= Tilted parallel-plate capacitor at fixed charge
{title2=$C=\epsilon_0s^2\log[(a+\delta)/(a-\delta)]/(2\delta)$}
For square plates with a linearly varying gap from $a-\delta$ to $a+\delta$, the local-strip approximation gives the displayed <capacitance> for $|\delta|<a$. Its expansion is $C=(\epsilon_0s^2/a)[1+\delta^2/(3a^2)+O(\delta^4/a^4)]$. An isolated capacitor has fixed <electric charge>, so $U=Q^2/(2C)$ decreases as $|\delta|$ increases. A sinusoidal small tilt therefore produces a leading energy variation at twice the tilt frequency.
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