Free charge
= Free charge
In macroscopic <electromagnetism>, <free charge> is the part of the <charge density> represented explicitly as $\rho_f$, separately from <bound charge> associated with the <electric polarization> of matter. The <electric displacement field> satisfies $\nabla\cdot\mathbf D=\rho_f$. The word free does not imply zero <force> or zero <energy>: it distinguishes this source from the polarization response in the chosen material description.