A parallel-plate capacitor with distance of separation between the plates, d, is

Question

A parallel-plate capacitor with distance of separation between the plates, d, is charged and then disconnected from the battery. The plates are then pushed together to decrease their separation to (1/2)d. Which of the following statements is true? The energy stored in the capacitor is twice the original level O The energy stored in the capacitor is four times the original level O The energy stored in the capacitor is one quarter times the original level O The energy stored in the capacitor is half the original level O The energy stored in the capacitor is not changed.

Explanation / Answer

The energy stored in the capacitor is half the original level.

C =Q/V =A/d

As d d/2 ==========> C 2C

Now energy stored = E = 0.5CV^2 = Q^2/2C

So when d d/2 ==========> C 2C ====> E E/2

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