complete physic as
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Physics (9702)
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Units and Dimensions
Physical QuantitiesPhysical quantity is physical property that can be measured.
Base Quantity
Seven arbitrarily chosen physical quantities are called base quantities and its units
are known as base units.
Basic Quantity Base Units Symbols
Length Metre m
Mass Kilogram kg
Time Second s
Electric Current Ampere A
Temperature Kelvin K
Amount of Substance Mole Mol
Luminous Intensity Candela cd
Derived Quantites
All physical quantities other than base quantities are called derived quantities, and
its unit is called derived unit. For example speed is a derived quantity and m/s is its
derived unit:
Quantity Unit Name Symbol
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Prefix Factor Symbol
Pico 1012 p
Nano 109 n
Micro 106
Milli 103 mCenti 102 c
Deci 101 d
Kilo 103 k
Mega 106 M
Giga 109 G
Tera 1012 T
MeasurementsReading
Reading is a single determination of the value of an unknown quantity. It is the
actual reading taken during an experiment.
Measurement
Is the final result of the analysis of a series of readings.
Vernier Calipers
1. Read the value of the main scale which is just to the left of 0 reading on theVernier scale (12 mm in this case).
2. Read the value of the Vernier scale which exactly coincides with the mainscale (3 in this case).
3. The total reading would be
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Micrometer screw gauge
Reading = Linear Scale reading + (Coinciding Circular scale reading x 0.01) mm
= 2.5 + (46 x 0.01)
= 2.5 + 0.46
= 2.96 mm
Systematic Errors
Uncertainties in the measurement of physical quantities due to instrument, faults in
the surrounding conditions or wrong assumptions made by the observer.
Size of error is roughly constant and measurement obtained is either always greateror always less than the actual value.
Examples of Systematic Error
1. Zero Error2. Reaction time of observer3. Due to instruments
a. A watch is fastb. Calibration at certain temperatures and used in under different
temperatures.
4. Errors due to wrong assumptions, e.g. value of g taken as 10m/s2Systemic Errors cannot be reduced by taking average.
Systematic Errors can be reduced by
Taking measurements carefully Using different instruments Using different methods Checking zero error Checking stopwatch with another stopwatch
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Random Error
Uncertainties in measurement made by the observer or person who takes the
measurement.
Random error can be positive or negative, and its magnitude is not constant.
Example of Random Error
1. Errors due to parallax error2. Changes in temperature during an experiment3. Different pressure applied when closing the gap of micrometer screw gauge
when measuring a soft object.
Random Errors can be reduced by
Taking average
Precision
A measurement is said to be precise if mean deviation of readings is smaller(the
readings are close to each other and not spread over a long range).
Diameter Mean Diameter Deviation Mean Deviation
0.38
0.02
0.024
0.36 0.04
0.40 0.00
0.44 0.040.42 0.02
Precision can be improver my:
Using a hand lens. A plane mirror behind the pointer. The observer reads the scale when the
pointer is directly on top of its image in mirror. This is done to avoid parallax
error.
AccuracyA measurement is said to be accurate if it is close the actual value.
Accuracy is given by the percentage error. The Smaller the percentage error, the
higher the accuracy.
Calculating Percentage Error
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Precise and
Accurate
Accurate but not
Precise
Precise but not
Accurate
Not Precise and
not Accurate
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Calculating Compound Errors
If
If If
[ ]If
[ ]If
[ ]
Addition, Subtraction, Multiplication and Division of constants have no effects on
compound errors.
Vectors
Scalar Quantities
A scalar quantity is one which can be described fully by just stating its magnitude.
For example: mass, time, length, temperature, density, speed, energy and volume.
Vector Quantity
A vector quantity is one which can only be fully described if its magnitude and
direction are stated.
For example: displacement, velocity, acceleration, force, momentum, magnetic flux
density and electric intensity.
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Addition of Vectors
1. Parallelograms of vectors
2. Triangle of Vectors (head to tail)
Resolving a Vector
Vertical Component of R = R sin
Horizontal Component of R = R cos
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Triangle of Forces
If three forces acting on a point can be
represented in magnitude and
direction by the three sides of a triangle
taken in order, then the three forcesare in equilibrium.
Static Equilibrium
Moment of a Force
Turning effect of force about an axis is called torque or moment of a force. SI unit of
moment is Nm.
