SAT Subject Physics Practice Question 105: Answer and Explanation

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Question: 105

5. In the figure below, a permanent bar magnet is pulled upward with a constant velocity through a loop of wire.

Which of the following best describes the direction(s) of the current induced in the loop (looking down on the loop from above)?

A. Always clockwise
B. Always counterclockwise
C. First clockwise, then counterclockwise
D. First counterclockwise, then clockwise
E. No current will be induced in the loop.

Correct Answer: C


By definition, magnetic field lines emerge from the north pole and enter at the south pole. Therefore, as the north pole is moved upward through the loop, the upward magnetic flux increases. To oppose an increasing upward flux, the direction of the induced current will be clockwise (as seen from above) to generate some downward magnetic flux. Now, as the south pole moves away from the center of the loop, there is a decreasing upward magnetic flux, so the direction of the induced current will be counterclockwise.

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