Lecture
The theory of the electromagnetic field, or electrodynamics, – is the theory of electromagnetic processes in various media, encompassing a large set of physical phenomena in which the main role is played by interactions between charged particles by means of the electromagnetic field (EMF). The electromagnetic field – is a special kind of matter, qualitatively distinct from substance. It is characterized by its ability to propagate in space and to carry electromagnetic energy. The EMF manifests itself through its effect on charged particles. The theory of the electromagnetic field is assigned an important role in modern views on the structure of matter. According to modern concepts, matter is built from elementary particles, atoms, and molecules. The theory of the structure of matter at the micro level falls within the competence of quantum physics, one of whose branches is quantum, or microscopic, electrodynamics. At the same time, in many practical cases, especially in engineering when designing machines and devices, it is convenient to regard substances as continuous rather than divided into atoms. The parameters of a substance are in this case characterized by certain averaged quantities, such as specific density, permittivity, and permeability. The electromagnetic field is likewise considered to be continuously distributed in space in this case. This approach is called macroscopic, and the theory of the electromagnetic field built within its framework is called macroscopic electrodynamics. The adopted assumptions are justified when all distances from the field sources to the point under consideration are significantly greater than the size of molecules, and the characteristic time of change of the EMF (for example, the oscillation period) is large compared with the period of intramolecular processes. We will study macroscopic electrodynamics, since it is precisely this that is convenient to apply when considering physical phenomena occurring in radio-engineering devices and apparatus. Macroscopic electrodynamics is also often called classical electrodynamics, since it was with its construction that the understanding of the nature of electromagnetic phenomena began.
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