A uniform magnetic field directed into the page occupies a region of width L.  A conducting coil of width L moves at constant speed v, from left to right, through the field.

From the instant that the coil enters the field until the instant that the coil leaves the field, which of the following best describes the direction of the current induced in the coil?

A. Anti-clockwise

B. Clockwise

C. Anti-clockwise then clockwise

D. Clockwise then anti-clockwise

C

It would seem that most candidates intuitively chose between A and B, with only those who understood Lenz’s Law selecting C or B. The essence of the solution is that as the square is entering the field the flux linkage is increasing, while as it is leaving the linkage is decreasing. Hence neither A nor B can be correct. And since the direction of the current is such as to try to reduce the change in flux linkage, it should set up a magnetic field out of the page as the loop enters the field. Hence C is the correct answer.

Candidates clearly need more practice in the application of Lenz’s Law in a variety of different situations.