Question

Consider a metal ring kept on a horizontal plane. A bar magnet is held above the ring with its length along the axis of the ring. If the magnet is dropped freely, the acceleration of the falling magnet is:
$(g$ is acceleration due to gravity)

• A. More than $g$
• B. Equal to $g$
• C. Less than $g$
• D. Depend on mass of magnet

Answer 1

The correct option is C Less than $g$

As the magnet falls, the magnetic flux linked with the ring increases. According to the Lenz’s law, the induces current in the ring will be in such a way that it will oppose the motion of the falling magnet i.e. the magnet will face retardation.
Hence, the magnet falls with an acceleration less than $g$, i.e. $a<g$.

Also, the direction of current in the ring will be anti-clockwise, as seen by an observer above the ring.

Hence, option ($C$) is the correct answer.

Why this Question? Key point: The direction of induced current is such that it opposes the relative motion between the ring and the magnet.