This chapter discusses two major effects that arise when electric and magnetic fields are changing in time: the “electromagnetic induction” of an additional electric field by changing magnetic field, and the reciprocal effect of the “displacement currents”- actually, the induction of an additional magnetic field by changing electric field. These two phenomena, which make time-dependent electric and magnetic fields inseparable (hence the term “electromagnetism”1), are reflected in the full system of Maxwell equations, valid for an arbitrary electromagnetic process. On the way toward this system, I will make a pause for a brief review of the electrodynamics of superconductivity, which (besides its own significance), provides a perfect platform for a discussion of the general issue of gauge invariance.
1 It was coined by H. Ørsted in 1820 in the context of his experiments – see the previous chapter.
Thumbnail: The magnetic field of a current-bearing coil, illustrating field lines. (CC BY 4.0; Y. Qing).