A movable parallel-plate capacitor initially stores energy E with plate separation d. It is electrically isolated, and the separation is increased to 2d. Neglect fringing. The final stored energy is
With fixed Q, stored energy is $Q^2/(2C)$. Replacing C by C/2 doubles the energy to **2E, A**. The extra energy is the mechanical work done in pulling the attracting plates apart.
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