[QUE/EM-04014] EM-PROBLEM

Prove that the potential \(\phi(0)\) at the center of a charge-free spherical volume \(V\) is the equal to average of \(\phi(r)\) over the surface \(S\) of the sphere. Using 

1. Green’s reciprocity relation.
2. Green's identity\[\iiint d^3r (f \nabla^2 g − g\nabla^2 f ) = \iint \hat{n} · (f \nabla g − g\nabla f) dS\]