in the form of ohm’s law.

Statement:

“The current flowing through an electrical conductor is directly proportional to the voltage applied across the ends of the conductor by keeping the temperature and other physical condition constant.”

Mathematically:

I∝ V ( Physical state and Temperature= constant )

⇒ I = 1R

V 1R

=Constant

⇒ V = I R

Where R is constant of proportionality and is called resistance of the conductor.

Limitation of Ohm’s law:

Ohmic conductors or linear conductors:

The conductors which obey Ohm’s law are known as Ohmic conductors.i-e. I ∝ V Examples of Ohmic conductors are metals and some alloys at constant temperature.

Metallic conductors:

When we plot I-V graph of metallic conductors at constant temperature, it

gives straight line with a slope of

1R as shown in the figure. The resistance


of the Ohmic conductor does not change when the potential difference across it changes, by keeping the temperature constant.

Non-Ohmic conductors:

The conductors which do not obey Ohm’s law are known as non-Ohmic conductors. The I-V graph of such material is nonlinear. For example filament of an electric bulb, thermistors and diodes etc.

Filament of an electric bulb:

The I-V graph for a filament bends over as I and V increases as shown in the figure. This means that the slope decreases with increase of voltage and is not a straight line. Thus the resistance of the tungsten wire filament increases as the current rises its temperature and make it white hot.

Thermistor:

A thermistor is a type of resistor whose resistance varies significantly with temperature. The I-V graph of thermistor bends upward as shown in the figure. This means that the resistance decreases sharply with the rise in temperature. Thermistors are made up of semiconductors.

Resistance:

The property of a substance to oppose the flow of current passing through it is called resistance.

Mathematically:

When potential difference V is applied across the end of a conductor due to which current I flows through it. The resistanceR offered by the conductor is given by.

R = V

I

Explanation:

It can be seen that if the value of resistance R is increased for a certain value of potential V then the value of current would be small. It means that the opposition to the flow of current would

be large. On other hand, if R is small then the opposition to the flow of current will be small and large

current will flow through the material. Thus R is the measure of resistance of the conductor.

Unit:

The SI unit of resistance is ohm represented by Greek letter omega (Ω )

Ohm:


The resistance of a conductor will be one ohm when one volt potential difference is applied across its ends and the current flowing through it is one ampere.

1 Ω = 1 V

1 A

Specific resistance or resistivity:

The resistance of a material of unit length having unit area of cross-section.

Explanation:

As we know that in a conductor electrons do not move in straight line path but they repeatedly collide with the metal atoms. At certain temperature the resistance of a conducting wire depends upon its length, area of cross section and nature of the material. Laws of resistance states that, the resistance of a conductor is

1. Directly proportional to its length.

R ∝ L−−−−−−−12. Inversely proportional to cross sectional area.

R ∝ 1

A−−−−−−−2

Combining equation 1 and 2 we get

R ∝LA

⇒R = ρLA

Where ρ (rho) is the intrinsic property of the conductor and is known as specific resistance or

resistivity. If L=1m and A = 1 m2 then R = ρ

The SI unit of specific resistance is ohm- meter (Ω−m )

Combination of resistors:

The arrangement of resistances in a circuit which affects the flow of current through each resistor and the voltage drop across each resistor is known as combination of resistors. Resistors are connected in two basic ways; in series and in parallel.

1. Series combination of resistors: In series combination the resistances are connected in such a way that the flow of current through each resistor is one after the other. Thus same amount of current flow through all resistances.Explanation:

Consider we have three resistances R1 ,R2 and R3 be connected in series as shown in the

figure. Let I be the current flowing through them and V be the potential difference applied

across its ends. V1, V2 and V3 are the potential differences across R1 ,R2 and R3 such that

V = V 1 + V 2 +V 3

From ohm’s law we know that


V = I Re

⇒V 1 = I R1

⇒V 2 = I R2

⇒V 3 = I R3

Where Re is the equivalent resistance of the combination. Putting the values in equation 1 we get.

V = V 1 + V 2 +V 3

I Re = I R1 + I R2 + I R3

I Re = I ( R1 + R2 + R3 )Re = R1 + R2 + R3

Hence when a number of resistances are connected in series, the equivalent resistance of the combination is equal to the sum of the individual resistances.

2. Parallel combination of resistances: In parallel combination of resistances the resistances are connected in such a way one end of each resistance is connected to one common point and the other end to the other common point so that there are different paths for the flow of current.Explanation:

Consider we have three resistances R1 ,R2 and R3 be connected in parallel as shown in

the figure. Let current “I ” reaching to point A divide itself into three parts I 1 ,I 2 and

I 3 along

R1 ,R2 andR3 respectively such that

I = I 1 + I 2 + I 3−−−−−−−−1

Let V the potential difference between A and B and Re be equivalent resistance of the combination we know from Ohm’s law that;

V = IR

⇒ I 1 = VR1

⇒ I 2 = VR2

⇒ I 3 = VR3

Putting all the values in equation 1 we get

VR

=VR1

+VR2

+VR3

VR

= V (1R1

+1R2

+ 1R3 )

1R

= 1R1

+1R2

+ 1R3

Hence when a number of resistances are connected in parallel, then reciprocal of the equivalent resistance of the combination is equal to the sum of reciprocals of the individual resistance.


Direct current (D.C.) and Alternating current (A.C.):

Direct current:

The current which always flow in same direction is known as direct current.

Explanation:

The current which we get from a cell or a battery is direct current because it always flows in

the same direction. The polarity of direct current is fixed. Direct current in short form can be written as

D.C.

Alternating current:

If the current changes direction after equal interval of time, it is called alternating current.

Explanation:

Consider a resistance R whose A end is connected to the alternating voltage source while the B

end is earthed (at zero potential) as shown in the figure.

The voltage at the end A varies with time is represented by the graph. We observe that the graph

between the time intervals 0-T, T-2T, 2T-3T is exactly the same. The time interval after which the

voltage repeats its value is known as time period.

During the time interval 0-T/2, the potential of the end A is positive with respect to end B. so in

this interval the current flows from A to B. It starts from 0, gradually increases to maximum value and

finally decreases to 0. The wave form is called positive half cycle.

During the interval T/2 – T, the potential of end A gets negative with respect to end B. so in this interval the current flows from B to A. It starts from 0, gradually increases to maximum value and finally decreases to 0.

The wave form is called negative half cycle.

This positive half cycle and negative half cycle repeats its selves again and again in equal interval of time. Such a current is known as alternating current. The number of cycles in one second is known as its frequency. The frequency of A.C used in our houses is 50 cycles per second.

Circuit:

The path required for the continuous flow of current is known as circuit.


Circuit components:

The components which are connected in a circuit are known as circuit components. For example switches, batteries, resistors, etc.

Switches:

It is an electric circuit control element with two discrete states i.e on and off or closed and open. As long as the switch is open no current flows through the circuit having infinite resistance. When the circuit is closed the current flow in the circuit having zero resistance. It is denoted by

.

