Consider an electromagnetic wave propagating through a region of empty space. How is the energy density of the wave partitioned between the electric and magnetic fields?

Explanation:

To assemble a distribution of charges, the work or energy required against the Coulomb repulsion of like charges is given by :

where, is the resulting electric field due to the distribution of charges.

Similarly, the work required to get currents going against the back emf,

where, is the resulting magnetic field.

The integrations are volume integrals. Hence, the total energy stored in the electromagnetic fields, per unit volume is given by

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