Equilibrium of a body
A body is in equilibrium if:
1. The resultant force is zeroAND
2. Sum of Moments of forces about any axis is zeroCouple
A couple consists of two equal and opposite forces when its line of action is not
along the same line.
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Centre of Gravity
The center of gravity G of an abject is the point where the line of action of weight
passes.
It is the point where the weight of the object appears to be acting.
Stability
An object is stable if
1. It has a larger base.2. Its center of gravity is lower.
Kinematics
Three types of motion
Translatory motion
Also known as linear motion is motion along a straight line.
Rotatory motion
Spinning or turning motion that takes place around an axis, without a change in
linear position is called rotatory motion.
Vibratory
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When an object is displaced from its fixed position and made to move to and fro
periodically, it is known as vibratory motion. A vibratory motion happens when a
particle is vibrated. Vibratory motion is also called as oscillatory motion
DistanceTotal length covered by a body during its motion. Scalar quantity SI unit is m.
Displacement
Shortest Distance covered. Vector quantity Si unit is m.
Speed
Distance covered per unit time. Scalar quantity SI unit is m/s.
Uniform/Constant speed
Equal distance covered in equal interval of time
Velocity
Displacement traveled per unit time. Vector quantity Si unit is m/s.
Acceleration
Rate of change of velocity. Vector quantity Si unit is m/s2.
Equations of Motion
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Distance-Time Graph
Gradient (slope) of Distance-Time graph shows velocity
Velocity-Time Graph
Area under Velocity-Time Graph shows distance covered. Gradient (slope) of Velocity-Time Graph gives acceleration.
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Projectile Motion
Horizontal component of velocity stays same.
Vertical component of velocity changes due to gravitational force.
Dynamics
Force
Force can be defined as rate of change of momentum. It is a vector quantity and its
SI unit is N.
Inertia
It is a property of material objects due to which they oppose any change in their
original state. It depends on mass of body.
Newtons First Law of Motion
A body continues its initial state of rest or uniform motion unless it is compelled to
change that state by an external force.
Newtons Second Law of Motion
When an unbalanced force acts on a body its produces acceleration in the body in
the direction of force. The magnitude of the acceleration is directly proportional to
the applied force and inversely proportional to the mass of body.
Newton
A force is said to be one newton when it produces an acceleration of 1 m/s2 in a
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body of mass 1 kg.
Newtons Third Law of Motion
To every action there is equal and opposite reaction.
Momentum
Linear momentum of a body is the product of its mass and its velocity.
It is a vector quantity and SI unit is Ns or Kg m/s.
Principle of Conversation of Momentum
The total linear momentum of an isolated system is constant.
Elastic Collision
A collision in which total kinetic energy stays constant.
Relative speed of approach is equal to relative speed of separation.
If bodies of same mass collide their speeds are interchanged.
Inelastic Collision
A collision in which total energy stays constant but kinetic energy changes.
Work
Work done is product of force and the distance in the direction of force.
It is scalar quantity and its SI unit is Joule, J.
Joule
Work done is said to be 1 J when the force of 1N displaces a body though 1 m in its
direction.
Energy
Energy is ability to do work.
Kinetic Energy
Energy possessed by a body due to its motion
Potential Energy
Energy possessed by a body due its physical condition or motion
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Internal Energy
Principle of Conservation of Energy
Total energy of an isolated system stays constant.
Power
Power is the rate of doing work. Its SI unit is watt or J/s.
Efficiency
Electricity ASElectric Current
Current is rate of flow of electric charge.
SI unit is Ampere, A.
where n = number of electrons
and e = charge on one electron
Potential Difference
The potential difference between two points in a circuit is the work done to move a
unit charge from one point to another.
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EMF
EMF of a source is the work done to move a unit charge around a complete circuit.
Ohms Law
Ohms law states that the current passing though the conductor is directly
proportional to the potential difference across it, provided that temperature and
other physical conditions stays same
Ohmic Conductor
Ristance Constant
ThermistorResistance
decreases
Filament lamp
Resistance Increases
Resistance
It is the ratio between V and I flowing through a circuit.
It is the ratio between V and I flowing
through a circuit.
Increasing temperature of a wire leads
to increasing resistance.
Factors on which Resistance depends
Nature of material. Resistivity of conductor. Cross-sectional area A. Inversely proportional Length l. Directly proportional
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Series combination of resistors
Same current passesthrough all resistors.