Resistors:

Resistors are the devices which provide resistance so that when connected in a circuit they reduce the current to a desired value. They are made up of wire coils of special alloys or from carbon.

The symbol of resistor is . Variable resistor is used in electronics having symbol of Batteries: Batteries are the devices which make a continuous flow of current in a circuit. It is a combination of many cells. They converts chemical or solar energy to electrical energy and are denoted

by .

Measuring instruments:

1. Galvanometer: Galvanometer is an instrument by which we can detect the presence of current in a circuit. In order to detect current in a circuit galvanometer is connected in series. If its needle shows deflection, it would indicate the presence of current in the circuit. Galvanometer is also used to detect the current between two points in a circuit.

2. Ammeter: Ammeter is an instrument which is used for measuring large amount of current. Ammeter is modified form of galvanometer. A galvanometer can be modified into ammeter by connecting low shunt resistance in parallel. In order to measure current in a circuit ammeter should be connected in series.

3. Voltmeter: A voltmeter is an instrument used for the measurement of potential difference across two points in an electrical circuit. It is connected in parallel to the branch across which the potential difference is to be measured. A galvanometer can be modified to voltmeter by connecting high shunt resistance in series.

Electric energy:

The energy which we get due to the flow of electrons is known as electric energy.

Explanation:

Consider a charge “Q” passes between two points having potential difference of “V ” volts. Work “W” is to be done in moving the charge from one point to another which is stored in the form of potential energy. Which is given by;

V =WQ

⇒ W = Q V---------1

Also we know from the definition of current i.e. the rate of flow if charge through a conductor.


I =Qt

⇒Q = I t−−−−−−−−2

Putting in equation 1 we get.

W = I t V------------3

If current is passing through a resistor R and potential difference across its ends is V volts. Then from Ohm’s law we have;

V = I R

Putting in equation 3 we get;

W = I t ( I R )W = I 2 R t

Also from Ohm’s law we know that I = V

R putting in above equation we get;

W = (VR )2

R t

W =V 2

R2R t

W = V 2 tR

Electric energy is generated by generators and cells, which can be utilized for different functions in various resistors.

Unit of energy: T he SI unit of energy is joule. However on commercial scale kilowatt-hour is used

which is equal to3 .6 ×106 J .

Electric power:

The rate at which work is done in an electric circuit is called electric power.

Mathematically:

P =W

t−−−−−1

Also we know that W =Q V putting in equation 1 we get.

P =Q Vt

⇒ P = (Qt ) V ∴Qt

= I

⇒ P = I V

Also we know that W = I 2 R t putting in 1 we get;


P =I 2 R tt

⇒ P =I 2 R−−−−−2

From Ohm’s law we have I=V

R putting in 2 we get;

P =(VR )2

R

⇒P = V 2

R

Unit of power:

The SI unit of power is watt.

Short Questions

1. Electric current is a flow of charge, why two wires rather than a single is used to carry current?

Ans: The movement or flow of electricity along a wire is similar to the flow of water along a pipe. Water will flow from one end of the pipe to the other when the two ends have a difference in level or height.

In a similar way, electricity will flow along a wire when the two ends have an electrical difference in level i.e, potential difference. Therefore, for the flow of electric current two wire i.e, live wire and neutral are required. The potential of live wire is higher than that of the neutral wire.

2. Which solids are good electrical conductors and which are good insulators? How will do these substance conduct heat?

Ans: Metals are good electrical conductors, because the electric current can pass through metals very easily. For example, copper, silver, gold, aluminum etc. On the other hand plastic, dry wood, rubber etc are good electrical insulators, because they do not allow electric current to pass through them.

The good electrical conductors are also called good heat conductors, because the heat can pass through them easily. While the heat cannot pass through good insulators, so they are also known as good heat insulators.

3. State how the resistance of a wire would change, i. If its length is increased.

ii. If its diameter is decreased.

Ans:

We know that the resistance of a conductor is given by,

R= α L/A ---------------- (1)

In eq (1), A= area of cross- section of wire= πr²

A=π r2 = π(d/2)² radius = Diameter/2 => r=d/2

A = πd²/4 ------------- (2)Putting eq (2) in eq (1), we get,


R = α L/ ¿²/4) = 4 α L / πd² R = 4 α L/ πd² ------------------- (3)

i. Eq (3) shows that if the length ‘L’ of the wire increases, then its resistance will also increase.ii. Eq (3) also shows that the resistance ‘R’ of the conductor is inversely proportional to the

diameter of the wire. So if the diameter of the wire decreased then its resistance will increase.4. To reduce the brightness of a light bulb an auxiliary resistance be connected in series with it or in

parallel? Why?

Ans: In order to reduce the brightness of a light bulb, the auxiliary resistance should be connected in series. It is because,

(1) In series combination, the equivalent resistance increases.(2) In series combination, the rate of loss of heat decreases.(3) In series combination, the potential difference is divided into parts.

5. A number of light bulbs are to be connected to single power outlet. Will they provide more illumination if connected in series or in parallel? Why?

Ans: The bulbs should be connected in parallel for getting more illumination. It is because,

(1) In parallel combination the potential difference across each bulb is the same i.e, the potential difference remains undivided.

(2) The equivalent resistance decreases in parallel combination.(3) By switching ‘OFF’ any bulbs, there is no effect on the brightness of the other bulbs.

6. What is meant by the statement that an electric kettle has power of 2 K watt?

Ans: if an electric kettle is marked as 2 K watt. Then it means that its power is 2000 watt. When an operating voltage of 240 volts is applied across its heating element then it will convert electrical energy into heat energy at the rate of 2000 j/sec.

(1 watt =1joule/see)

7. When an incandescent lamps burn out they usually do so just after they are switched on. Why?

Ans: In incandescent lamps the light is amend (generally tungsten). In these lamps only a small percentage of electrical energy supplied is converted into light. The major part of electrical energy is converted into thermal energy that is why they may burn out when switched on.

8. Why is it possible for a bird to perch on a high voltage wire without being electrocuted?

Ans: it is possible for a bird to perch on a high voltage wire without being electrocuted. It is because the whole body of the bird sitting on a high voltage bare wired due to zero potential different no current passes through the body of the bird therefore the bird is not electrocuted.

9. Why is it dangerous to turn on a light when you are in the bath tub?

Ans: The human body is a good conductor of electricity i.e. heavy current may pass through his/her body when touches a live wire. Therefore when a person in a bath tub tries to turn on the switch of a bulb he may get an electric shock. Thus for safety measure he should not do so.


CHAPTER NO: 15 ELECTROMAGNETISM

Q1: What ways are electric and magnetic fields similar? In what ways are they different?

Ans: Similarity of electric and magnetic field:

I. Both the fields are represented by lines of force.II. In both the fields lines of force show the direction i.e. both the field are vector field.

