SAT Subject Physics Practice Question 65: Answer and Explanation

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Question: 65

2. Two 1 kg spheres each carry a charge of magnitude 1 C. How does FE, the strength of the electric force between the spheres, compare with FG, the strength of their gravitational attraction?

A. FE < FG
B. FE = FG
C. FE > FG
D. If the charges on the spheres are of the same sign, then FE > FG ; but if the charges on the spheres are of opposite sign, then FE < FG.
E. Cannot be determined without knowing the distance between the spheres

Correct Answer: C


The strength of the electric force is given by , and the strength of the gravitational force is . Since both of these quantities have r2 in the denominator, we simply need to compare the numerical values of kq2 and Gm2. There’s no contest since


we see that kq2 > Gm2, so FE is much stronger than FG.

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