A thin spherical conducting shell of radius R has a charge q. Another charge Q is placed at the centre of the shell. The electrostatic potential at a point p a distance from the centre of the shell is
For a conducting spherical shell, the electric field and potential inside the shell are due solely to the charge Q at the center. Outside the shell, the potential is the sum of the potentials due to Q and the charge q on the shell. At a distance R/2 from the center, the potential contributions from Q and q add up to give the given expression.