A small conducting sphere of radius r is lying concentrically inside a bigger hollow conducting sphere of radius R. The bigger and smaller spheres are charged with Q and q (Q > q) and are insulated from each other. The potential difference between the spheres will be
`1/(4pi epsi_0) (q/r - Q/R)`
`1/(4pi eipsi_0) (Q/R + q/r)`
`1/(4pi epsi_0) (q/r - q/R)`
`1/(4pi epsi_0) (q/R - Q/r)`