Electromagnetic Induction

This MCQ is based on the topic of Electromagnetic Induction(Electricity and magnetism were considered separate and unrelated phenomena for a long time. The phenomenon in which electric current is generated by varying magnetic fields is appropriately called electromagnetic induction).

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Two coils of self inductance 2 mil and 8 mil are placed so close together that the effective flux in one coil is completely linked with other. The mutual inductance between these coils is :

4 mH 16 mH 10 mH 6 mH

The flux linked with a coif at any instant t is given by ∅ = 10t² – 50t + 250. The induced emf at t = 35 is:

10 V 190 V -190 V -10 V

The energy stored in a 50 mH inductor carrying a current of 4 A is:

0.1 J 0.4 J 0.04 J 0.01 J

The self-inductance L of a solenoid of length l and area of cross-section A, with a fixed number of turns N increases as

l and A increase. l decreases and A increases. l increases and A decreases. both l and A decrease.

A transformer is used to light 100 W and 110 V lamp from a 220 V mains. If the main current is 0.5 A. Then the efficiency of the transformer is:

11% 50% 80% 90%

The magnetic flux (∅) linked with a coil is related to the number of turns (N) ofthe coil as:

f ∝ N f ∝ N-1 f ∝ N2 f ∝ N-2

A metal conductor of length 1 m rotates vertically about one of its ends at angular velocity 5 rad s-1 . If the horizontal component of earth’s magnetism is 2 × 10-5 T, then e.m.f. developed between the two ends of the conductor is:

5 µV 50 µV 5 mV 50 mV

coil of L = 8.4 mH and R = 6 Ω is connected to a 12 V battery. The current in the coil is 1.0 A at approx’time of:

500 s 20 s 35 ms 1 ms

A metal rod is forced to move with constant velocity along two parallel metal rails, connected with a strip of metal at one end across a magnetic field (B) of 0.5 T, pointing out of the page. The rod is of length 45 cm and the speed of the rod is 70 cm/s. The rod has a resistance of 10 Ω and the resistance of the rails and connector is negligible. What is the rate at which energy is being transferred to thermal energy?

0.225 W 22.55 W 2.25 × 10^-4 W 2.25 × 10^-3 W

A bar of length 2m is said to fall freely in a magnetic field of magnitude 50 T. What is the motional emf in the bar when it has fallen 40 meters?

700 V 2100 V 2800 V 1400 V

Induced emf and motional emf are exactly the same.

True False

A bar of length 0.15 m slides along metal rails at a speed of 5 m/s. The bar and rails are in a magnetic field of 40 T, pointing out into the page. The resistance of two resistors in parallel is both 20 Ω, and the resistance of the bar is 5 Ω. What is the current in the bar?

1 A 2 A 3 A 5 A

A bar of length 0.7 m slides along metal rails at a speed of 1 m/s. The bar and rails are in a magnetic field of 20 T, pointing out into the page. Calculate the motional emf.

0.7 V 7 V 14 V 1.4 V

The role of inductance is equivalent to:

Inertia

Force

Energy

Momentum

A circular coil expands radially in a region of magnetic field and no electromotive force is produced in the coil. This can be because

The magnetic field is constant.

The magnetic field is in the same plane as the circular coil and it may or may not vary.

The magnetic field has a perpendicular (to the plane of the coil) component whose magnitude is constant.

There is a constant magnetic field in the perpendicular (to the plane of the coil) direction.

Which of the following will not induce an emf in a coil in the absence of an external voltage source?

The coil being in a time-varying magnetic field.

The coil moving in a time-varying magnetic field.

The coil moving in a constant magnetic field.

The coil is stationary in external spatially varying magnetic field, which does not change with time.

A metal plate is getting heated. Which one of the following is not the possible cause?

A direct current is passing through the plate.

It is placed in a time-varying magnetic field.

It is placed in a space-varying magnetic field, but does not vary with time.

A current (either direct or alternating) is passing through the plate.

There are two coils A and B as shown in figure. A current starts flowing in B as shown, when A is moved towards B and stops when A stops moving. The current in A is counterclockwise. B is kept stationary when A moves. We can infer that

There is a constant current in the clockwise direction in A.

There is a varying current in A.

There is no current in A.

There is a constant current in the counterclockwise direction in A.

When the magnetic field is parallel to a surface, then the magnetic flux through the surface is:

Infinite

Zero

Small but not zero

Large but not infinite.

The core of a transformer is laminated because:

Rusting of core may be prevented

Ratio of voltage in primary and secondary may be increased.