1. The problem statement, all variables and given/known data A point charge +Q is placed at the centre of an isolated conducting shell of radius R. Find the electrostatic potential energy stored outside the spherical shell if the shell also contains a charge +Q distributed uniformly over it. 2. Relevant equations E=kQ/r2. dU/dV=(1/2)εoE2, where V is volume, U is potential energy. 3. The attempt at a solution The charge +Q inside the conductor will induce -Q charge on the inside surface, which further leads to +2Q charge on the outer surface of the shell. Therefore E=2kQ/r2. My question is, why have we used the formula dU/dV=(1/2)εoE2. How did we derive it? Isn't it equal to the energy of a charged capacitor. Sorry if this a bad question, I just want to know how did we derive this formula. Thanks.