Is it necessary that a capacitor storage charge? I am a little bit confused. For example: if I take two concentric conducting shells and a positive point charge at the centre. Both the surfaces (outer surface of the the smaller shell and the inner surface of the larger shell) are equipotential and have zero net charge but as electric field lines exist between the surfaces the volume stores energy and if this energy is discharged then it will act like a capacitor although it doesn't conatin any charge. Please tell me where I am wrong.


In the case you described, if there's no charge on the surfaces there's nothing to stop the electric field from leaking out. In this case the charge inside does not affect the surfaces at all - you just have a standard uncharged capacitator, and a free charge that just happen to be in the same place. Moving the charge around just shifts its enlectric field from one place to another, so you cannot extract any energy from moving this charge. Assuming there's nothing else it can interact, then from the point of view of the electric circuit this charge may as well not exist at all.

In another case, if the charge in the middle causes an opposite charge to gather on the surfaces (the charges on the surfaces are equal, they are just opposite to the charge in the middle), then whatever keeps the charge in the middle should be considered one terminal and the two surfaces should be considered the second terminal (a single one, just disjoined). The surfaces are still equipotential, but they do hold charge. In this case you can extract the energy from the system by manipulating the amount of charge.

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  • $\begingroup$ It means that capacitor should store energy and charge not necessarily charge. $\endgroup$ – Soumyadip Roy May 19 at 16:12

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