SAT Subject Physics Practice Question 78: Answer and Explanation

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Question: 78

5. Which of the following would be half as great at a distance of 2R from a source charge than it would be at a distance of R from the charge?

I. Electric force on another charge

II. Electric field due to the source charge

III. Electric potential due to the source charge

A. I only
B. II only
C. III only
D. I and II only
E. I, II, and III

Correct Answer: C


5 C Both electric force and electric field obey inverse square laws. The equation for electric force is F = , and the equation for electric field is E = . Thus, both of these would be one-quarter as great at twice the distance. However, electric potential is just an inverse. The equation for electric potential is (notice that the r is not squared). Thus, at twice the distance, it would be half as great. Thus, only Roman numeral III is true.

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