V = V1 + V2 + V3 R = R1 + R2 + R3
Parallel Combination
V stays same
Thermistor
Thermistors are devices made from
semiconductors, break the rule we've
just explained (typical) and reduce
their resistance as temperatureincreases. This is because the extra
energy makes the atoms release
electrons, allowing them to move more
easily, this in turn reduces the
resistance.
Light Dependent Resistor LDR
Light-dependent resistors also decrease their resistance when energy is given, but
this time the energy needs to be given as light energy.
Internal Resistance
Cell is not perfect, some energy is wasted inside the cell, this is due to resistance
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inside the cell.
Where:
V= Voltage across external circuit.
E= emf of cell
I= Current through cell
r= Value of internal resistance
Ir= P.d across internal resistor
Kirchhoffs First Law
The vector sum of current entering a junction in a circuit is zero.
This law follows the principle of
conservation electric charge.
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Kirchhoffs Second LawAround a closed loop, the vector sum of the Emf is equal to the vector sum of the
products of current and resistors.
This law is based on law of conservation
of energy.
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Elasticity
ElasticityIt is the ability of a body which enables it to regain its original dimensions when the
deforming force acting on it is removed.
Elastic Limit
Materials are only elastic up to a certain limit known as elastic limit. Beyond this point
material would not return to its original dimension when deforming force is removed.
Hooks Law
Within the elastic limit the extension or compression is directly proportional to the
applied force.
Stress
It is the force applied per unit area of cross-section. Its unit is N m-2 or Pa.
Strain
It is defined as extension per unit length. Strain does not have a unit.
Young ModulusThe ratio of stress and strain is known as Young Modulus. Its unit is N m-2 or Pa.
(
)
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Gradient of Stress-Strain graph gives
youngs modulus.
Area under force-extension graph gives
Strain Energy
Behaviour of a wire under stress
Limit of proportionality
It a point beyond which extension is not proportional to force and stress is notproportional to strain.
Elastic Limit
Slightly beyond limit of proportionality is elastic limit. Up to elastic limit, any work done
can be fully recovered.
Elastic Deformation
Wire returns to its original length.
Plastic Deformation
After elastic limit is exceeded, wire does not return completely to its original length.
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Yield point
At lower yield point plastic flow begins. Atomic planes in the wire slide over each
other, and wire narrows down uniformly.
Beyond upper yield point extension is permanent and a small amount of forceapplied causes great extension.
Ultimate tensile strength
It is the point of maximum stress where wire has its greatest strength.
Beyond ultimate tensile strength wire narrows unevenly, forming necks and
eventually breaks.
Brittle Material
Do not show plastic deformation and break immediately once elastic limit is
exceeded. For example glass.
Ductile Material
Undergo plastic deformation before breaking
Glass Copper
Rubber Polythene
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Experiment to determine the Young modulus of a metal wire
States of Matter
Solid
Made up of molecules arranged closely in fixed pattern. Molecules vibrate about their mean positions The force of attraction between molecules are very strong Solids have fixed shape
Liquids Made up of molecules close together. Molecules are able to slide over each other. Takes shape of container.
Gas
Molecules are in constant random motion. Intermolecular distance is very large compared to size of molecule.
Gas molecules collide with each other and with walls of container (elastically)exerting pressure.
Crystalline Solids
Atoms are arranged in fixed repetitive manner over a long distance. For example
NaCl.
Amorphous Solids
Also known as non-crystalline solids, it lacks long-range order characteristic of crystal
(atoms are repeated only over a short range).
Waves
Displacement
Amplitude
Phase Difference
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Period
Frequency
Wavelength
Speed
Progressive waves distribute energy from a point source to a surrounding area. They
move energy in the form of vibrating particles or fields
Transverse Waves
vibrations are perpendicular to the wave motion - so if the wave is travelling
horizontally, the vibrations will be up and down. For example, light and water.
Longitudinal Waves
vibrations are parallel to the wave motion - so if the wave is travelling horizontally,
the particles will be compressed closer together horizontally, or expanded
horizontally as they go along (we call the expanded bit a rarefaction). The particle
movement is a series of compressions and rarefactions. For example, sound and
some earthquake waves.
Diffraction
Diffraction is the spreading out of waves as they pass through a gap or by an
obstacle.
