The classical theory of electrodynamics is built upon Maxwell's equations andthe concepts of electromagnetic (EM) field, force, energy, and momentum, whichare intimately tied together by Poynting's theorem and by the Lorentz forcelaw. Whereas Maxwell's equations relate the fields to their material sources,Poynting's theorem governs the flow of EM energy and its exchange betweenfields and material media, while the Lorentz law regulates the back-and-forthtransfer of momentum between the media and the fields. An alternative forcelaw, first proposed by Einstein and Laub, exists that is consistent withMaxwell's equations and complies with the conservation laws as well as with therequirements of special relativity. While the Lorentz law requires theintroduction of hidden energy and hidden momentum in situations where anelectric field acts on a magnetized medium, the Einstein-Laub (E-L) formulationof EM force and torque does not invoke hidden entities under suchcircumstances. Moreover, total force/torque exerted by EM fields on any givenobject turns out to be independent of whether the density of force/torque isevaluated using the law of Lorentz or that of Einstein and Laub. Hiddenentities aside, the two formulations differ only in their predicted force andtorque distributions inside matter. Such differences in distribution areoccasionally measurable, and could serve as a guide in deciding whichformulation, if either, corresponds to physical reality.
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