1. The problem statement, all variables and given/known data If the electric potential of the inner shell is 10 volts and that of outer shell is 5 volts, then the potential at the center will be? 2. Relevant equations The radius of inner shell = a (given) The radius of outer shell = 2a (given) 3. The attempt at a solution Having read that electric field at the center of a shell is zero, I assume, the potential is constant throughout like in the case of a conducting sphere, and hence the answer is 10 volts. But I could get this answer only after a lot of manipulation, and I'm still not clear about it. Also, if someone can help me out with why the electric field inside a shell is zero? Doesn't necessarily have to be a conducting shell.