where and are the electric field and displacement vectors, respectively, and are the magnetic field and induction vectors, respectively, is the total electric charge density, and the conduction current density.

Eqns. (2.1) and (2.2) and eqns. (2.3) and (2.4) each build a subsystem of their own. They are linked together by the relations

where and are the permittivity and permeability, respectively. Eqn. (2.5) and eqn. (2.6) are valid only in materials where no piezoelectric or ferroelectric phenomena occur. In semiconductors the frequency dependence of and can be neglected.