[QUE/EM-04010] EM-PROBLEM

Use Green’ reciprocity theorem to prove “mean value theorem”: the average of \(\phi(\vec{r})\) over a spherical surface \(S\) that encloses a charge-free volume is equal to the potential at the center of the sphere:\[ \frac{1}{4\pi R^2} \iint \phi(\vec{r}) dS = \phi(0) \]