First slide

Lenz's law and conservation of energy

Question

A small bar magnet is allowed to fall vertically towards a copper ring which is held stationary with its plane horizontal. The axis of the ring passes through the mid-point of the bar magnet. If ‘a’ be the acceleration of the ring, then

Moderate

A

a < g
B

a = g
C

a > g
D

relation between ‘a’ and ‘g’ depends upon the diameter of the ring.

Solution

As the bar magnet is horizontal and symmetrically placed about the axis of the ring, at any instant of time, the net magnetic flux linked with ring is zero and net rate of change of flux is also zero. So no emf and no current is induced in the ring and the magnet is neither repelled nor attracted by the ring. Hence a = g.

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