Consider three plastic balls (A, B, and C), each carrying a uniformly distributed charge equal to either +Q, -Q or zero, and an uncharged copper ball (D). A positive test charge (T) experiences the forces shown in the figure when brought very near to the individual balls. The test charge T is strongly attracted to A, strongly repelled from B, weakly attracted to C, and strongly attracted to D.

Assume throughout this problem that the balls are brought very close together. What is the nature of the force between balls A and C?

(A) strongly attractive
(B) strongly repulsive
(C) weakly attractive
(D) neither attractive nor repulsive

Since the positive test charge T is strongly attracted to A, the net charge of A must be negative. C is made of plastic, this is an insulator material, this means that it can be polarized. Since the test charge T is weakly attracted to C, the charge on C must be zero. Therefore C can be polarized by the presence of an electric field such as that generated by A, so this will generate a weak electrical attraction between A and C.