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A parallel plate capacitor is attached to a battery which maintains a constant voltage difference V between the capacitor plates. While the battery is attached, the plates are pulled apart so their separation increases. The electrostatic energy stored in the capacitor decreases.

How is this decrease in energy possible even though the external force does positive work in pulling the oppositely charged plates?

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    $\begingroup$ If the energy didn't go into the capacitor, it must be somewhere else. Is there anywhere else the energy could go? $\endgroup$ – BowlOfRed Aug 31 '16 at 5:27
  • $\begingroup$ You might notice that this is not the behavior when the plate is disconnected from the battery before separating the plates. $\endgroup$ – BowlOfRed Aug 31 '16 at 5:31
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If the voltage across the plates is the same and the capacitance decreases then the charge on the plates must decrease.

So current flows into the positive terminal of the battery and work must be done for this to happen. If the battery is rechargeable then that work results in an increase in the chemical energy stored in the battery.
Heat will also be dissipated in the leads because they have resistance.

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