Conductor

Properties of conductors
1 There is no electric field inside a conductor.
Otherwise, the charges in the conductor would move.
2 Charge exist only on the surface of a conductor.
Otherwise, there would be electric fields inside.
3 All points of a conductor are at the same potential.
Since V=-grad E and E=0, the potential must be constant.
Conductors are materials such as copper, aluminium, silver... The fact that electric field do not penetrate into conductors explains why light, which is an electromagnetic wave, does not penetrate into a metal.

Inside a conductor, only one sign of charge can exist. That is, either the charge is positive or negative. And inside a conductor, the charge can move freely.

2009-08-28 18:17:03 補充：
Now, same charges repel, so the charges try to stay away from each other as far as possible.

As they can move freely in the conductor, the further apart from each other they can stay is to stay on the surface of a conductor.