SAT Subject Physics Practice Question 173: Answer and Explanation
19. Two particles have unequal charges; one is +q and the other is –2q. The strength of the electrostatic force between these two stationary particles is equal to F. What happens to F if the distance between the particles is halved?
A. It decreases by a factor of 4.
B. It decreases by a factor of 2.
C. It remains the same.
D. It increases by a factor of 2.
E. It increases by a factor of 4.
Correct Answer: E
Since Coulomb’s law is an inverse-square law (that is, F is inversely proportional to r2), if r decreases by a factor of 2, then F increases by a factor of 22 = 4.