70987

Interpretation

Now the first term represents the ratę of increase of stored energy per unit volume. The

second term represents the ratę of working per unit volume of the field on the conduction

currents J. These currents might result in the dissipation of energy in the resistance of the medium, or might, if flowing in a pair of external terminals, result in the supply of electric power to or removal of electric power from the field system. The third term must,

by conservation of energy, represent the ratę of flow of energy per unit volume out of the

volume by electromagnetic means. In fact the energy flows out in the form of photons, but

we are not working to a microscopic scalę, so quantization of the field and such conditions

do not concern us. To us it looks as if energy is being continuously transported by the field in an amount div (E x H) per unit volume, per unit time.

Now by Gauss' theorem l

s

(E xH) ds =

z

v

div(ExH)dv. (7.13)

So we identify ExH as the power flow per unit area across any surface.