Conducting and Insulating Conductors: why uniform and 'surface' charge distribution?

[Kuhan]

Why is it that when you have an insulator, the charges are uniformly distributed but in a conductor, the charges are only at the surface?

 

[mfb]

Insulators can have their charge everywhere, and in many real setups you will find them on the surface only, too.

Conductors cannot have net charges inside in equilibrium - those charges would induce a field in the conductor, which would lead to a current flow, and that violates the equilibrium condition.