Same as problem 4 except the coil A is made to rotate about a vertical axis. No current flows in B if A is at rest. The current in coil A, when the current in B (at t = 0) is counterclockwise and the coil A is as shown at this instant, t = 0, is

(a) constant current clockwise.

(b) varying current clockwise.

(c) varying current counterclockwise.

(d) constant current counterclockwise.

Answers (2)

The answer is the option (a)

Here Lenz’s law is followed which states that, the direction of induced emf (current) is in such that it opposes the cause that produced it. It is derived from the law of conservation of energy.

The current of coil B is given to be counter-clockwise, so if this current is induced so as to oppose its cause than the current in the coil A should be in constant clockwise direction. And it is also supported by the fact that when A is stationary then there is no current induced in coil B.

Key concept: In this problem, the Lenz’s law is applicable so let us introduce Lenz’s law first. .
Lenz’s law gives the direction of induced emf/induced current. According to this law, the direction of induced emf or current in a circuit is such as to oppose the cause that produces it. This law is based upon law of conservation of energy.
When the current in coil B (at t= 0) is counter-clockwise and the coil A is considered above it. The counter clockwise flow of the current in coil B is equivalent to north pole of magnet and magnetic field lines are eliminating upward to coil A. When coil A starts rotating at t = 0, the current in A is constant along clockwise direction by Lenz’s rule.

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