A metal sphere of radius R1 has a charge Q1. It is enclosed by a conducting spherical shell of radius R2 that has a charge -Q2.
(a) the potential V1 of the inner surface is k (Q1/R1 - Q2/R2)
(b) the potential V2 of the outer surface is k ((Q1 - Q2)/R2)
(c) the potential difference V1 - V2 is kQ1
(d) if Q1 = 0 then V1 = V2
