A parallel plate capacitor is made of two square parallel plates of area $A$ , and separated by a distance $d < < \sqrt A $ . The capacitor is connected to a battery with potential $V$ and allowed to fully charge. The battery is then disconnected. A square metal conducting slab also with area $A$ but thickness $\frac {d}{2}$ is then fully inserted between the plates, so that it is always parallel to the plates. How much work has been done on the metal slab by external agent while it is being inserted?
A parallel plate capacitor is made of two square parallel plates of area $A$ , and separated by a distance $d < < \sqrt A $ . The capacitor is connected to a battery with potential $V$ and allowed to fully charge. The battery is then disconnected. A square metal conducting slab also with area $A$ but thickness $\frac {d}{2}$ is then fully inserted between the plates, so that it is always parallel to the plates. How much work has been done on the metal slab by external agent while it is being inserted?
- A
$ + \frac{1}{4}\,\frac{{{ \in _0}A}}{d}{V^2}$
- B
$ - \frac{1}{2}\,\frac{{{ \in _0}A}}{d}{V^2}$
- C
$ + \frac{1}{2}\,\frac{{{ \in _0}A}}{d}{V^2}$
- D
$ - \frac{1}{4}\,\frac{{{ \in _0}A}}{d}{V^2}$
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