試題詳解

16. Consider a system consisting of a conducting sphere and a conducting spherical shell that are concentric. The radius of the sphere is a； the radii of the inner and outer surfaces of the shell are 2a and 3a, respectively, as shown in the figure. If the sphere carries a net charge of -Q and the shell carries a net charge of +2Q, what is the electric potential at a distance of a/2 from the center when the system is in electrostatic equilibrium? Assume that the zero of potential is at infinity. k is the Coulomb's constant.

(A) -kQ/a
(B) -2kQ/a
(C) 2kQ/a
(D) kQ/3a
(E) -kQ/6a