SAT Subject Physics Practice Question 218: Answer and Explanation

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Question: 218


A loop of metal wire containing a tiny lightbulb is attached to an insulating handle and placed over a coil of wire in which a current can be established by a source of emf and controlled by a variable resistor. The plane of the top loop is parallel to the plane of the bottom coil. Which of the following could NOT cause the bulb to light?

A. Rotating the handle 90° while keeping the plane of the top loop parallel to the plane of the bottom coil
B. Raising the handle up and away from the coil
C. Lowering the handle down toward the coil
D. Decreasing the resistance of the coil
E. Increasing the resistance of the coil

Correct Answer: A


B, C, D, and E all change the magnetic flux through the loop containing the light bulb, thus inducing an emf and a current. However, just swinging the handle over as described in choice A changes neither the area presented to the magnetic field lines from the bottom coil nor the density of the field lines at the position of the loop. No change in magnetic flux means no induced emf and no induced current.

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