Cracking the SAT Physics Subject Test

CHAPTER 9 DRILL

1. D Electrostatic force obeys an inverse-square law: FE. Therefore, if
   r increases by a factor of 3, then FE decreases by a factor of 3^2 = 9.


2. C The strength of the electric force is given by , and the strength of

the gravitational force is. Since both of these quantities have r^2 in the

denominator, we simply need to compare the numerical values of kq^2 and

Gm^2. There’s no contest since

and

we see that kq^2 > Gm^2 , so FE is much stronger than FG.

3. C If the net electric force on the center charge is zero, the electrical repulsion
   by the +2q charge must balance the electrical repulsion by the +3q charge:


4. E Since P is equidistant from the two charges, and the magnitudes of the
   charges are identical, the strength of the electric field at P due to +Q is the
   same as the strength of the electric field at P due to −Q. The electric field
   vector at P due to +Q points away from +Q, and the electric field vector at P