Poynting's theorem

Theorem in physics showing the conservation of energy for the electromagnetic field

In electrodynamics, Poynting's theorem is a statement of conservation of energy for electromagnetic fields that was developed by British physicist John Henry Poynting. It states that in a given volume, the stored energy changes at a rate given by the work done on the charges within the volume, minus the rate at which energy leaves the volume. It is only strictly true in media that is not dispersive, but can be extended for the dispersive case.

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