III. In both fields the lines of force shows the magnitude (strength) of the field.

IV. The changing of one field generates the other filed.

Differences of electric and magnetic field:

I. Strength of electric field is measured in ampere while strength of magnetic field is measured in gauss (G) or tesla.

II. An electric field may do work on charge particle while a magnetic field does not work.III. Electric field lines are generated on positive charges and terminate on negative charge

while magnetic field lines in case of a magnet, are generated at North Pole and terminate on South Pole.

2. A wire carrying a current is placed in a magnetic field B. (a) under what circumstances, if any, will the force on the wire be zero? (b) Under what circumstances will the force on the wire be maximum?

Ans: (a) if a wire carrying current is placed to the magnetic field B. the force on the wire is zero. It means

θ = 0


F = B I L sin θF = B I L sin 0F = 0

b) If a wire carrying current is placed perpendicular to the magnetic field B, then the force on the wire is maximum. It means

θ = 90

F = B I L sin θF = B I L sin 90Fmax = B I L

3. What would happen if the primary winding of transformer were connected to a battery?

Ans: Transformer is a device in which alternating e.m.f in primary coil induces an alternating e.m.f in the secondary coil.

If two primary winding of a transfer is connected to a battery nothing would happen because battery is a source which provides only direct current and direct current cannot produce any charge in primary coil. As a result no induced e.m.f produces in the secondary coil.

4. What are the similarities between motor and generator? What are the differences?

Ans: Similarities:

1. Both are consist of coils2. In both carbon brushes are used 3. Both have magnetic poles.

Differences:

1. A motor converts electric energy into mechanical energy while a generator converts mechanical energy into electrical energy.

2. Motor is based on the principal that current carrying coil in a magnetic field is acted upon by a torque, which tends to rotate the coil while in case of generator if the coil is placed in a uniform magnetic field, it will rotate.

3. Motor coil is connected to split ring while generator coil is connected to slip ring.

1 Explain the left-hand rule to determine the direction of force on a current carrying wire placed in a magnetic field.

Ans: The magnetic force on a current carrying wire can be demonstrated by the following experiment. Pass a straight wire through magnetic field between the poles of a U- shaped magnet. The ends of the wire are connected through a switch to the terminal of a battery. You


can observe that by turning on the switch, the current carrying wire is immediately pushed to a side. Increasing the current in the wire increases the push. Reverse the current in the wire, the direction of the push in also reversed. Reverse the direction of the magnetic field by exchanging the positions of the N and s-pole, the magnetic force on the wire is reversed. On the basis of this experiment, Michael faraday discovered that when a wire and a magnetic field are at right angle, a force acts on the wire due to the interaction of the fields. The force is proportional to three factors;

I. The magnetic field “B”II. The current “I” in the wire

III. The length of the wire “L”

The force acting on wire is proportional to the product of L, L and B.F=ILB

The direction of the force acting on wire can be determined by Fleming’s left hand rule.

According to this rule, stretch the thumb, forefinger, and the middle finger of the left hand mutually at right angles to each other. If the forefinger points in the direction of the magnetic field, the middle finger in the direction of the current, then the thumb would indicate the direction of the force acting on the wire.

2. Explain how a wire and a strong magnet generate an electric current.

Ans: Electricity can be generated by changing magnetic fields in a coil of electrically conducting wire.

Material used for this purpose is strong magnetic, coil of electrical wire wrapped around a tube ammeter to measure electric current. Take a strong magnet and fit it inside the wire coil tube. Slide the magnet through the wire coil, in one direction and use an ammeter to observe the strength and sign (positive or negative) of electrical current produced.

Moving a magnet through a coil of wire-or moving a coil of wire by a magnet-induces electrical current in the wire. The direction that the current flows through the wire depends on the relative direction of motion between the magnet and the wire coil.

The strength of the electrical current depends on:

1) The strength of the magnet,2) The number of wraps of wire in the coil, and 3) How quickly the magnet and coil move past each other.


The more wraps of wire the coil is made from, the more electrical current is generated. If you think of each turn of wire in the coil as a single, independent loop, or ring, each of which will have electrical current induced by the passing magnet, then you can understand how a bunch of rings together produce more current.

3 Sketch and describe a D.C motor.

Ans: D.C motor: D.C motor is a device, which converts electrical energy into mechanical energy.

Principal: the principal of an electric motor is that a current carrying coil in a magnetic field is acted upon by a torque, which tends to rotate the coil.

Construction: An electric motor consists of a armature which is a coil of N turns wound on an iron core. The armature can rotate about an axis perpendicular to the magnetic field of an electromagnet. The terminals of the coil are connected to split ring (commutator) R. Brushes b, b’ supply current from the battery commutator reverses the supplied current every half rotation.

Working: When current I is passed in the armature coils them the coil is acted upon by a torque until the plane of the coil is perpendicular to the field, where torque becomes zero, at this instant, the split-ring system reverse the direction of the current and the armature continues to rotate. The rotating armature provides mechanical power.

Uses: Electric motors are used in electric fans, water pumps, electrics drills, washing machines, radar, communication devices, telephone, television etc.

4 Explain the phenomenon of mutual induction.

Ans: Mutual induction: The phenomenon, in which current is induced in a circuit due to a change of current in another circuit, is known as mutual induction. OR

The phenomenon in which induced e.m.f is produced in the secondary coil when current is changing the primary coil is known as mutual induction.

Explanation: In the given figure are two coils placed near each other. The coil A is placed in battery circuit and is called primary coil. The coil B is connected with galvanometer and is called the secondary coil.

According to the principle of electromagnetic induction a changing current in one coil can produced an induced e.m.f in a neighboring coil. So starting, stopping or varying current in a


primary coil a will produced e.m.f in coil B. The current in coil B lasts only as long as current in the coil A is changing.

5. Sketch and describe an A.C generator.

Ans: A.C Generator: A.C generator is a device, which converts mechanical energy into electrical energy.

Principle: The principle of A.C generator is based on electromagnetic induction. An induced emf is produce by changing magnetic flux through the coil.

Construction: A generator consists of a rectangular coil of N turns wound on an iron core. The assembly of coil plus cylinder is called armature. It can rotate about an axis OO’ perpendicular to a uniform magnetic field of flux intensity ‘B’. Ring R,R’ are connected to the terminals of coil. Brushes bb’ slide along the rings and connect the coil to the external circuit.

Working: Suppose in the beginning the coil is vertical with its side A upward and side B downward. The sides A and B are moving parallel to the field, so rate of change of flux through the coil is zero and induced e.m.f is zero as well. After completing one right angle to the field and rate of change of flux is maximum. In this way the rotation of the coil in a magnetic field generator an alternating induced e.m.f.

Uses: AC generators are installed at Tarbela, Mangla and Warsak etc and are used to convert the kinetic energy of water falls into electrical energy. In petrol and diesel operated AC generators, the chemical energy is converted into electrical energy.


6 Write note on transformer.

Ans: Transformer: AC transformer is a mutual inductance electrical device which steps up or steps down Ac voltage.

Principle: A transformer operates on the principle of mutual induction. A changing current in a primary coil can cause an induced e.m.f and induced current in a neighboring secondary coil.

