SAT Subject Physics Practice Question 101: Answer and Explanation

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Question: 101

1. A metal rod of length L is pulled upward with constant velocity v through a uniform magnetic field B that points out of the plane of the page.

What is the potential difference between points a and b ?

A. 0
B. vBL, with point a at the higher potential
C. vBL, with point b at the higher potential
D. vBL, with point a at the higher potential
E. vBL, with point b at the higher potential

Correct Answer: E

Explanation:

Since v is upward and B is out of the page, the direction of FB on the free electrons will be to the left, leaving an excess of positive charge at the right. Therefore, the potential at point b will be higher than at point a, by ε = vBL (motional emf).

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