Question

A metallic ring with a small cut is held horizontally and a magnet is allowed to fall vertically through the ring then the acceleration of the magnet is
A. Always equal to g
B. Initially less than g but greater than g once it passes through the ring
C. Initially less than g but less than g once it passes through the ring
D. Always less than g

Answer 1

Concept of induced emf in accordance with Lenz law is to be used.
As there is a cut in the ring, no current will flow as the circuit is incomplete due to the cut.

Complete step by step answer:
We know that whenever magnetic flux linked with a closed circuit changes, so emf is induced in it which lasts only as long as the change in flux continues. This phenomenon is called electromagnetic induction.
According to Lenz law, the direction of induced current is such that it opposes the cause by which it produces i.e. it opposes the change in magnetic flux.
So, when a magnet is permitted to fall vertically along the axis of the metallic ring, then the upper face develops the same polarity as the pole of the magnet facing the ring, so, polarity developed on the upper face of the ring will be the same as that of the magnet. This is in accordance with Lenz law. So, the acceleration of the magnet is less that ‘g’
But as there is cut in the metallic ring so although emf is induced but there will be no induced current in it. This is because for current to flow, the circuit should be closed, so, there will be no opposition to motion of magnet. Hence, the magnet will fall freely under the acceleration due to gravity.
So, acceleration of the magnet in this case will always equal to g.

So, the correct answer is “Option A”.

Note:
Remember without closed circuit, there will be no induced current.
The magnet will fall freely under the acceleration due to gravity.
Hence, non-applicability of Lenz law.