Construction: A typical 6transformer has an iron core on which two coils are wound, primary coil with Np turns and secondary with Ns turns. The primary coil is connected to A.C voltage Vp and output Vs is obtained across the secondary coils terminals.

Working: The AC in the primary coil produces a changing magnetic flux in the iron core. The changing flux through the secondary coil give rise to an induced e.m.f Vs in the secondary coil. The voltage and number of turns in primary and secondary coils are related by equation.

Types:

1. A transformer that raises the input voltage Vp to a higher voltage Vs has Ns>Np and is called a step up transformer.

2. A transformer that decreases the input voltage Vp to a lower voltage Vs has Np>Ns and is called a step down transformer.

Uses:

1. For charging voltage in A.C circuit.2. In the designs of circuits for commununication, in radio, television, telephone, radar

etc.3. In transmission and distribution of electrical power from the distant generating plant

to industrial and household users.

7 What are magnetic effects of a steady current?

Ans: following the magnetic effects of a steady current.


I. Field due to a straight wire: To investigate the magnetic field about a current carrying wire, place a wire vertically through a piece of card board. Pass an electric current through the wire. Sprinkle iron fillings on the cardboard around the wire. Tap the card board until the fillings arrange themselves in concentric circles. The circular lines indicate that the line of force form closed loops. The intensity of the magnetic field about the wire baries directly as the magnitude of the current flowing in the wire. The magnetic field of this current can also be traced by means of a magnetic compass. To find the direction of the magnetic field about the wire, use the right hand rule. The rule states that if “the current carrying wire is grasped by the right hand with the thumb in the direction of the current; the fingers encircle the wire in the direction of the field.”

II. The magnetic field around a coil: The strength of the magnetic field step up by a piece of wire carrying a current can be enhanced at a specific location it the wire is formed into a loop. The magnetic field inside the loop is created by the current flowing thought the wire. The magnetic field runs in the same direction all around the loop. Apply the right hand rule around the loop. The fingers always point in the same direction inside and outside the loop. Notice in (fig) that the magnetic field lines enter at the left side of the current loop and exit at the right. Thus, one side of the loop acts as through it were the north pole of magnet and the other act as a south pole. The fact that the field setup by such a current loop bears a striking resemblance to the field of a bar magnet.

III. The magnetic field due a solenoid: If a long, straight wire is wrapped into a coil of several closely spaced loops, the resulting device is a solenoid or, as it is often called, an electromagnet. This device is important is many application since it acts like a magnet only when it carries a current. The magnetic field inside a solenoid increases


with the current and is proportional to the number of coils per unit length. Note that the field lines inside the solenoid are merely parallel, uniformly, spaced and close together. This indicates that the field inside the solenoid is uniform and strong. The exterior field at the sides of the solenoid is non uniform and is much weaker than the field inside. The field lines enter at one end of the solenoid and emerge at the other. This means that one end of the solenoid acts as though it were the north pole of a magnet whiles the other and acts as though it were the South pole fig(a). The polarity of a current carrying solenoid can be determined by the following rule. Hold the solenoid in your right hand by curling the fingers in the direction of the current; the stretched thumb would indicate towards the North Pole Fig(b).

8 Explain torque on a current carrying coil in a magnetic field.

Ans: Torque is exerted on a current loop placed in magnetic field. The results of this analysis will be of great practical value when discuss galvanometer and generators.

Consider a rectangular loop ABCD carrying current “1” in the presence of an external uniform magnetic field as show in fig. the forces on the sides AD and BC are zero because these wires are parallel to the field. The magnitude of the forces on sides AB and DC, however is

In sides AB and DC the direction of current is at right angle to the field. According to Fleming” left hand rule, the forces acting on these perpendicular sides would act horizontally in the opposite direction (fig). The magnitude of these forces would be equal. Under the action of these two equal but opposite forces a couple would act on the coil due to which it would start rotating.


9 Write a note on moving coil loud speaker.

Ans: Most Loudspeakers are of the moving coil type, which uses the fact that a current carrying conductor experiences a force when in a magnetic field. A loud speaker consists of a permanent magnet with a central cylindrical pole and a surrounding ring pole which creates a strong magnetic field in the gap between the poles, a short cylindrical coil which can move backward and forward through a short distance in the magnetic field and whose turns are at right angles to the field; and a paper cone attached to the coil.

When alternating current from say, a radio or a record player passes through the coil it is forced to move in or out depending on the current direction according to Flemings” left hand rule. The paper cone moves with the coil and thus sets up sound waves in the surrounding air of the same frequency as that of the alternating current.

10 Write note on electromagnetism.

Ans: The magnetism produced due to the flow of current is known as electromagnetism. This shows that magnetic effect can be produced by electric effect. This was first of all proved by a Dutch scientist Oersted in 1820. He discovered that when current passes through the conductor then besides electric field it can also produce a magnetic field where as a moving charge produces both electric and magnetic field.

Ousted experiment can be practically demonstrated as. Take a long current carrying wire and pass it through a card board. Both of the wires are attached to the terminals of the battery. Some iron fillings are sprinkled over the card board. When the current is allowed to pass


through the wire, then the iron fillings will be attracted by the wire. Which shows that magnetic field is produced around the wire.

CHAPTER NO 16 INTRODUCTORY ELECTRONICS

Electronics:

The branch of applied physics which deals with the development of electron emitting devices and their utilization is called electronics.

Q4. What do you mean by analogue?

Ans: Electronics can be classified into two main branches: analogue electronics and digital electronics. In practical application, a combination of both branches is often utilized.

Those quantities whose values vary continuously or remain constant are known as analogue quantities. For example, the temperature of the air in the atmosphere is continuously changing during day and night. The branch of electronics consisting of such circuits, which processes analogue quantities, is called analogue electronics.

Q. Explain construction, working principle and use of C.R.O

Ans: Cathode-ray Oscilloscope:

Cathode-ray tubes have become part of everyday life. They can be found in the screens of television sets and computer monitors. In the physics laboratory, we use the cathode-ray tube in the oscilloscope to study waveforms.

The cathode-ray oscilloscope (C.R.O) consists of the following components:

i. The electron gun.ii. The deflecting plates.iii. A fluorescent screen.i. The electron gun: It consists of a grid, which is connected to a negative potential. The

more negative is the potential, the more electrons will reach the anode and the screen. The number of electrons reaching the screen determines the brightness of the light. Hence, the negative potential of the grid can be used as a brightness control. The other feature in the electron gun is the use of the anode. The anode at positive potential


accelerates the electrons and the electrons are focused into fine beam as they pass through the anode.

ii. The deflecting plates: Two pairs of plates are fitted in C.R.O. Voltages can be applied to these plates so as to deflect the electron beam. The Y-plates will cause deflection in the vertical direction when a voltage is applied across them. On the other hand, the X-plates will cause the electron beam to be deflected in the horizontal direction if a voltage is applied across them.

iii. The fluorescent screen: The screen is coated with a fluorescent salt, for example, zinc sulphide. When the electrons hit the screen, it will cause the salt to produce a flash of light and hence a bright spot on the screen.

