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A rectangular coil is pulled directly away from a fixed straight wire carrying a steady current, as shown. In which sense does the induced current flow in the coil?

1. Anti-clockwise
2. Clockwise
3. Zero
4. It may be clockwise or anticlockwise depending on the coil's speed

Correct answer: Clockwise

Solution

The field of the straight wire is strongest near it and falls off as 1/distance. As the coil moves away, the flux through it (into the page, in the standard figure) decreases. By Lenz's law the induced current flows so as to oppose this decrease, i.e. to reinforce the into-the-page flux, which requires a clockwise current in the coil.

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