SAT Subject Physics Practice Question 78: Answer and Explanation
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.