Is it necessary that a capacitor stores charge? The definition of capacitor given in books is that it store electric energy. So is it possible that the capacitor does not store charge but stores energy only?
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$\begingroup$ Related question by OP: Is it necessary that a capacitor stores charge? $\endgroup$– Alfred CentauriCommented May 19, 2020 at 17:54
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$\begingroup$ A capacitor doesn't store NET charge, but it definitely stores negative charge on one plate and positive charge (a lack of negative charge) on the other plate. $\endgroup$– David WhiteCommented May 19, 2020 at 20:40
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$\begingroup$ @DavidWhite, isn't that an answer? Please see David Z's answer here: Do you find that answers in comments are a problem? $\endgroup$– Alfred CentauriCommented May 19, 2020 at 21:48
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3 Answers
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If you'll take some time to search this site for capacitor related questions, you'll probably find that I and others have often pointed out that capacitors store energy and not electric charge.
A charged capacitor has stored energy due to the work required to separate charge, i.e., the plates of the capacitor are individually charged but in the opposite sense ($+Q$ on one plate, $-Q$ on the other).
Yes, you'll often read phrases like "A capacitor stores electric charge". This is just plain wrong.
However, you'll also read phrases like "$Q$ is the charge on the capacitor". Literally, this is wrong. However, as long you understand that $Q$ is the charge that has flowed from one plate to the other, you'll stay out of trouble.
Bottom line, a charged capacitor is electrically neutral (in 'normal' operation). To say that a "capacitor is charged" is to use charged in the same sense as when we say that a "battery is charged". We mean that there is energy stored.
Given the good natured push-back in the comments, I thought I would do a quick Google search. So, for what it's worth...
answered May 19, 2020 at 17:51
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$\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$– Chris ♦Commented May 20, 2020 at 21:26
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It depends on what the capacitor is used for:
- In some cases it is indeed a way of storing energy, similar to the battery. It however allows for higher transfer of this energy, although a rather short storage time.
- Capacitors may be used as a way of creating high electric fields. In this case the potential difference between the plates is more crucial than the energy involved.
- Finally, by far the most frequent use is in $LC$-circuits, that are part of any generators/receivers of the electromagnetic waves of radio and tv frequencies. In this case the charge is probably the most important variable.
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$\begingroup$ Vadim, why is point 3 different from point 1? An LC circuit is analogous to a mass-spring system. Energy flows back and forth between the inductor and capacitor, i.e., the capacitor is used as an energy storage element analogous to the spring (in the impedance analogy) while the inductor is analogous to the mass. Both are energy storage elements that form a resonant system. $\endgroup$ Commented May 19, 2020 at 18:20
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$\begingroup$ What is storage? In LC circuit energy oscillates, just like current, voltage, charge - I don't think we need to give it special preference. In fact, I don't think we should choose between energy and dynamical variables - these are different ways of looking at things. $\endgroup$– Roger V.Commented May 19, 2020 at 19:12
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$\begingroup$ Vadim, let me try to be more clear. The OP seems to be asking if it is true that the "capacitor stores energy but not charge". You've started your answer with "It depends on what the capacitor is used for" and, in point 3, seem to be stating that charge is stored when the capacitor is used in an LC circuit. But, in your response to my comment, you focus on circuit variable which seems to miss the point entirely . Have you misunderstood the question? $\endgroup$ Commented May 19, 2020 at 19:19
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$\begingroup$ I interpret the question differently from you. $\endgroup$– Roger V.Commented May 19, 2020 at 19:51
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To put my thoughts into an answer instead of comments, it depends on what you mean by "store charge". If you mean "is there a net charge stored across the entire capacitor", then this is not correct. However, if by "store charge" you mean "there is excess charge stored on each plate", then you would be correct. Of course, the issue is that some people might take "store charge" to mean the former and not the latter case, so it is less confusing to say "capacitors store energy"$^*$, or as Khan Academy puts it: "capacitors store an imbalance of charge".
$^*$Of course, you need excess charge "stored" in certain locations to allow for this energy to be stored in the first place ;)
answered May 19, 2020 at 18:41
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2$\begingroup$ I like this approach of summarizing a comment thread with an answer. I should do that more often. $\endgroup$ Commented May 19, 2020 at 19:22
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2$\begingroup$ @AlfredCentauri I'll let you do it only if you memorialize me in your profile ;) $\endgroup$ Commented May 19, 2020 at 19:23