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12th Class Chapter 4 Physics Short Questions

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Question: 1
Does the induced emf always act to decrease the magnetic flux through a circuit?
Answer: 1
1-46
No.the induced emf always oppose the cause that produce it.If the magnetic flux through the circuit is increasing then induced emf acts to decrease the magnetic flux.
it the magnetic flux through the circuit is decreasing,then induced emf acts to increase the magnetic flux.
Question: 2
State Faraday's law of electromagnetic induction.
Answer: 2
2-46
It states that
"The average emf induced in a conducting coil of N loops is equal to the negative of the rate at which the magnetic flux through the coil is changing with time".
Question: 3
Name four methods to produce induce emf.
Answer: 3
3-46
An induce emf is produced in the loop if the magnetic flux through it changes.The methods to produce induced emf are:
  1. A bar magnet is moved towards the coil.
  2. By changing the area of the coil in a constant magnetic field.
  3. A coil of constant area is rotated in a constant magnetic field,
  4. The coil is placed in the magnetic field of an electromagnet.

Question: 4
State the Lenz's law and define Henry.
Answer: 4
4-46
Lenz's Law: It states that the direction of induced current is always so as to opposite the change which cause the current.
Henry: If the current in the primary is changing at the rate of one ampere per second and the emf induced across the ends of the secondary coil is one volt then the mutual inductance is called one henry.
Question: 5
Is it possible to change both the area of the loop and magnetic field passing through the loop and still have bo induced emf in the loop?Explain briefly.
Answer: 5
5-46
Yes,it the plane of the loop is kept parallel to the direction of the magnetic field magnetic flux through the coil will be zero,no emf will induced in the loop either by changing its area or by changing the magnetic field.
Question: 6
Write two similarities and two difference between motor and generator.
Answer: 6
6-46
Similarities:
  1. Construction of a motor is similar to a generator.
  2. In both,magnetic field is provided by an electromagnet.
Differences:
  1. Generators convert mechanical energy into electrical energy while motor converts electrical energy into mechanical energy.
  2. In generator,the armature coil is rotated in the magnetic field and the current is the output.While in motor,armature is connected to battery,which rotated the armature.
Question: 7
Define self induction and also write its SI units.
Answer: 7
7-46
The phenomena in which the changing current in a coil induce an emf in itself is called the self-induction.Its SI unit is VsA-1.It is also called as henry(H).
Question: 8
When the primary of a transformer is connected to A.C mains,the current it is very small if the secondary circuit is open.Explain.
Answer: 8
8-46
If the secondary circuit is open then output power will be zero,Because output power is always slightly smaller than the input power,therefore a very small value of current is being drawn by a primary coil of transformer from AC units,
Question: 9
In a certain region the earth's magnetic field points vertically down,When a plane flies due north,which wing tip is positively charged?
Answer: 9
9-46
When the plane flies due north in the earth magnetic field directed vertical downward,then electron will experience force in east direction,Thus west wingtip of the plane is positively charged.
Question: 10
Define motional emf and write its formula.
Answer: 10
10-46
If a conductor moves through a magnetic field then due to change in magnet flux,an emf is induced across the ends of the conductor which is known as motional emf.
e=vBL sin theta.
Question: 11
Define induced emf and induced current.
Answer: 11
11-46
If a conductor moves through a magnetic field then due to change in magnetic flux,an emf is induced across the ends of the conductor which is known as back emf.If the circuit is close it will cause an electric current which is called as induced current.
Question: 12
Does the induced current depend on the resistance of the circuit?
Answer: 12
12-46
Yes the induced current depend upon the resistance of the circuit.
By ohm's law
I=e/R
Thus the induced current depends on the resistance of the circuit.
Question: 13
What is back motor effect in generators?
Answer: 13
13-46
Back motor effect is the opposing torque produced due to the induced emf in the coil.When coil rotated a current is drawn through the coil.The magnetic field exerts two equal and opposite forces on left and right side of the current carrying coil,These forces are such that they produce a counter torque that oppose the rotational motion of the coil.This effect is known as back motor effect in generators.
Question: 14
Can an electron at rest be set in motion with magnet?Explain
Answer: 14
14-46
No an electron at rest can not be set in motion with a magnet because at rest electrons only have electric field which can not interfere with magnetic field to cause any force which can produce motion in electrons.
Question: 15
In a transformer there is no transfer of charge from the primary to the secondary.How is then the power transferred?
