Bihar Board - Class 12 Physics - Chapter 6: Electromagnetic Induction Short Answer Question

1. What is the magnetic flux state of its unit?

Answer: The magnetic flux through any surface placed in a magnetic field is the total number of magnetic lines of force crossing the field is the total number of magnetic lines of force crossing the surface is normally , denoted by ϕ or ϕB

2. What are the methods of generating induced emf?

Answer:

• Induced emf by changing the magnetic field B

• Induced emf by changing the area of coil

• Induced emf by changing the relative orientation of the coil and the magnetic field

3. What is the lenz’ s law ?

Answer: According to Lenz's law , the polarity of the induced e.m.f. is such that it opposes the change in the magnetic flux responsible for its production.

4. Why is the core of a transformer laminated?

Answer:
The core of a transformer is laminated because of preventing eddy current being produced in the core.

5. How can the self-inductance of a given coil having N number of turns, area of cross-section A and lengths l be increased?

Answer:
The self-inductance can be increased by the help of electric fields. It does not depend on the current through the circuit but depends upon the permeability of material from which the core is made up of.

6. State Lenz’s law. The closed loop PQRS is moving into a uniform magnetic field acting at right angles to the plane of the paper. State the direction in which induced current flows in the loop.

Answer:  Lenz’s law states that when there is the change in magnetic flux linked with the coil, a cause of its production.
The direction of induced current in the loop will be anticlockwise i.e., along SRQP

7. When a magnet is moved towards a suspended wire loop as shown in figure. Evaluate the direction of induced current in the loop. State the law used by you for this evaluation.

Answer:  When the south pole of a magnet moves towards a suspended wire loop the direction of induced current is such that the surface of the wire loop facing south pole of the moving magnet behaves as south pole so that it may oppose the cause by which it is produced. This is in accordance with Lenz’s law.

 8. At an airport, a person is made to walk through the doorway of a metal detector, for security reasons. If the person is carrying anything made of metal, the metal detector emits sound. On what principle does this detector work ? Explain.

Answer:  This work on the principle of electromagnetic induction.
There are two coils, primary and secondary. Under ordinary conditions appreciable magnetic induction does not take place and the alarm circuit attached with secondary is not activated. As soon as a metal is placed between the coils eddy currents are produced in it. Eddy currents being sinusoidal in nature induce e.m.f. in the secondary coil which is sufficient to activate the alarm circuit.

9. A pair of adjacent coils has a mutual inductance of 1.5 H. If the current in one coil changes from 0 to 20 A in 0.5 s, what is the change of flux linkage with the other coil?

Solution:

Hers, M = 1.5H; dl = 20-0 = 20A;

dt = 0.5 s

Now φ = MI

Or dφ= Mdl=1.5 x 20 = 30Wb.

10. Consider a metallic pipe with an inner radius of 1 cm. If a cylindrical bar magnet of radius 0.8 cm is dropped through the pipe, It takes more time to come down than It takes for a similar
unmagnetised cylindrical iron bar dropped through the metallic pipe. Explain. 

Answer:
For the magnet, eddy currents are produced in the metallic pipe. These currents will oppose the motion of the magnet. Therefore the magnet's downward acceleration will be less than the acceleration due to gravity g. On the other hand, an unmagnetised iron bar will not produce eddy
currents and will fall an acceleration g. Thus the magnet will take more time.

11. A wire in the form of a tightly wound solenoid is connected to a DC source, and carries a current. If the coil is stretched so that there are gaps between successive elements of the spiral coil, will the current increase or decrease? Explain.

Answer:

The current will increase. As the wires are pulled apart the flux will leak through the gaps. Lenz’s law demands that induced emf resist this decrease, which can be done by an increase in current.

12. A solenoid is connected to a battery so that a steady current flows through it. If an iron core is inserted into the solenoid, will the current increase or decrease? Explain.
Answer:
The current will decrease. As the iron core is inserted in the solenoid, the magnetic field increases and the flux increases. Lenz’s law implies that induced emf should resist this increase, which can be achieved by a decrease in current. However, this change will be momentarily.

.
13. Consider a metal ring kept (supported by a cardboard) on top of a fixed solenoid carrying a current /. The center of the ring coincides with the axis of the solenoid. If the current in the solenoid is switched off, what will happen to the ring? 