1. The smaller the gap the greater the diffraction.2.
The longer the wavelength the greater the diffraction.
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Wavelength remains same before and after the diffraction.
Coherence
Coherent waves are waves with a constant phase difference. (Note: They don't haveto be in phase for this to be true.) They will have the same frequency and
wavelength (they are normally produced from one source).
When two waves meet they will interfere and superpose. After they have passed
they return to their original forms. This is true if they are coherent or not.
At the point they meet, the two waves will combine to give a resultant wave whose
amplitude (or intensity) may be greater or less than the original two waves.
The resultant displacement can be found by adding the two displacements together:
This is called the Principle of Superposition.
If two waves of the same type and the same frequency combine so that the crest of
one coincides with the trough of the other, they will completely cancel each other
out. This is called destructive interference.Alternatively, the two waves could
combine when their crests coincide; then there would be constructive
interference and the resultant amplitude would be equal to the sum of the separate
amplitudes:
Superposition will occur whether waves are coherent or not. (However, if the waves
are coherent, they will interfere to produce a fixed pattern.)
The same rules apply... the resultant displacement at any point is always the sum of
the separate displacements of the wave at that point:
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Diffracting Grating
Standing/Stationary Waves
Polarization
Transverse waves can oscillate in any plane. Polarization is the process by which the
oscillations are made to occur in one plane only.
This is done by passing the waves through a 'grid' so that only the waves that can fit
through the slits can continue through:
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Longitudinal waves cannot be polarised.
Circular Motion
Angular Displacement
Angle subtended by an arc at the
center of the circle. It is measured in
radians.
Radian
Angular displacement is said to be one radian when length of arc is equal to the
radius of circle
Angular Velocity
Rate of change of angular displacement
Frequency f
The number of rotation per second made by the rotating object
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Centripetal Force
It is the force which compels the body to move in a circle. Centripetal force always
directs towards the center of the circle. During circular motion whatever be theforces acting on the body their resultant is centripetal force.
Motion in Horizontal Cirle
In case of horizontal circle tension in the string provides necessary centripetal force
so
Motion in Vertical Cirlce
At the top of ci rc le:
Tension and weight are in same
direction
At the bottom of ci rc le:
Tension and weight are in opposite
direction
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Centripetal Acceleration
Acceleration caused by centripetal force.
Universal Gravitation
Newtons law of Universal Gravitation
Newtons law of Universal Gravitation states that the force of attraction between
two objects is directly proportional to the product of the masses of the objects and
inversely proportional to the square of the distance between them.
G is known as universal gravitation constant. Relationship between G and g
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Gravitational Field
It is the force field around a mass, when another mass is introduced into the field it
experiences a force.
Gravitational Field Strength
The gravitational field strength, g at a point in a gravitational field is defined as the
gravitational force of attraction per unit mass at that point.
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Gravitation Potential
The gravitation potential, V at a
point in a gravitation al field is
the work done by the
gravitational attraction to bring
a unit mass from infinity to that
point.
The gravitation potential at
infinity is assumed to be zero
and its value decreases as mass
approaches closer to earth
(becomes negative). It is scalar
quantity and its unit is J/Kg
Equipotential Surface
The surface where all points on it has the same gravitation potential.
Gravitational Potential Energy
The gravitational potential energy U of a body of mass m at a point in gravitational
field is defined as work done to bring the body from infinity to that point. Its unit is
joule J.
Motion of Satellite
Gravitational attraction provides centripetal force so:
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Energy of Satellite
( )
( )
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Geostationary Satellite
Satellite which orbit around the earth above the equator with period of 24 hours.
These satellites orbit in the same direction as rotation of earth (west to east). These
satellites provide continuous link for the communication. These satellites do not need
to be tracked.
Simple Harmonic Motion
A body is said to be in simple harmonic motion if its acceleration is directly
proportional to its displacement from fixed point (equilibrium position) and is always
directed towards that fixed point.
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Relation SHM and Circular Motion
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Energy of particles executing SHM
Kinetic energy is maximum when x=0
Potential Energy is maximum when x=x0
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Damping Effect
The effect of resistive forces in removing energy in vibration object in form of heat to
the surrounding.
Damped Oscillations
Oscillations where the amplitudes become smaller and smaller are known as
damped oscillations.
