Starting with Gauss's law for electricity (also one of Maxwell's equations) in differential form, one has ∇ ∇ ⋅ ⋅ D = ρ ρ f , where ∇ ∇ ⋅ ⋅ is the divergence operator, D is the electric displacement field, and ρ f is the free- charge density (describing charges brought from outside).