E is generated by all charges: div(E) = ρ = ρ_f + ρ_b D is generated by the free charges only: div(D) = ρ_f The quantity that makes an appearance in

E is generated by all charges: div(E) = ρ = ρ_f + ρ_b D is generated by the free charges only: div(D) = ρ_f The quantity that makes an appearance in the Lorentz force law (hence acts on test charges) is E. Admittedly, things get muddled because the distinction E vs D does double duty: We also associate E with (specific) forces, and D with fluxes. Depending on your system of units, this necessitates the introduction of a conversion factor.