Light Damping
If the resistive forces are small then system is said to be lightly damped. Amplitude of
oscillation is decreases gradually. It is only after a large number of oscillations that
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the system stops oscillating. Amplitude decreases exponentially with time.
Critical Damping
If the damping is just sufficient to prevent oscillation, but yet not too great to
indefinitely delay returning to original position then the system is said to be critically
damped.
Heavy Damping
If the resistive forces are very large then the system is said to be heavily damped.
After displacement return to original position takes a very long time.
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Advantages of damping
Suspension system in car. Good system is one which is slightly under critical damping.
Forced Oscillations
Oscillations which are under the influence of an external periodic force are known
as forced oscillations. The frequency of such a system is the same as the frequency
of the external periodic force (known as drivers frequency).
ResonanceThe phenomenon in which the amplitude of the oscillating system becomes very
large when drivers frequency becomes equal to the natural frequency of the
oscillating system is known as resonance.
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The sharpness of the peak depends upon the resistive forces acting on the system,greater the resistive force smaller the amplitude of the vibration and flatten will be
the peak.
One effect of the damping is to decrease the effect of resonance so greater the
damping smaller the amplitude of vibration at the resonance frequency.
Advantages of Resonance
1. Radio tuning circuit: When tuned, radio waves have same frequency asfrequency of the electrical oscillations in the circuits of receiver.
2. Cooking of Food in microwave oven: In ovens microwave with the frequencysimilar to the natural frequency of water molecules are used.
3. MRI:Disadvantages of Resonance
1. Vibrating parts of the mechanical equipment might break.2. Collapse of suspension bridge.
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Motion of mass attached to a spring
Hooks Law: F=Kx
When mass is released it accelerates towards equilibrium position.
Hence motion is SHM
Comparing with
Motion of mass attached to two similar springs.
When the mass is in equilibrium the extension in each spring is equal to e.
When the mass is displaced a small distance x to the right, the magnitude of
restoring force is given by.
When mass is released it accelerates towards equilibrium position
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Hence motion is SHM
Electrostatics
Coulombs Law
Coulombs Law states that the force between two point charges is directly
proportional to the product of the charges and inversely proportional to the square
of the distance r between them.
0 = Epsilon knot = Permittivity of free space or vacuum = 8.85 x 10-12 Fm-1
It the charges are not in vacuum then
r=Relative permittivity of that medium with respect to vacuum. Its value is always
>1.
Electric Field
It is a force field which exists around a charge when another charge is introduced in
experiences a force.
It is a vector and is represented by lines of force. The direction of electric field is from
positive to negative.
Electric Field Strength
Electric field strength E at a point in an electric field is the force per unit charge
acting on a positive test charge (q) placed at that point. Its a vector quantity and
its unit is N/C or V/m.
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point charge.
Inside the sphere electric field E=0.
Capacitor
A capacitor is a device for storing electric charge.
It consists of two parallel metal plates with an insulator (known as dielectric) present
between the plates.
Capacitance
Capacitance is the ability to store charge. It is the ratio between the charge Q
stored on any one plate of capacitor and the potential difference V across it. SI unit
is Farad F.
Capacitance depends on
A = Area of the plate d = Separation between plates 0 = Permittivity of free space (nature of dielectric)
Combination of Capacitors
Series Combination
Charge stored on each capacitor is same = Q V=V1 + V2 + V3
If only two capacitors then Parallel Combination
Potential difference across each capacitor is same = V Q = Q1 + Q2 + Q3 C = C1+ C2 + C3
NOTE:
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Energy stored in charged capacitor
Forces in Magnetic Field
Force on Charged Particles
Where:
Q=charge of particle
v=velocity of particle
B=Magnetic flux density
=Angle between velocity and magnetic field
Force on Current Carrying conductor
Where:
I=current passing through conductor
l=length of conductor
B=magnetic field density
=Angle between direction of flow of current and magnetic field
If the particle goes undeflected
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When a charged particle enters a magnetic field we now know it will be forced to
change direction. If it stays in the field it will continue to change direction and will
move in a circle. The force produced will provide the centripetal force on themoving particle.
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Flemings Left Hand Rule
Force of repulsionbetween current
carrying parallelconductors when
current flowing is in
opposite direction.
Force of attraction
between current
carrying parallel
conductors whencurrent flowing is in the
same direction.
