A conductor A with a cavity as shown in the figure is given a charge Q. Find the net charge appear on the inner surface of the cavity.

Assume a Gaussian surface that is lying wholly within a conductor and enclosing the cavity. The electric field intensity (E) acting inside the charged conductor is zero.Let q be the charge inside the conductor and ε0 be the permittivity of free space. Now, according to Gauss's law, we have the electric fluxϕ=E→⋅ds→=qε0where E=0 ; thus q=0. Therefore, the charge inside the conductor is zero. That is, the charges appears on the outer surface of the conductor.