Uses: A cathode ray oscilloscope (CRO) is used to see the waveform of a repetitive electron signal. The signal is amplified or attenuated as required and used to deflect an electron beam in the vertical direction. This electron beam is deflected in the horizontal direction at a suitable speed. The electron beam impinging on a screen enables the viewer to see the wave shape of the signal.

Q2. What are logic gates? Draw symbol and truth table of the following logic gated.

(a) AND gate(b) OR gate(c) NOT gate(d) NAND gate(e) NOR gate

Ans: Logic gates: The electrical circuits That Perform one or more logical operations on one or more input signals are called logic gates.

Precious articles, cash and impotent documents etc are kept in safes. The door of a safe is opened and closed by a certain order of a set of switches. A systematic ordered system is called a logic gate. If the buttons of switches are pressed in right order, the door of the safe will open. However, if the buttons are pressed in wrong order, the alarm begins to ring.

(A) AND gate:

The AND gate has two inputs switches A and B. The bulb Q will light if both the switches are closed. This will allow current to flow through the bulb, illuminating the filament. The AND gate is an electronic circuit that gives a high output (1), only if all its inputs are high. A dot (.) is used to show the AND operation.

(B) OR gate:


The OR gate consists of two input switches A and B. the bulb Q will be light if either A or B is switch closed. This allows the current to flow through the bulb, illuminating the filament when either of the switches is closed. The OR gate is an electronic circuit that gives a high output (1) if one or more of its inputs are high. A plus (+) is used to show the OR operation.

(C) NOT gate: The NOT gate is an electronic circuit that produces an inverted

version of the input at its output. It is known as inverter. If input variable is A, then the inverted output is known as NOT A. this is

shown as A .Its symbols and truth table are given in the figure.

(D) NAND gate: This is a NOT-AND gate which is equal to an AND gate followed by a

NOT gate. The outputs of all NAND gates are high if any of the inputs are low. The symbol is an AND gate with a small circle on the output. The small circle represents inversion.

(E) NOR gate: This is a NOT-OR gate which is equal to an OR gate followed by

a NOT gate. The inputs of all NOR gates are low if any of the inputs are high. The symbol is an OR gate with a small circle on the output as shown in the fig. The small circle represents inversion.

Q3. Explain the difference between analogue and digital electronics.

Ans: Analogue electronics:

The branch of electronics consisting of such circuits, which processes analogue quantities, is called analogue electronics.

Those quantities who values very continuously or remain consist are known as analogue quantities. For example, the temperature of air in changing during day and night

Digital electronics:

The branch of electronics, which provides the data in the form of a maximum and minimum voltage signals, is known as digital electronics. In this type of data, the changes are not continuous. In fact, one


part of the data quite distinct from the other part of the data. Modern telephone system, radar system, naval and other communication system of military importance is using digital electronics.

Q4: Write a note on thermionic emission.

Ans: In a metal, each atom has a few outer electrons which move randomly through the material as a while. We can these electrons as free electrons because they can move freely through the metal. An electron travelling outwards on the surface cannot escape as it is held back by the attractive forces of the atomic nuclei. When a metal is heated, however, some of the electrons may gain enough energy to escape from the surface of the metal. This effect is known as thermionic emission.

Thermionic emission can be produced by electricity heating a fine tungsten filament.

Q5. Write a note on electron gun. How the path of the free electron os effected by the (a) electric field (b) magnetic field

Ans: An electron gun is used to investigate the properties of electrons beams. It consists of a glass tube at a very low pressure. The electrons are produced by thermionic emission from a tungsten filament heated by a 6v supply. A high positive potential (several thousand volts) is connected to a cylindrical anode (+). The electron will be accelerated to the anode in a fine beam of electrons. This electron beam is called as cathode rays as they are produced from the cathode or negative electrode (-). The path of these free electrons can be affected by electric and magnetic field.

(a) Deflection by electric field: An electric field can be set up by applying a potential difference across two parallel metals placed horizontally some distance apart. When an electron beam passes between the two plates, it can be observed. The reason being that electrons are attracted by the positive plate and repelled by the negative plate.

(b) Defection by magnetic field: A magnetic field is applied at right angles to the beam of electrons by having two poles of the beam of electrons by having two poles of the magnet placed as shown is fig: it can be observed that the electrons beam is deflected by magnetic field. Since electrons are negative, the beam of electrons would be equivalent to a positive or conventional current in the opposite direction.

Q6: what are the uses of logic gates?

Ans:. Logic gates are in fact the building block of digital electronics. Every digital product, like computer, mobile, calculators even digital watches, contains logic gates, logic gates are also used to perform different types of hobs in digital electronics i.e. house safety alarm etc.

Wireless door and window alarm:

To prevent that your child do not walked outside the front door without telling you, a special type wireless alarm can prevent such a risk and keep your child safety inside. When the door is opened the alarm emits a continuous 120DB sound (until the door is shut) to tell you that your child is at risk of leaving. Thus digital electronic has facilitated us in all respect of our daily life.


CHAPTER NO 17 INFORMATION AND COMMUNICATION TECHNOLOGY

Information technology:

The scientific method of storing, protecting, processing, transmitting, receiving and retrieving information is called information technology.

Telecommunication:

The method that is used to communicate informations to far off places instantly is called telecommunication.


Telephone, radio, T.V, fax, telex machine and machine are a few of contemporary devices used for telecommunication

Q1. Define I.T.

Ans: Information technology:

The scientific method of storing, protecting, processing, transmitting, receiving and retrieving information is called information technology.

Modern science and technology are very rapidly discovering and inventing new techniques, skills and knowledge. Sometimes back, telephone was the source by means of which you could talk to your relatives across the sea. Today, there are various means of communication, for example, Fax and computer internet. Presently, the world has becomes a small well-informed and well connected village which is called a global village.

Q2. Write input devices of a computer.

Ans: In the computer, calculation and instruction are inserted by an input unit. Usually a key broad, similar to a type writer is used for this purpose. The different functions of the keyboard are also controlled by a special instrument called a mouse. There are several other methods to feed information into the computer, which includes floppy disks and CD’s. Input devices send the received data to CPU so that its arrangement and analysis may be done.

Q3. What do you mean by graphic designing?

Ans: The process to draw a required line or pictures on a computer screen using mouse or keyboard is called the graphic designing.

Designs of the building or components can also be drawn with help of computer. This process is named as computer aided designing (CAD). Three dimensional (3D) colored pictures can be draw by this process which can be checked by rotating it at different angles. Moreover different colors can also be selected of the pictures. This process because of accuracy and comport is very popular in industrial field.

Q4. ATM stands for what?

Ans: ATM stand for automated teller machine or automatic teller machine. An ATM for deposits, withdrawals, account information and other types of transaction often through interbank networks.

Q5. SIM stands for what?