Answer: 15
15-46
The power will be transferred because the two coils of transformer are magnetically linked i.e, the change of flux through one coil is linked with the other coil.
Question: 16
Write down any one method used for the production of induced emf.
Answer: 16
16-46
Induced emf can be induced by electromagnetic induction.When a conductor is moved through a magnetic field the electric current flow through the circuit,The emf produced in the conductor is called induced emf and the current generated is called induced current.
Question: 17
Differentiate between motor and generator.
Answer: 17
17-46
Motor: A motor is a device which converts electrical energy into mechanical energy.
Question: 18
A glass rod of "L"Length is moving perpendicular to the applied magnetic field P with velociety v.Explain briefly about the induced emf in it.
Answer: 18
18-46
Induced emf is zero.As glass rod is insulator and there are no free electrons to be shifted from one extreme to the other.Therefore there will be no effect of perpendicular magnetic field.The induced emf is only produced by moving a conductor across the magnetic field.
Question: 19
What is back emf in motors?
Answer: 19
19-46
When to coil motor rotates across the magnetic field by the applied potential difference V,an emf is induced in it.The induced emf is in such a direction that opposes the emf running motor.Due to this reasons,the induced emf is called back emf of the motor.The magnitude of the back emf increase with the speed of motor.
Question: 20
When the primary of a transformer is connected to AC mains the current in it increase when secondary circuit is closed.Explain why?
Answer: 20
20-46
When the secondary circuit is closed the output power increase.To produce the power ,transformer will draw large current from the A.C mains to increase its primary power.
Question: 21
Why is split ring is used in DC generators in place of slips rings?
Answer: 21
21-46
The split rings are two halves of a ring that acts as commutator.When the current in the coil is zero and is about to change direction,the split ring also changes the contacts with the carbon brushes,Therefore the output remains the same direction although the current is not constantan magnitude.
Question: 22
Define electromagnetic induction.
Answer: 22
22-46
When a conductor is moved through a magnetic field,the electric current flow through the circuit,The emf produced in the conductor is called induced emf,and the current generated is called induced current.This phenomena is known as electromagnetic induction.
Question: 23
Can step up transformer increase the power level?
Answer: 23
23-46
No,a step up transformer can not increase the power level,in actual transformer,due of dissipation of energy in the coil the output power is always less than input power,Therefore a step up transformer can't increase power level.
Question: 24
Name the factor upon which the self inductance depends.
Answer: 24
24-46
It depends upon induced emd and time rate of change of current in the coil.It also depend upon the number of turns of the coil,its area of cross section and the core material.
Question: 25
What is electromagnet?Mention two practical example of electromagnet.
Answer: 25
25-46
When a specimen of iron is placed inside a current carrying solenoid,it becomes and electromagnet.Examples are
  1. Transformers
  2. Motors
Question: 26
Define mutual inductance and write at least two factors at which it depends.
Answer: 26
26-46
The ratio of average emf induced in the secondary to the time rate of changing current in the primary is called mutual inductance.
It depends upon numbers of turn of the coil,area of cross section of the coil closeness of coil and nature of the core materials.
Question: 27
If area of the loop and magnetic field both are changing and still have no induced emf.Explain why?
Answer: 27
27-46
If the plane of the loop is kept parallel to the direction of the magnetic field,magnetic field through the coil will be zero,no emf will be induced in the loop either by changing its are or by changing the magentic field.
Question: 28
What happen to the current of a circuit if a load resistance of the circuit is much less than the power transferred?
Answer: 28
28-46
The greater the load the larger the current is supplied by the generator,When the load resistance of the circuit is much less,small current is supplied by the generator.
Question: 29
Can DC motor be turned into DC generator?What changes are required to be done?
Answer: 29
29-46
Yes ,a D.C motor can be converted into D.C generator.For this armature of the motor is to be coupled with some rotating body.Due to rotation ,magnetic flux through the coil changes so and emf will be induced at the output.Hence the motor acts as a generator.
Question: 30
Define mutual induction.
Answer: 30
30-46
The phenomena in which the changing current in one coil induces an emd in another coil is called the mutual induction.
Question: 31
What is D.C motor?Write down part of D.C motor.
Answer: 31
31-46
D.C motor is a device which converts electrical energy into mechanical energy.Main part of D.C motor are commutator ,armature and permanent magnet or electromagnet.
Question: 32
A suspended magnet is oscillating freely in the horizontal plane.The oscillation are strongly damped when a metal plate is placed under,The magnet.Explain why its occurs.
