of these four equations are, respectively:
(1) electric field diverges from electric charge, an expression of the Coulomb force,
(2) there are no isolated magnetic poles, but the Coulomb force acts between the poles of a magnet,
(3) electric fields are produced by changing magnetic fields, an expression of Faraday's law of induction, and
(4) circulating magnetic fields are produced by changing electric fields and by electric currents, Maxwell's extension of Ampère's law (q.v.) to include the interaction of changing fields.
The most compact way of writing these equations in the metre-kilogram-second (mks) system is in terms of the vector
operators div (divergence) and curl. In these expressions the Greek letter rho, r, is charge density, J is current density, E is the
electric field, and B is the magnetic field; here, D and H are field quantities that are proportional to E and B, respectively. The four
Maxwell equations, corresponding to the four statements above, are: (1) div D = r, (2) div B = 0, (3) curl E = -dB/dt, and (4) curl H = dD/dt + J.