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JEE Advanced · 2020 · 27 Sep 2020 · Paper 1 · Electromagnetic Induction

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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)-
  1. (A)rotate in the direction opposite to the direction of magnet's motion
  2. (B)rotate in the same direction as the direction of magnet's motion✓ Correct
  3. (C)not rotate and its temperature will remain unchanged
  4. (D)not rotate but its temperature will slowly rise

Answer: (B)

Solution

When the magnet is revolved around the axis, the changing magnetic flux through the aluminium disc induces eddy currents (by Faraday's law). According to Lenz's law, these eddy currents create a magnetic field that opposes the relative motion between the magnet and the disc. This opposition manifests as a force that tries to reduce the relative motion, causing the disc to rotate in the same direction as the magnet's motion. This device is known as Arago's disk.

JEE Advanced · 2020 · 27 Sep 2020 · Paper 1 · Single correct option

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