Question

A light disc made of aluminium ( a non - magnetic 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

Consider a small hypothetical loop just below the north pole of the magnet . As the magnet moves , the number of field lines crossing the loop decreases , as a result of which flux through the loop decreases.
According to Lenz law the upper face of the loop should behave as south pole and try to oppose the relative motion between the bar magnet and the loop.
The same situation holds true for all the infinite number of hypothetical loops .
However, the relative motion can be opposed only if the entire disc starts moving in the same sense as the bar magnet.

Hence, option (b) is the correct answer.

Note: This apparatus is called Arago's disk