1. The problem statement, all variables and given/known data A non-conducting spherical shell is uniformly charged. The electrostatic potential [itex]\phi[/itex] at the centre of the sphere is [itex]\phi[/itex]1 = 200V The potential at distance r = 50cm from the centre is [itex]\phi[/itex]2 = 40V Find the radius of sphere: a 2. Relevant equations I seem to have missed this lecture. 3. The attempt at a solution I understand that the potential (V) is a function of r (distance from centre) and satisfies: V(0) = 200V V(50) = 40V V(a) = 0V I've been reading up on this, but I can't seem to make sense out of this question. I can't seem to find a case for non conducting spherical shell. Would someone mind providing me a formula and clear steps to solve this problem? Thank you!