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A long straight wire carries a current from A to B that is increasing in magnitude. A nearby conducting loop lies in the same plane, beside the wire. The induced current in the loop will

1. have a direction such that its own field opposes the increasing flux of the wire
2. have a direction such that its own field aids the increasing flux of the wire
3. be zero because the wire and loop are coplanar
4. oscillate between clockwise and anticlockwise

Correct answer: have a direction such that its own field opposes the increasing flux of the wire

Solution

The straight wire produces a magnetic field that threads the coplanar loop. As the current from A to B increases, the magnetic flux through the loop increases. By Lenz's law the induced current must flow so that its own magnetic field opposes this increase, i.e. it tries to maintain the original (smaller) flux. The exact clockwise/anticlockwise sense depends on which side of the wire the loop sits, but in all cases the induced current opposes the increasing flux. The current is steady (not oscillating) and is non-zero even though the wire and loop are coplanar.

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