Magnetic field pattern in case of a current carrying solenoid
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X= Current into the paper
0 = Current out of the paper
Magnetic flux density
Magnetic flux density is the force per unit length on a straight conductor carrying
unit current normal to the magnetic field. Its unit is Tesla, T.
Tesla
Magnetic flux density is said to be 1 Tesla when force of 1N is experiences by a
conductor of 1m carrying a current of 1A placed normal to the magnetic field.
Ac current
The main feature of AC is that the direction of flow changes from one direction to
another.
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Full-Wave Rectification
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The output is not pure DC so, put a capacitor (known as reservoir capacitor) across
(parallel to) the resistor and when the supply voltage across the resistor drops
towards zero the capacitor delivers some extra current through the resistor in the
correct direction. This is called 'smoothing'
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Hall Effect
It is the phenomenon in which transverse potential difference is set up in a
conductor carrying current in a magnetic field.
Hall Probe
Hall probe uses hall effect to measure magnetic flux density. It consists of small piece
of semi-conductor through which constant current I is flowing. A sensitive voltmeter is
connected to the terminals to measure Hall p.d. Hall probe must be positioned
perpendicular to the magnetic field. Hall probe is calibrated by placing it in known
magnetic field (B0) and noting p.d (V0) produced.
Advantages of Hall Probe
Size of Hall probe is very small and can be used to measure magnetic fluxdensity in small isolated areas.
It is very sensitive so it can measure small changes in magnetic flux.
Electromagnetic Induction
Effect of producing an electric current using magnetism is known as
electromagnetic induction.
The current produced is known as induces current.
Magnetic flux through an area A perpendicular to magnetic field of flux density Bis given by:
Magnetic flux linkage through a coil of N turns with its plane perpendicular to themagnetic field density B is given by:
SI unit of magnetic flux is Weber (Wb).
Magnetic flux density
Magnetic flux per unit area.
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SI unit is Wb/m2.
Faradays Law
When the magnetic flux linkage though a circuit changes, the emf produced is
directly proportional to the rate of change of flux linkage.
Lenzs LawThe direction of the induced current is such as to oppose the change which gives
rise to it.
Lenzs law is a direct consequence of the principle of conservation of energy. The
electrical energy produced in electromagnetic induction is the result of the
transformation of mechanical energy in the form of work done into electrical
energy.
Negative sign in the equation implies that induced emf opposes the rate of change
of magnetic flux linkage.
-t graph, B-t graph and I-t graph are same.
Mutual Induction
Mutual Induction is the phenomenon in which E.M.F. is induced in one coil due to
the variation of current in nearby coil.
Self-Induction
Self-induction is phenomenon in which an E.M.F is induced in the same coil (Primary
Coil) in which current is changing.
Self-induction results in Back E.M.F or Eddy Current
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Transformer
Transformer is a device used to increase or decrease the A.C voltage.
Soft Iron core is used:
So that magnetic field lines can pass through easily. Because it is easy to magnetize and de magnetize.
Where
Np and Ns are the number of turns in Primary and Secondary coil respectively
B is the magnetic flux density
A is the Cross-Sectional area of core
Transmission of A.C current
It is recommended to transmit the power at high voltage to minimize power loss.
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Threshold Frequency, f0
The minimum frequency of incident radiation required to cause the emission of
photoelectrons from the metal surface is known as threshold frequency.
Work Function
The minimum energy needed for an electron to escape from metal surface.
Einstein Photoelectron Equation
Note:
No photoelectron emits whatever the intensity of light be, if the frequency of
incident radiation is less than threshold frequency
After threshold frequency when the intensity of elimination is increased the number
of photoelectrons emitted per second also increases (i.e. photoelectric current
increases).
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The increase in photoelectric current is due to higher rate of emission of
photoelectrons and not because the photoelectrons have more energy.
Maximum kinetic energy does not change unless the fof the incident radiation or
metal surface is changed.
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Experiment to find Maximum Energy
A reverse p.d. is applied (cathode is connected with positive terminal), the p.d. is
slowly increased until the reading of Ammeter falls to zero. This stopping p.d. (Vs) isrecorded by the voltmeter.
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Momentum of a Photon
Radiation Pressure
Radiation pressure is the pressure exerted upon any surface exposed to
electromagnetic radiation
As photon is absorbed by the surface the final momentum is zero so
If n= number of photons incident per second
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Radioactivity
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