Question

A metallic ring is held horizontally and a magnet is allowed to fall vertically through the ring, then the acceleration of this magnet is

• A. Equal to $g$
• B. More than $g$
• C. Less than $g$
• D. Sometimes less and sometimes more than $g$

Answer 1

The correct option is C Less than $g$

When the magnet falls vertically down, the magnetic flux through the loop increases.
So, according to Lenz's law, current will be induced in the loop in such a way that the magnetic field due to the current will oppose the increase in magnetic flux.
i.e current in the loop is induced in an anticlockwise direction.


Or, we can consider that the loop (with induced current) behaves like a magnetic with reverse polarity to the magnet which is moving down.



Then the north pole of the loop magnet will repel the north pole of the bar magnet. And due to this repulsion, the acceleration of the vertically falling bar magnet will be less than $g$

Once the magnet has passed through the loop and is falling further, the magnetic flux through the loop due to the magnet actually starts decreasing. Therefore, to oppose the change in magnetic flux, current will be induced in clockwise direction in the loop which leads to the loop behaving like a magnet with polarity as shown in the figure.



The north pole of the loop will then attract the south pole of the falling bar magnet and due to this attraction, the acceleration of the bar magnet is less than $g$ again. Thus, in all cases, the acceleration in less than $g$. Option (c) is correct.