Free current

A free current is the part of the macroscopic electric current assigned to mobile or externally supplied electric charges, rather than to the bound motion represented by polarization and magnetization. In a conductor, drifting conduction electrons provide a familiar example.

In macroscopic Maxwell's equations, the free-current density appears as $\nabla \times \mathbf{H}={\mathbf{J}}_{\mathrm{f}}+\partial \mathbf{D}/\partial t$. Here $\mathbf{H}$ is the magnetic field strength and $\mathbf{D}$ the electric displacement field. The separation depends on the material model; the total microscopic charge motion remains the source of the electromagnetic field.