SAT Subject Physics Practice Question 73: Answer and Explanation

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Question: 73

10.

The figure above shows four point charges fixed in position at the corners of a square. What charge would have to be present at the bottom right location for the electric field at the center of the square to be zero?

A. +Q
B. +Q
C. +2Q
D. +3Q
E. +4Q

Correct Answer: A

Explanation:

The individual electric field vectors at the center of the square due to the two negative source charges cancel each other out. So we simply need to make sure that the individual electric field vector at the center of the square due to the bottom-right source charge cancels the individual electric field vector at the center of the square due to the upper-left source charge. Since the upper-left source charge is +Q, the bottom-right source charge must be +Q also.

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