Answer:
When the current in the solenoid decreases a current flows in the same direction in the metal ring as in the solenoid. Thus there will be a downward force. This means the ring will remain on the cardboard. The upward reaction of the cardboard on the ring will increase.

14. In what sense are eddy currents considered undesirable in a transformer and how are these reduced in such a device ?

Answer:  In a transformer, a considerable amount of electric power would get consumed due to Joule’s heating effect, if large eddy currents produced in its core are not reduced.
To reduce eddy currents, we have identical slotted iron strips. Coat them with varnish for insulation and join together to form a thick core of a transformer. As resistance of such a core is much larger than that of a single thick core, eddy currents get reduced to a good amount.

15. What are eddy currents ? How are they produced ? Give two important applications of eddy currents.

Answer:  Eddy currents are the currents induced in the body of a conductor.
When the magnetic flux linked with a conductor changes, eddy currents are produced.
Two important applications of eddy currents are in electromagnetic damping and induction motor.

16. Define the term ‘self-inductance’. Give its unit. Write an expression for the energy stored in an inductor when a steady current ‘I’ is passed through it. Is this energy electric or magnetic ?

Answer:  Self inductance of a coil is defined as the emf induced in the coil when the current passing through it changes at the unit rate. Its SI unit is Henry (H) when a current I flows through an inductor of inductance L, the energy stored in it is
                                                U = 12 Ll2
The energy is stored as magnetic energy.

17. Take a strong cylindrical electromagnet connected to the AC source and place a light metallic disc at the top of it. When the current is switched on the disc is thrown up in the air.
(a) Why does the disc go up?
(b) How is the repulsive force produced?

Answer:
(a) Induced currents are produced in the disc which acts in such a manner that it opposes the increase of magnetic flux through it. As a result, the disc is thrown up.

(b) When the circuit is switched on, the current starts growing through the solenoid. As the current grows, the magnetic lines along the axis of the solenoid and hence magnetic flux through the disc also increases from zero to some finite value. Induced currents are produced in the disc which opposes the increase of magnetic flux through it. Hence the disc is thrown up.

18. When a metallic block moves in a magnetic field induced currents are developed in the body of the block.

(a) What is this type of current called?

(b) Which law gives the direction of such currents?

(c) What is the strength of the induced current?

Answer:

(a) Eddy currents

(b) Lenz’s law

(c) Since the metallic block has very small resistance, the strength of eddy current will be very large.

19. A magnet is moved towards a coil. What is the source of electric current induced in the coil?

Answer:
Mechanical energy used to move the magnet towards the coil is converted into electrical energy and induced current is produced in the coil.

20.Why self induction is called inertia of electricity?

Answer:

Because self induction opposes the variation in current in an electric circuit which is analogous to inertia of a body which opposes any change in its position.

21. What is the principle of electromagnetic damping?

Answer:
When a conductor is made to rotate in a magnetic field, the magnetic flux linked with the conductor changes and induces e.m.f. and hence induced current is produced. The direction of induced current is such that it always opposes the cause of its production in accordance with Lenz’s law, hence it opposes the rotation of conductor and stops its motion. This is called electromagnetic damping.

22.Magnetic flux is a misnomer. Explain.

Answer:
The term flux is generally used when something is flowing out of certain material, for example, water coming out of a tap is called flux of water. But in case of magnetic flux, there is nothing that really flows through the surface. Hence it is a misnomer.

23.A wire in the N-S direction is pin dropped freely. Will any potential difference be induced across its ends?

Answer:
No, because there is no change in the horizontal component and vertical component of earth’s magnetic field due to the fall of the wire.

24.If the wire kept in E-W direction be dropped freely, then, is there any potential difference induced across its ends?

Answer:
Yes, because the horizontal component of earth’s magnetic field is intercepted by the wire and hence potential difference is induced across the ends of the falling wire.

25. A solenoid with an iron core and a bulb are connected to a d.c. source. How does the brightness of the bulb change, when the iron core is removed from the solenoid?

On removing the iron core from the solenoid, the magnetic flux linked with the coil decreases and induced e.m.f. is produced, hence the brightness of the bulb will increase.