Question

A bar magnet is held vertically with its upper end a little bit below the center of a horizontal metal ring. The upper end of the magnet is its north pole (see Fig.29.12). The bar magnet is now dropped. An observer views the ring from above its center. To this observer, how will the induced current in the ring behave as the magnet falls?

The current will flow counterclockwise and be increasing.
The current will flow counterclockwise and be decreasing.
The current will flow clockwise and be increasing.
The current will flow clockwise and be decreasing.
The induced current will be zero.

Answer 1

According to Lenz's law, the direction of the current induced by the change in magnetic field is such that the magneticfield produced by this current will oppose the original magnetic field.

The induced emf is defined as foll ows:

$$ \varepsilon=-\frac{d \Phi_{\mathrm{B}}}{d t} $$

The magnetic field strength decreases when the magnet is dropped. In this case, the north end of the magnet is below the loop and is moving away from the ring, so the flux through the ring is directed upward and is decreasing in magnitude. Thus, the induced current will flow counterclockwise in order to produce an upward flux through the ring and oppose

the increasing flux due to the magnet.Therefore, the current decreasing in a counter - clockwise direction (or anti-clockwise direction).

Answer 2

The current will flow clockwise and be decreasing. Because As the magnet approaches from the top, first thesouth pole of the magnet induces a current in the ring and from Lenz's law a clockwise current flows in the ring and the ring behavesas a south pole so as to oppose the motion of the magnet.

Next as the magnet passes through the ring and Recedes from the ring, again the receding north pole induces aclockwise current, following Lenz's law, and the ring still behaves as a south pole, Now Attracting the magnet. Since the magnet is receding theinduced current is decreasing in the circuit.