SAT Subject Physics Practice Question 65: Answer and Explanation

Next steps

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

Explanation:

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

and

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

Previous       Next