The simplest form the normal derivative can take is that of a constant, namely 1/ S, where S is the surface area of the surface. The surface term in the solution becomes ∫ ∫ S φ φ ( x ′ ) ∇ ∇ ′ G ( x , x ′ ) ⋅ ⋅ d σ σ ^ ^ ′ = ⟨ ⟨ φ φ ⟩ ⟩ S where ⟨ ⟨ φ φ ⟩ ⟩ S is the average value of the potential on the surface. This number is not known in general, but is often unimportant, as the goal is often to obtain the electric field given by the gradient of the potential, rather than the potential itself.