Magnet going upward through a loop, direction of current

Question

This a question that bothers me from a SAT Physics review book.

So in the question, there is a bar magnet with north pole up and south pole down going upwards through a loop of wire and (from my understanding) inducing a current from the emf created by change in magnetic flux.

The question asks what direction is the current and whether it changes direction. My teacher believes it will always be clockwise, but the answer from Princeton review says that it will first become clockwise, then switch to counterclockwise.

I personally think my teacher is right since the change in magnetic flux is in the same "direction" (NORTH pole approaching loop, and then SOUTH pole leaving loop), thus always generating a clockwise current.

So which is the correct answer? Any explanations would greatly help, thanks so much.

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The direction of the magnetic field through the loop does not change but at first the magnetic flux linked with the loop is increasing and later the magnetic flux is increasing. — Farcher, May 30 '18 at 05:02

score 1 · Answer 1 · answered Jun 04 '18 at 10:11

This

the change in magnetic flux is in the same "direction"

is incorrect. The magnetic flux starts off at zero, increases to a maximal value, and then decreases back to zero. This is impossible to achieve with a monotonic derivative, i.e. the rate of change of the flux needs to change sign partway through the interaction.

Magnet going upward through a loop, direction of current

1 Answers1