where Q is the total charge, and R is the radius of the sphere. The value of a C can be approximately calculated by using this equation to calculate the potential energy, using an empirical nuclear radius of R ≈ ≈ r 0 A 1 3 and Q = Ze. However, because electrostatic repulsion will only exist for more than one proton, Z 2 becomes Z ( Z − − 1 ) :