Electrostatic energy affected by dielectric medium

Question

An isolated metallic object is charged in vacuum to a potential $V_0$ using a suitable source, it's electrostatic energy being $W_0$. It is then disconnected from the source and immersed in a large volume of dielectric with dielectric constant $K$. The electrostatic energy of the sphere in the dielectric is:

I know that in a general case, electrostatic energy $E=\frac{Q^2}{8\pi\epsilon_0R}$. I always believed that an outside dielectric medium has no affect on the internal electrical energy, since it cannot affect the charge distribution. However, this question's answer is not $W_0$...

My question has a situation similar to this question but that question does not talk about energy changes. Please can anyone elaborate exactly why the metallic object's electrostatic energy changes. Thank you!

Answer 1

Electrostatic energy does not only depend on the charge on the object. It depends on the work done in bringing charge to it from infinity, where the potential energy is presumed to be zero.

The polarization of the dielectic medium affects the electric field between the surface of the metal object and infinity. Therefore it also affects the work done in bringing charge from infinity.