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A magnet approaches a coil along its axis, inducing an emf epsilon in the coil. If the coil now also moves toward the magnet with the same speed (both approaching), what is the induced emf?

1. epsilon/2
2. epsilon
3. 2*epsilon
4. 4*epsilon

Correct answer: 2*epsilon

Solution

Induced emf is proportional to the relative velocity. Initially the relative speed is v (emf = epsilon). When both approach at v, relative speed is 2v, so the emf doubles to 2*epsilon.

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