72 Energy may e stored up in the field in a different way, namely by the action of electromotive force in producing electric displacement. The work done by a variable electromotive force, P, in producing a variable displacement, f, is got by integrating [integral]Pdf from P=0 to the given value of P. Since P=kf eq<sup>n<\sup> (E) this quantity becomes [integral] kfdf = [half]kf<sup>2<\sup> = [half]Pf Hence the intrinsic energy of any part of the field, as existing in the form of electric displacement is <s>[capital sigma] k/2(f<sup>2<\sup> + g<sup>2<\sup> + h<sup>2<\sup>)[delta]V<s> [half][capital sigma](Pf + Qg + Rh)dV The total energy existing in the field is therefore E = [capital sigma]{1/8[pi]([alpha][mu][alpha] + [beta][mu][beta] + [gamma][mu][gamma]) + [half](Pf + Qg + Rh)}dV (I) The first term of this expression depends on the magnetization of the field and is explained on our theory by actual motion of some kind. The second term depends on the electric polarization of the field and is explained on our theory by strain of some kind in an elastic medium. (73) I have on a former occasion* attempted to describe a particu[lar] kind f motion and a particular kind of strain so arranged as to account for the phenomena. In the present paper I avoid any hypothesis of this kind, and in using such words as electric momentum and electric elasticity in reference to the known *On Physical Lines of Force. Philosophical Magazine 18612
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Manuscript details
 Author
 James Clerk Maxwell
 Reference
 PT/72/7
 Series
 PT
 Date
 1864
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Cite as
J. C. Maxwell’s, ‘Dynamical theory of the electromagnetic field’, 1864. From The Royal Society, PT/72/7
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