Problem 3.138

A spherical shell of radius $R_{1}$ with a uniform charge $q$ has a point charge $q_{0}$ at its centre. Find the work performed by the electric forces during the shell expansion from radius $R_{1}$ to radius $R_{2}$

A=\frac{q\left(q_{0}+q / 2\right)}{4 \pi \varepsilon_{0}}\left(\frac{1}{R_{1}}-\frac{1}{R_{2}}\right)

Electric Capacitance, Energy of an Electric Field