Abstract

Standard textbooks on classical electrodynamics frequently operate with the notions of free and bound currents (charges). Alternative terminology of external and induced currents also exists. However, a clear physical definition of these physical objects is rarely given. The term “free current” can refer in some cases to the conductivity current, which is subject to constitutive relations in a material sample. In other cases, free current refers to the current that is completely extrinsic to a given material sample and is assumed to be known a priori or manipulated by the experimentalist at will. Although one can argue that all currents flowing in material media are subject to some constitutive relations, there is a clear distinction in the construction of the classical electrodynamics between the external and induced currents. The aim of this paper is to clarify this distinction while pointing out that the traditional distinction between free and bound currents is arbitrary and can be abandoned. In addition, the paper considers some relevant fundamental questions of classical electrodynamics, including the derivation of macroscopic Maxwell’s equations, the properties of the external currents, and the physical interpretation of some auxiliary fields such as the field of polarization P.

© 2018 Optical Society of America

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