Question

If a hollow metal sphere is given a charge Q and another conducting sphere is placed inside it concentrically. A charge q is placed at the centre. The charge that appears on the surface B of the inner sphere is

• A. -q
• B. +q
• C. zero
• D. +Q

Answer 1

The correct option is B

+q

If you take a Gaussian surface through the material of the second conductor, the flux must be zero (as the net electric field inside is zero). But the Gauss law says that if the flux through a surface is zero then the charge enclosed must be zero. And it is for that reason that surface A gets negative charge –q so that the net charge enclosed becomes zero. Now as the inner sphere had zero charge to begin with, the outer surface i.e. surface B gets positive charge q