A conducting sphere carrying charge Q is surrounded by a spherical conducting shell.
a. What is the net charge on the inner surface of the shell?
b. Another charge q is placed outside the shell. Now, what is the net charge on the inner surface of the shell?
c. If q is moved to a position between the shell and the sphere, what is the net charge on the inner surface of the shell?
d. Are your answer valid if the sphere and shell are not concentric?

Explanation

(a) Facing surface have equal and opposite charges
(b)