= Magnetic-to-electric energy ratio in a driven disc capacitor
{title2=$\overline U_B/\overline U_E=(\omega a/c)^2/8$}
For thin parallel circular plates of radius $a$ and separation $L$, the quasistatic axial field $E_z(t)$ induces $H_\phi=\epsilon_0r\dot E_z/2$. Integrating cycle-averaged vacuum <energy> <densities> for sinusoidal drive gives $\overline U_B/\overline U_E=(\omega a/c)^2/8$. The same fields have radial Poynting <flux> $S_r=-\epsilon_0rE_z\dot E_z/2$, whose leading net inward <power> equals the time derivative of stored electric <energy>. The small ratio diagnoses the quasistatic regime.
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