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Two coaxial circular coils A and B face each other. The current i in coil A can be varied. Which statement is correct?

1. A and B repel each other when i is increased
2. A and B attract each other when i is increased
3. there is neither attraction nor repulsion when i is changed
4. whether A and B attract or repel depends only on the current direction, not on whether i increases or decreases

Correct answer: A and B repel each other when i is increased

Solution

When i in A increases, the magnetic flux through B rises. By Lenz's law, B develops an induced current that opposes this increase, i.e., it creates a magnetic field opposing A's field. The two coils then present like magnetic poles to each other, so they repel. (Conversely, decreasing i would cause attraction.) Therefore increasing i produces repulsion.

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