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A thin disc of aluminium (a non-magnetic metal) lies horizontally and can spin freely about its central vertical axis. A powerful magnet is positioned vertically above the disc, offset from the axis. When this magnet is revolved in a circle about the disc's axis, what happens to the disc?

1. rotate in the direction opposite to the direction of magnet's motion
2. rotate in the same direction as the direction of magnet's motion
3. not rotate and its temperature will remain unchanged
4. not rotate but its temperature will slowly rise

Correct answer: rotate in the same direction as the direction of magnet's motion

Solution

As the magnet revolves, it produces a time-varying magnetic flux in the aluminium beneath it, inducing eddy currents. By Lenz's law these currents oppose the change, i.e. they oppose the relative motion of the magnet with respect to the disc. The only way to reduce this relative motion is for the disc to chase the magnet, so the disc rotates in the same sense as the magnet's revolution (it can never quite catch up, like an induction motor's slip).

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