A charge Q is kept on a spherical shell A of radius R and a charge q is kept on a spherical shell B of radius r inside the shell A. The potential difference between outer and inner shell is

$\frac{q}{4 \pi \varepsilon_0}\left(\frac{1}{R}-\frac{1}{r}\right)$

Potential at R = $\frac{1}{4 \pi \varepsilon_0}\left[\frac{Q}{R}+\frac{q}{R}\right]$

Potential at r = $\frac{1}{4 \pi \varepsilon_0}\left[\frac{Q}{R}+\frac{q}{r}\right]$

Potential difference = $\frac{q}{4 \pi \varepsilon_0}\left[\frac{1}{R}-\frac{1}{r}\right]$