Answer: 32
32-46
The oscillating magnet produces change of magnetic flux close to it.The metal plate placed below it experiences the change the magnetic flux,As the result eddy currents are produced inside the metal.According to Lenz's law these eddy currents oppose the cause which produce it,So the oscillation of magnet are strongly damped.
Question: 33
What is meant by back emf?Write its SI units.
Answer: 33
33-46
The emf induced across the ends of a coil due to the changing current in the same coil is called self induced emf or back emf.Its SI unit is Volt(V).
Question: 34
How does Lenz's law explain law of conservation of energy phenomena of electromagnetism induction?
Answer: 34
34-46
When a rod is moving in the magnetic field towards right,an induced current flows through the loop in anticlockwise direction,Since current carrying rod experience a magnetic force opposite to that velociety.An external force equal in magnitude and opposite in direction must be applied to keep the rod moving with constant velocity.This dragging force provides the energy for the induced current to flow,This energy is the source of induced current.Thus electromagnetic induction is exactly according to law of conservation of energy.
Question: 35
Can a transformer be used with D.C? Explain
Answer: 35
35-46
No As transformer works on the principle of electromagnetic induction,which is produced by A.C and not by D.C,To induced a voltage in the secondary coil it is necessary to have magnetic flux change.
Question: 36
What is DC motor?Write its principle.
Answer: 36
36-46
AD.c motor is a device which converts D.C electrical energy into mechanical energy.The basic principle of electric motor is that a current carrying coil placed in magnetic field experiences a torque which is given by t=NIAB cos theta.
Question: 37
Define self induction and mutual induction.
Answer: 37
37-46
The phenomena in which changing current in a coil induced a emf in itself is called self-induction.
The phenomena in which changing current in one coil induces an emf in another coil is called mutual induction.
Question: 38
Define the SI unit of mutual inductance.
Answer: 38
38-46
If current is changing at the rate of one ampere per second and the emf induced across the ends of the coil is one volt then inductance is called one Henry.Its SI units are VsA-1.
Question: 39
What changes are required to turn the D.C motors into a D.C generators?
Answer: 39
39-46
  1. In order to convert to convert DC motor into SC generator,the magnetic field must be supplied by the permanent magnet and not by electromagnet.
  2. An arrangement to rotate the coil armature should be provided and battery must be removed.
Question: 40
What is the working principle of transformer?Explain
Answer: 40
40-46
The transformer work on the principle of mutual induction between two coils.The transformer consists of two coils of copper electrically insulated from each other,wound on the same iron core,The coil to which AC power is supplied is called primary and that from which power is delivered to the circuit is called secondary.
Question: 41
What changes are required to turn the D.C motor into D.C generators?
Answer: 41
41-46
  1. In order to convert DC motor into DC generator the magnetic field must be supplied by the permanent magnet and not by electromagnet.
  2. An arrangement to rotate the coil armature should be provided and battery must be removed.
Question: 42
How can the power losses be minimized in a transformer?
Answer: 42
42-46
The power can be minimized in transformer by implementing following considerations.
  1. Core should be assembled from the laminated sheet of a material whose hysteresis loop are is very small.
  2. The insulation between lamination sheets should be perfect so as to stop the flow of eddy current.
  3. The resistance of the primary and secondary coils should be kept minimum.
Question: 43
What is mean by commutator in D.C generator?
Answer: 43
43-46
Commutator consists of two split rings or two halves of a single ring.Each half of split ring is connected to each end of the rotating coil,It helps to maintain the output in the same direction by inverting lower halves if sine curve.
Question: 44
How the power is lost due to eddy current in a transformer and how this loss can be minimized?
Answer: 44
44-46
The induced currents that are set up in the core of transformer in the direction perpendicular to the flux are known as eddy currents,It results in power dissipation and heating of the coral material.The insulation between lamination sheets should be perfect so as to stop the flow of eddy currents.
Question: 45
Can an electric motor be used to derive and electric generator with output from the generator being used to operate the motor?
Answer: 45
45-46
No,it is not possible.Because if it possible,it will be a self operating system without getting energy from some external source and this is against the law of conservation of energy.
Question: 46
Distinguished between AC generator and transformer.
Answer: 46
46-46
AC generator: AC generator is a device which produce AC voltage by converting mechanical energy into electrical energy.

12th Class Physics Chapters Short Questions