Ans: SIM stands for subscriber identification module. They are used in mobile sets. SIM is used for talking to other people. It can also store number in its memory.

Q.1. What is a computer? Write the parts of computer.

Computer:


Computer is an electronic machine which after analyzing and arranging the given information, presents it in a very short time.

All the work is done by the computer in the light of those instructions which called “program” and is saved in its memory. Computer preserves this program and information in its memory so long as we desire.

Parts of computer:

The important parts of a computer are given bellow:

1. Input devices:

In the computer, calculations and instructions are inserted by an input unit. Usually a keyboard, similar to a type writer is used for this purpose. The different functions of the keyboard are also controlled by a special instrument called a “mouse”. There are several other methods to feed information into the computer, which includes floppy discs and CDs. Input devices send the received data to CPU so that its arrangement and analysis may be done.

2. Central processing unit:

It is a type of a brain of the computer which performs the whole job. This part comprises of a control unit and memory unit.

3. Output device:

The internal working of the computer is represented by the output unit. It resembles a television set which is called a monitor. All the processes can be observed on its screen while the print out of the result can be taken on a paper with the help of an attached printer. Moreover output can be recorded on a floppy disc or CD’s.

Fast working of the computer, accurate solution of the given information, large memory and capability of deriving results is increasing its importance day by day and perhaps, there is no department left where man is not benefiting its uses.

Q4. Write the importance of I.T

Ans: Information Technology Important in Business:

There are many businesses which are in need of the software packages for satisfying their operational as well as functional needs. For fulfilling this requirement, these companies sign deals with the software manufacturing companies.

Information technology is useful in ensuring the smooth functioning of all the departments in a company such as the human resource department, finance department, manufacturing deportment and in security related purposes. The companies in the automobile manufacturing sector are able to get rid of any sort of errors or mistakes in the proper functioning of the tools used for designing and manufacturing purposes.

Due to the development of the information technology sector, the companies are being able to keep themselves aware of the changes in the global markets.

It plays an important role in easily solving the mathematical problems and in the project management system. It has a great use in the automated production of sensitive information, automated up-gradation of the important business processes and the automated streamlining of the various business processes. It has also played an important role in the areas of communication and automated administration of entire systems.


Importance of information technology in education:

Importance of information technology in educational sector is well-known. It helps the students as well as the teachers in studying the course material easily because of fast access. Studying the subjects with the help of online libraries and dictionaries has made grasping and increasing the knowledge easy for he students. The inclusion of information technology in the syllabus in schools, colleges and universities has helped them in grasping the subject well and getting their basics cleared. Since, many educational centers have the online grading system; it has been a boon for the parents of the children to keep a tab on their performance. Parents can also get the details of the attendance record of their child in schools.

Q.5 What is the importance of e-mail and internet?

Ans: By using internet, now we can communicates in a fraction of seconds with a person who is sitting in the other part of the world. Today for better communication, we can avail the facilities of e-mail.

1. Information: The biggest advantage that internet offering is information. The internet and the world wide web has made if easy for anyone to access information, and it can be of any type, as the internet is flooded with information.

2. Business: World trade has seen a big boom with the help of internet, as it has becomes easier for buyers and sellers to communicate and also to advertise their sites.

3. Social Networking: Today social networking sites have become an important part of the online community. Almost all users are members and can use if for personal and business purposes.

4. Entertainment: On internet we can find all forms of entertainment from watching films to playing games online almost anyone can find the right kind of entertainment for themselves.

5. E-Commerce: Is the concept used for any type of commercial maneuvering or business deals that involves the transfer of information across the globe via internet.

6. Shopping: It has become a phenomenon associated with any kind of shopping, almost anything. It has got a real amazing and range of products form household need to entertainment.

7. Services: Many services are now provided on the internet such as online banking, job seeking, purchasing tickets for your favorite movies, and guidance services on array of topic in the ever aspect of life, and hotel reservations and bills paying.

Q.2 How does a cell phone work?

Ans: Inside your cell phone, there is a compact speaker, a microphone; a keyboard, a display screen, and a powerful circuit board with microprocessors that make each phone a miniature computer. When connected to a wireless network, this bundle of technologies allows you to make phone calls or exchange data with other phones and computers around the world. The components operate so efficiently that a light weight battery can power your phone for days.

Today, cell phones fit in the palm of your hand, weigh only a few ounces, and offer features such as color graphic, musical ring tones and voice-activated dialing. Only a few years ago, the electronic in

this sleek device would have filled a large briefcase. A cell phone is really versatile radio. Much like a walkie talkie, a cell phone receives and sends radio signals. Because these radios connect into a network, cell phones offers much more, the ability to call any telephone anywhere in the world, internet access and data services.

Q3 : How fax machines work?

Ans: A fax machine is used to send documents to other locations without exchanging the physical versions of the document. The information is instead converted to an electronic signal and sent through a phone line to the other location. Scanning and email capabilities are slowly replacing fax


machine, but it is still a necessary device for most offices, especially in businesses where documents need to be signed.

There are several components required for a successful fax machine process. First, there must be two different fax machines that can communicate with each other, each with their own private phone number, known as a fax number. Fax numbers are dialed to send faxes to a particular location, just like inputting a phone number. Smaller offices have only one fax machine, so receiving multiple orders can take some time, most fax machines only print out one page at a time. There are various models of fax machines, and generally only those of the same generation can communicate with one another, although some modern models are backwards compatible.

1. Sending Faxes : When sending a document through a fax machine, the user puts the paper in a scanner device and inputs the correct fax number. The scanning machine then reads the paper, dividing it into lines of information that show where the paper is dark and where it is white. Most fax machines send only black and white information, through some models can also read color. Once the document has been scanned, the fax machine sends the information through the phone line to the fax machine specified by the fax number.

2. Receiving Faxes: Fax machine are created to respond to incoming faxes automatically. They receive the incoming signal, and translate it back into black and white (or color) markings. The information is then replicated by the fax machines printer section. The information tells the printer what part of the paper should be black and which should be white, dot by dot. More modern fax machines can send detailed information for a high dot-per-inch quality, but traditional fax machines have around 200 dpi quality.


Chapter No: 18 RADIOACTIVITY

Nuclear physics:

The branch of physics which deals with the study of properties and structure of nucleus is known as nuclear physics.

Structure of atom:

The atom consists of two parts, the central dense part having positive charge is called nucleus, which consists of protons and neutrons. The outer part consists of circular orbits in which electrons revolve around the nucleus. There are three fundamental particles present in an atom i.e. proton, neutron and electron. The outer part consists of circular revolves around the nucleus. Nucleus is spherical in shape. The diameter of nucleus is 10-14m and that of atom is 10-10m.

The number of protons and neutrons inside the nucleus are known

as nucleons. The mass of proton is 1 .67×10−27kg and its charge is equal to

+1 .6×10−19C. Neutron is a neutral particle having no charge and its mass of

neutron is slightly less than that of proton. The mass of electron is

9 .11×10−31kg and its charge is −1 .6×10−19

C. In a neutral atom the numbers of electrons are equal to the number of protons. Thus atom as whole posses no net charge.

