A conducting sphere A of radius a, with charge Q, is placed concentrically inside a conducting shell B of radius b. B is earthed. C is the common centre of the A and B

The potential difference between A and B is $\frac{1}{4\pi {\epsilon }_0}Q\left(\frac{1}{a}-\frac{1}{b}\right)$

The potential at a distance r from C, Where  $a<r<b, \mathrm{is} \frac{1}{4\pi {\epsilon }_0}\frac{Q}{r}$

The field is at a distance r from C, Where  $a<r<b, \mathrm{is} \frac{1}{4\pi {\epsilon }_0}\frac{Q}{r^2}$

The potential at a distance r from C, Where $a<r<b, \mathrm{is} \frac{1}{4\pi {\epsilon }_0}Q\left(\frac{1}{r}-\frac{1}{b}\right)$