Following the dielectric analogy (Section 5.1), this is akin to Maxwell’s first equation, Gauss’ law, in terms of free charge density ρf, i.e., ∇.[μ𝜖0E] = ρf, where E is the electric field and μ 𝜖0E = D is the electric displacement field. See [56Jump To The Next Citation Point] for a thorough discussion of the analogy.