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Two identical coaxial circular current loops carry equal currents i flowing in the same sense. If the loops are moved closer together, what happens to the current in each loop (assuming each is part of a circuit maintaining the same conditions)?

1. the current in each will decrease
2. the current in each will increase
3. the current in each will remain the same
4. the current in one increases and in the other decreases

Correct answer: the current in each will decrease

Solution

As the loops approach, the mutual flux through each loop (due to the other, same-sense current) increases. By Lenz's law the induced EMF opposes this increase, i.e. it opposes the existing current, so the current in each loop decreases.

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