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.