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)
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JEE Advanced 2020 · Paper 1, Physics question 2
Answer and worked solution
Official answer: B
Revolving the magnet sweeps a changing flux over the part of the disc under it, which induces eddy currents in the aluminium.
By Lenz's law the currents oppose the relative motion between magnet and disc, so the force on the disc pulls it along with the magnet, as in an induction motor. The disc rotates in the same direction as the magnet. (The currents also heat it slightly, but it does rotate.)
(B)