two electrostatic questions, plz help, I'm so confused?

1. When the sphere is brought close to an earthed metal plate, -ve charges are induced on the plate. These induced -ve charges lowers the potential of the metal sphere (i.e. the potential of the sphere is lower than if the sphere is completely isolated, without the presence of the earthed plate).

When the sphere moves away from the earthed plate, the influence of the induced -ve charges on the sphere becomes less and less. The potential of the sphere is thus increased.

Just imagine that if the sphere is sufficiently far from the earthed plate, the influence of the induced -ve charges on the sphere becomes negligible. The potential of the sphere will restore to that of an isolated sphere.

2.When the charged sphere is inside the metal shell, charges of -q is induced on the inside surface of the shell and charges of +q is induced on the outer surface of the shell. The potential on the shell is determines by net charges enclosed by it, i.e. (+q)(from sphere) + (-q)(on shell inner surface) + (+q)(from shell outer surface) = +q (net charge).

When the charged sphere touches the inner surface of the shell, the charges of +q on the sphere neutralizes the induced charges of -q on the inner surface of the shell. As a result, only charges of +q resides on the outer shell surface. Since the net charge (+q) enclosed by the shell has not changed, its potential will not be changed.


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