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It depends on what you mean by "energy density". If you want to ascribe some energy to EM field in material medium in a general way, not requiring linear and lossless behaviour of the medium, the simplest way is to use the same Poynting theorem and formulae as in vacuum. Then density of EM energy is $$ \frac{1}{2}\epsilon_0E^2 + \frac{1}{2\mu_0} B^2 $$ and ...

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