1. The problem statement, all variables and given/known data Given two concentric spherical metal shells, with radii a and b (a < b), and surface charge densities Sa and Sb. Find the capacitance if Sa = - Sb. 2. Relevant equations C = Q/V 3. The attempt at a solution I would know how to solve this if the absolute values of the charges were equal, but here the densities are equal and not the charges. I don't know how the capaticance is defined in this case. How do I solve this?