The number of protons inside the nucleus is called atomic number or charge number. It is represented by “Z”. The number of protons and neutrons is called mass number. It is represented by

“A”. An atom “X” can be represented asz X A.

Isotopes: -

The atoms of an element having same atomic number but different mass numbers are called isotopes.

OR

Atoms of the same elements containing the same number of protons but different number of neutrons in their nuclei are called isotopes.

Explanation: -

All the atoms of an element contain the same number of protons and electron but they may have different number of neutrons.

Example; all the uranium atoms contain 92 protons and 92 electrons but the number of neutrons are different. Some uranium atoms contain 143 neutrons while other contains 146 neutrons. All the atoms


have same number of protons that’s why the atomic number of all the atoms of an element remains the same. But the mass number is different due to presence of different number of neutrons.

Mass number of uranium atom containing 143 neutrons.

A = No. of Protons + No of neutrons

A = 92 + 143

A = 235

Mass number of uranium containing 146 elements.

A = No. of protons + No of neutrons

= 92 +146

A = 238

The uranium elements has mainly two isotopes

92 U235 92 U238

Isotopes have same chemical properties due to presence of same number of electrons in its valence shell. But different physical properties due to presence of different number of neutrons.

Radioactivity: -

The spontaneous and uncontrollable disintegration of the nucleus of certain heavy atoms with the emission of alpha, beta and gamma rays/particles is called radioactivity.

Explanation: -

The heavy elements having atomic number Z>82 like uranium, thorium, radium etc. are unstable. To gain stability they emit some invisible radiations which affect the photographic plate just like ordinary light rays. These elements which emit radiation are known as radioactive element and this property of emitting invisible radiations is known as radioactivity. This phenomenon was first discovered by the French scientist Henri Becquerel in 1896.

For example; U238 disintegrates and its own and forms a new element Th234 by emitting alpha particle and energy in the form of gamma radiations.

92U 238 90 Th234 + 2 He4 + E

In above example uranium atom has been converted to Thorium atom element. When a heavier atom disintegrates its self it is known as natural radioactivity while the disintegration of artificial radioactive elements is known as artificial radioactivity.

Unit:

The SI unit of radioactivity is becquerel denoted by (Bq).

Becquerel:-

The activity of quantity of radioactivity element in which one nucleus decays per second is known as one becquerel.

Experiment:-


A small quantity of radioactivity element (radium) is placed in the cavity of lead block as shown in the figure. The radiations emitted by the radium were allowed to enter in a vacuum chamber in which a uniform magnetic field was applied. As soon as the radiation enters the magnetic field, they were deflected and gave three images on the photograph plates.

The radiations which were deflected towards the right are known as beta rays. The radiations deflected towards the left are known as alpha rays while the radiations which pass undeflected are known as gamma rays.

Properties of alpha, beta and gamma rays:

Alpha rays:

1. The α -rays consist of double positive charge having mass equal to four times of that of hydrogen atom.

2. The charge on α -rays/ particles is equal twice that of protons. These particles are in fact helium nuclei

3. The speed of α rays depends upon the radioactive element and ranges from 1 .4×107 m /s to

1 .8×107 m/ s .4. α -rays ionize the gas through which they pass.5. α -rays affect the photographic plate.6. They produce fluorescence in zinc sulphide.7. They are affected by electric and magnetic fields.8. At atmospheric pressure they travel a small distance in air 4cm to 5cm.

Beta rays:

1. β -rays carries negative charge. The mass and charge of β -rays is equal to that of electrons.

2. The ionization power β -rays is less than that of α -rays.3. They produce fluorescence in Barium Platinocynide.4. They are affected by electric and magnetic fields.5. They can pass through thick layer of matter up to 1cm.

6. They affect the photographic plate but the effect produced by β -rays is more than α -rays.7. They are emitted by radioactive element with a great speed.

Gamma rays:

1. They move with the speed of light.

2. The ionization produced is very smaller as compared to β andα -rays.

3. γ -rays affect the photographic plate but this effect is much higher than that of β -rays.

4. γ -rays also produce fluorescence in Barium platinocynide.

5. The penetrating power of γ -rays is much greater than β -rays.6. They are not affected by electric and magnetic fields. i.e. They are neutral.

7. γ -rays are such electromagnetic waves which are emitted by the nucleus.

Half Life of Radioactivity Elements:

The time interval in which half of the atoms in any given same decay into daughters elements is known as half life of the parent element.


Explanation:

As we know that any radioactive element emitting radiations is converted into another element with the passage of time. The original element is known as parent element while the new element formed is known as daughter elements. When an atom emits alpha particles then the new element formed is given by;

Z X A Z−2 X A−4 + α

Similarly its Beta particles are emitted then;

Z X A Z +1 X A +β

From above reactions we can say that the protons and neutrons present inside the nucleus are continuously changing to one another. When a neutron emits Beta particles it is converted into proton. Thus the charge number of the daughter element increases by one unit. We cannot estimate the life of a single atom. Therefore the time interval in which half of the parents decay into daughter elements is known as half life. Different elements have different half lives ranging from a fraction of second to 10 years. If we have 20,000 elements of a sample, then after first half life 10,000 will decay into daughters’ elements. While in the second half life 5000 will remain as parent elements.

Isotopes:

The atom of same element have same atomic number but different mass number are called isotopes. Hydrogen has three isotopes i.e. 1H1 ,1H2 and 1H3.

Radioisotopes:

The isotopes of an element which are unstable and decays by emitting alpha, Beta and gamma rays are known as radioisotopes.

Explanation:

Some isotopes in nature are stable but some are unstable. They decay and are radioactive such isotopes are known as radio isotopes. Many of them are naturally occurring while some are produce artificially by bombardment of sub atomic particles. Examples of artificially prepared radioisotopes are

11 Na24 , 27 Co62 , 53 I 131

Einstein's Mass Energy Equation:

In 1905, Einstein gave his theory of relatively which contained the idea that energy and matter are interchangeable according the Einstein mass energy equation i.e ,

E = m c2

Where E is the energy obtained as a result of mass 'm' when it is given the speed of light 'c' where 'c' is the speed of light and its value is 3 X 108 m/sec. From above equation it is clear that mass can be converted into energy and energy can be converted into mass.

Nuclear Fission:

The splitting of heavy nucleus into fragments with the emission of energy when bombarded by a neutron is called nuclear fission.

Explanation:


Nuclear fission was first discovered by Otto Hann and Fritz Strassman bombarding uranium 92U235

nucleus with slow neutrons .Due to which compound nucleus 92U236 were formed and thus broke into fragments with the release of two or three neutrons and a large amount of energy , This process is expressed as follows ;

0 n1 +92 U 235 92 U236

56 Ba141 + 36 Kr92 +3 0 n1 +E

Whereas, 56 Ba141 and 36 Kr92

represents the Barium and Krapton nuclei.

