Question

A light disc made of Aluminium (a nonmagnetic material) is kept horizontally and is free to rotate about its axis, as shown in the figure. A strong magnet is held vertically at a point above the disc away from its axis. On revolving the magnet about the axis of the disc, the disc will (figure is schematic and not drawn to scale)

• A. Rotate in the direction opposite to the direction of magnet's motion
• B. Rotate in the same direction as the direction of magnet's motion
• C. Not rotate and its temperature will remain unchanged
• D. Not rotate, but its temperature will slowly rise

Answer 1

The correct option is B Rotate in the same direction as the direction of magnet's motion

As the magnet rotates about the axis of the disc, the magnetic flux linked with the part of the disc lying just below the magnet will decrease. According to Lenz's law the disc will rotate in the direction of the rotation of the magnet to compensate for the decreasing magnetic flux.

Hence, the disc will rotate in the same direction as the direction of the rotation of the magnet.



$\text{Alternate method :}$
By Lenz's law, the disc also tries to move in the same direction because in the backward part of the disc the flux reduces as the magnet moves and as there is change in magnetic flux, so there is eddy current production which leads to production of heat These currents are such that it opposes the relative motion. So, disc will rotate in the direction of rotation of magnet.

Hence, option $\left(\text{B}\right)$ is correct.