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

(B) rotate in the same direction as the direction of magnet's motion

When the magnet is moved, it creates a state where the plate through the magnetic flux, due to which an electromotive force is generated in the plate and eddy currents are induced. These currents are such that it opposes the relative motion ⇒ disc will in the direction of rotation of magnet.

Note : This apparatus is called Arago's disk