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1/2 e E^2 for the electric field, with a similar expression for the magnetic (e is permittivity of vacuum). E^2 implies that the energy oscillates as a cosine squares function if we represent the E field as E_0.cos [ kx - wt].

But since cosine squared goes between 0 and 1, where does the energy go when it is at 0?? It cant be stored in the magnetic field since they are in phase in vacuum, so where does it "go"? I hope you can understand my question!

Secondly, in Poyntings theorem for the conservation of energy with EM waves, the kinetic energy density is given by E.J. Does this current density refer to a current that the E field has generated AND/OR one that was already there?

Cheers guys!!