Problem 3.136

A point charge $q=3.0 \mu \mathrm{C}$ is located at the centre of a spherical layer of uniform isotropic dielectric with permittivity $\varepsilon=3.0$. The inside radius of the layer is equal to $a=250 \mathrm{~mm},$ the outside radius is $b=500 \mathrm{~mm}$. Find the electrostatic energy inside the dielectric layer.