Question

A bar magnet is dropped along the axis of a copper ring held horizontally. The acceleration of fall is
A. equal to g at the place.
B. less than g.
C. more than g.
D. depends upon diameter of the ring and length of the magnet.

Answer 1

Hint: The above problem can be resolved using the fundamentals of Lenz law, and the basic concept employed under the Lenz law. In this problem, it is given that the bar magnet is made to fall from a height, and we need to determine the magnitude of acceleration during the freefall. When the magnet is dropped, then there will be variation in the magnetic field, and this will cause the development of emf.

Complete step by step answer:
The mathematical expression for the magnetic flux is given as,
\[\phi = B \times A\]…………………………………………. (1)
Here, B is the magnetic field and A is the area of the coil.
As the bar magnet is falling, then due to this fall, there will be some change in the magnitude of magnetic field taking place in the coil of area A.
And if the magnitude of magnetic field changes, then from equation 1, one can observe the change in the magnitude of magnetic flux linked with the coil. This will lead to the development of induced emf (E) and by Lenz’s law, the current will flow in the direction, such that it opposes the change in the magnetic field. This in turn will oppose the motion of the bar magnet by the force experienced on the magnet due to current flow.
Therefore, the net acceleration of the bar magnet during the fall is less than the gravitational acceleration g

Note:
Try to understand the fact that, when the magnetic field changes, the emf will induce. And this induced emf will cause the current flow in the direction, where the opposition in the magnetic field is observed.