A small sphere, which carries a charge $Q$, is located at the centre of a fixed uncharged conducting hollow sphere with outer and inner radii $R_{1}$ and $R_{2}\left(R_{2} < R_{1}\right)$. The sphere is enclosed by a concentric dielectric layer of permittivity $\varepsilon$ and the outer radius $R_{3}$ (see fig.).

What is the minimum work required to move away a small sphere to a distance much greater than $R_{3}$ through a narrow channel inside the conductor and the dielectric?

A S