A parallel-plate vacuum capacitor has 8.38 J of energy stored in it. The separat

Question

A parallel-plate vacuum capacitor has 8.38 J of energy stored in it. The separation between the plates is 2.30 mm. If the separation is decreased to 1.15 mm, what is the energy stored (a) if the capacitor is disconnected from the potential source so the charge on the plates remains constant, and (b) if the capacitor remains connected to the potential source so the potential difference between the plates remains constant? [Answers: 4.19J, 16.8J]

I need someone to show the work, so I know how to get to the answer. Thanks.

Explanation / Answer

Energy stored in a parallel plate capacitor is U= 1/2 CV2

and the potential V= Ed   d separation of the plates in the capacitor

energy stored in the capacitor is U= 1/2 C E2d2

                                                U is proportional to d

the energy stored in the capacitor after with d 1.15 mm

                                                          U2=2.09 J

a)If the capacitor is disconnected from the source of potential charge remains constanta up to some time only because the capacitor is already charged,

b)If the capacitor is connected to the source of potential , and the capacitor is charged fully then the potential reaches to saturation that is constant.

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