Maxwell's equations are the system of partial differential equations describing classical electromagnetism and therefore of
central importance in physics. In the so-called cgs system of units, Maxwell's equations
are given by

(1)

(2)

(3)

(4)

where
is the electric displacement field, is the free charge density, is the electric field, is the speed of light, is the magnetic field, is the magnetizing field, and is the free current density (cf. Purcell 1985, p. 330,
381, and 432; Jackson 1998, p. 818). As usual, is the divergence
and
is the curl.

In the MKS system of units, the equations are written

(5)

(6)

(7)

(8)

supplemented by

(9)

(10)

where
is the permittivity of free space, is the permeability of free space, is the electric polarization, and is the magnetic polarization or "magnetization"
(Griffiths 1998, pp. 278-279).