A charge q is distributed over two concentric hollow spheres of radii r and R(>r) such that the surface densities are equal. Find the potential at he common centre.

As given:Surface charge density is equal meanssigma1=sigma2Let q` charge on rAnd Q on RThen q`/4πr^2 =Q/4πR^2q`/Q =r^2/R^2. ......(1) Now potential at common centre isV= kQ/R + kq`/rTaking kQ commonV= kQ(1/R+ q`/Qr)V= kQ(1/R +r/R^2)V= kQ{(r +R)/R^2}