= Stress contribution to energy transport in a moving capacitor
{title2=$S_x=\gamma^2v(u_0+T_*^{xx})$}
For a static rest-frame <electromagnetic stress-energy tensor> with no <Poynting flux>, a <Lorentz transformation> to a laboratory where it moves along $+x$ gives $u=\gamma^2(u_0+\beta^2T_*^{xx})$ and $S_x=\gamma^2v(u_0+T_*^{xx})$. Thus <energy> flux includes stress as well as rest <energy density>. In an ideal <parallel-plate capacitor>, $T_*^{xx}=+u_0$ for motion perpendicular to the <electric field>, but $T_*^{xx}=-u_0$ for motion parallel to it. The former has $S_x=2\gamma^2vu_0$; the latter has zero interior flux, with <work> at the moving plates transferring <energy> at the boundaries. The field subsystem need not have an integrated <four-momentum> independent of simultaneity surface; include material support stresses for a closed system.
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