# Energy in EM wave

The reason the energy density of an EM wave is proportional only to the amplitude squared is because electromagnetic energy density U is:

U = (E2 + B2)/2

in units where c = &mu;0 = &epsilon;0 = 1

The average value of E2 is proportional to Emax2, it doesn't matter how fast E is varying, so the average energy density doesn't depend on &omega;.

Hurkyl

The electric field is also proportional to the acceleration of the charge. The larger the acceleration, the larger is the field.
-> http://electron9.phys.utk.edu/optics421/modules/m1/production.htm [Broken]

Hmmm.... If U &prop; E2, E &prop; a and a &prop; &omega;, how can Uavg not be dependent on &omega; ?

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Integral
Staff Emeritus