Consider a spherical conducting shell where all the charges reside on the surface. Using Gauss's Law, it can be found that the electric field inside the shell is zero. But why is the electric potential a constant? More over, why is the potential same as the potential on the surface of the shell? Please refer to the voltage plots in the following link. http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html Thank you.