I never really understood this... Lets say we have a positive point charge Q at the center of a spherical conducting shell of an inner radius Ri and an outer radius R0. Now my books says that an amount of negative charge equal to -Q must be induced on the inner shell surface (R=Ri), and that an amount of positve . charge +Q is induced on the outer shell surface (R=R0). Why is this? What would happen if a negative point charge was placed in the center? I would assume that the inner shell would have an induced positve charge, and the outer shell would have an induced negative charge. Again, why is this? Also what would happen if there was no charge at the center, and instead a charge was placed on the outside of the shell near the conducting surface?