= Solution
The unit <relative permittivity> and vacuum-valued <magnetic permeability> imply $\mathbf D=\epsilon_0\mathbf E$ and $\mathbf B=\mu_0\mathbf H$. The <electromagnetic energy> per unit volume is
$$
\boxed{U=\frac12\epsilon_0\mathbf E\cdot\mathbf E+\frac12\mu_0\mathbf H\cdot\mathbf H,\qquad[U]=\mathrm{J\,m^{-3}}.}
$$
The two contributions are the stored electric and magnetic <energies>. Although called $U$ in this paper, this quantity is an <energy density>; the total <electromagnetic energy> is $\int_VU\,dV$.
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