Within a spherical charge distribution of charge density ρ(r),N equipotential surfaces of potential V0,V0+∆V,V0+2∆V,..........V0+N∆V(∆V>0), are drawn and have increasing radii r_0, r_1, r_2,..........r_N, respectively. If the difference in the radii of the surfaces is constant for all values of V_0 and ∆V then :