# SAT Subject Physics Practice Question 173: Answer and Explanation

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**Question: 173**

**19.** Two particles have unequal charges; one is +*q* and the other is –2*q*. 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

**Explanation:**

Since Coulomb’s law is an inverse-square law (that is, *F* is inversely proportional to *r*^{2}), if *r* decreases by a factor of 2, then *F* increases by a factor of 2^{2} = 4.