An uncharged spherical conducting shell surrounds a charge −q at the center of the shell. Then charge +3q is placed on the outside of the shell. When static equilibrium is reached, the charges on the inner and outer surfaces of the shell are respectively
A) +q, −q.
B) −q, +q.
C) +q, +2q.
D) +2q, +q.
E) +3q, 0.

Question

Quizplus · Accepted Answer

When a charge is placed inside a conducting shell, the inner surface of the shell acquires a charge equal in magnitude but opposite in sign to the enclosed charge to ensure the electric field inside the conductor is zero. Thus, the inner surface gets a charge of +q to neutralize the effect of the −q charge inside. When +3q is placed on the outside, the total charge on the shell must equal +3q. Since the inner surface already has +q, the outer surface must have +2q to make the total charge on the shell +3q.

An Uncharged Spherical Conducting Shell Surrounds a Charge −Q at the Center