ABSTRACT
The Maxwell equations contain source terms being the charge densities and the currents. This chapter presents the physics behind these terms and derives their precise form. It examines that the charge and current formulas depend very much on the medium in which these charges and currents are present. Before starting to derive the constitutive equations one need to address another machinery, namely statistical physics. From a philosophical point of view, statistical physics is a remarkable part of natural science. The chapter considers the assumptions that lead to the Boltzmann transport equation. This equation serves as the starting point for deriving the formules for the constitutive equation for the currents in metals, semiconductors and insulators. Intrinsic semiconductors are insulators at zero temperature. This is because the band structure of semiconductors consists of bands that are either filled or empty. So far, people have been rather sloppy in classifying materials as being an insulator, semiconductor or metal.