In a fission reaction, the sum of masses of the produce nuclei and neutrons is less than the mass of the original nucleus. Thus the mass difference results in the release of energy according to Einstein’s mass energy equation.

Fission Chain Reaction:

In a fission process each nucleus emits about two to three neutrons. These neutrons may collide with the other uranium nuclei and cause fission in them. This process continues, more and more neutrons are emitted thus a chain is formed known as fission chain reaction.

If the chain reaction is not controlled, then a large a large amount of energy is released in the form of heat and kinetic energy of fragments in a very short time Example; explosion of atomic bomb. If the fission chain reaction is controlled then the desired energy can be produce which may be used for useful purpose i.e. for proton of electricity in nuclear reactor.

Fusion Reaction:

A reaction in which two light nuclei fuses together to form a heavy nucleus with the release of a large amount of energy is called nuclear fusion reaction.

Explanation:

Fusion reaction is the reverse of fission reaction. In fusion reaction the energy released is much more as compared to that of fission reaction. If an atom of deuterium is fused with an atom of Tritium then a helium nucleus or alpha particle is formed; nearly 17.6 MeV of energy is liberated. This process is expressed as,

1 H2 + 2 H3 2 He4 + 0 n1 + E

The mass of helium nucleus and neutron is less than that of deuterium and tritium. The deficiency of the mass converted to energy through Einstein mass energy equation.

E = m c2


It is very difficult to produce a fusion reaction. When nuclei are brought rear each other for fusion, work has to be done to overcome the electrostatic force of repulsion between the two nuclei, which requires greater amount of heat energy and can be achieved by fission reaction.

This reaction occurs in the inside of sun and stars. Efforts are being made to achieve energy through fusion reaction for peaceful purposes.

Radiation Hazards:

Nuclear radiations can be dangerous to human body, due to careless in its use because these radiations are invisible. They can damage our body cells due to its ionization property. Excessive radiations may cause cancer or incurable radiation may cause cancer or incurable radiation sickness to human being. If a person highly radiated may suffer from the following diseases.

1. Anemia2. Blood cancer3. Malignant tumor4. Cataracts (Optically of lens of eyes)

Precautions against Nuclear radiation:

1. We should stay away from radioactive source, because these radiation spread in all direction.2. The doctors should use the radiation for minimum possible during the treatment.3. Overdose of radiations should avoided4. The nuclear waste should be buried for away from the residential areas.5. We should build a thick wall concrete around the reactor to protect ourselves from radiations.6. We should use film badge and dosimeter during the determination of intensity of radiations.7. We should place the radioactive materials in a box lead.8. The person should check themselves medically on regular basis who is working in nucleus

laboratory.

Uses of radioisotopes:

Radio isotopes are very useful in medicine, Agriculture, Industry, Geological dating and archeology etc.

Medicines:

1) To study the working of internal organs. 2) For the treatment of many complicated diseases.3) To trace out circulation of blood in the body.4) For sterilizing instruments and surgical tools.5) To control cancer.6) To control excessive productions of white blood cells in the body.7) To trace out path of an element in the body of plants, animals and human beings.

Agriculture:

1) For preservation of fruits, vegetables and other food stuff.2) To kill bacteria and other harmful insects.3) Can be used as fertilizers.4) To study the process of photosynthesis in plants.5) To measure the rate of growth of roots of a plant.

Industry:


1. In the manufacturing of machine tools.2. To find the thickness of different materials.3. To locate and remove faults in machine.4. To trace out the leakage of pipes.5. To trace out the cracks in welded joints.6. To estimate the wear and tear in machine tools.

Geological dating:

U-238 decays to lead-206. The ratio of u-238 to lead-206 present in a rock can be used to determine the age of the rock.

Conceptual Questions:

Q1. What type of charge is present on.

(a) α -rays (b) β -rays (c) γ -rays?

Ans:

a) The α -particles are double positive charged particles. The posses low penetrating power and high ionization effect. They can be deflected by magnetic and electric field.

b) The β -particles are negatively charged particles. They posses high penetrating power and low ionization. The electric and magnetic field can affect the path of those rays/particles.

c) The γ - particles are neutral particles. There is no effect of electric and magnetic field on the path of those particles. They can travel with velocity of light and thus posses’ very high penetrating power and very low ionization effect.

Q.2: What are the sources of energy from sun and stars?

Ans: The temperature of sun and other stars is extremely very high. Due to which high temperature, fusion reaction occurs at sun surface and stars very easily.

During fusion reaction, tremendous amount of energy is released. Thus the fusion reaction is cheaper source of energy from sun and stars.

Q3: An element “X” having atomic number “92” and mass number 238 emits on d-particle to form another element “Y” What will be atomic number and mass number if element “Y”.

Ans: We Know that when a radioactive element emits d-particle then the parent element “X” is converted into a daughter element “Y” the atomic number of the daughter element is two less while its parent element is four less than that of the parent element so we have,

Thus the atomic number of “Y” will be “90” and its mass number will be 234.

Q4: Which of the following nuclei will be more stable and why?

(a) 92U235

(b) 16S32

Ans: The stability of nuclei depends upon the number of nuclei. Greater the number of nucleons less will be the stability of the nuclei and vice versa.


Thus “16S32” will be more stable than 92U235 because the number of nucleons in “16S32” is less than the number of nucleons in “92U235”

Q5: Complete the following reaction?

92 U 235 + 0n 1 56 Ba 139+ Kr +0n1 +q

Ans: The above reaction represents fission reaction in which neutron is bombarded on uranium and thus a compound nucleus is formed. Thus compound nucleus is very much unstable and splits to give barium “Ba”Krypton“Kr” three neutrons and energy.

We know that in every nuclear reaction the mass number and charge number of reaction and products remain the same. So in above reaction, the mass no of “Kr” will be ‘94’ and its charge number will be “36” The compound nucleus is represented by [92 U 236], so the complete reaction can be written as;

0 n1 +92 U 235 56 Ba141 + 36 Kr92 +3 0 n1 +q


Zahoor Ahmad

M.Sc. PHYSICS


FACUILTY MEMBERS


S.NO Teacher Name Qualifaction

1 Dr. Naveed Ahmad Ph.D Genetics2 Ayan Ullah M.Phil Statistics, B.Ed3 Wajid Ali M.Phil Mathematics4 Miftah Ullah M.B.A, 5 S. Khalid M.Phil Statistics,B.Ed6 Zahoor Ahmad M.Sc. Physics7 Ihsan Ilahi M.S Computer Science8 Ajmal Khan M.Sc Management,B.Ed9 Gulraiz Khan M.Sc Chemistry

10 Shehriyar M.S Human Resources11 Shahid Ali M.A English12 Hamid Iqbal M.Sc Zoology13 Shahid Khan M.Sc Physics14 Amir Shahzad M.A Islamiat15 M.Azeem M.A Islamiat, B.Ed16 Haroon Qadir M.Sc Statistics,B.Ed17 Adnan M.A Political Science18 Farooq Shah B.S. Computer Science