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Stored charge in a capacitor

  1. Aug 7, 2010 #1
    Hi All,
    I've been having a debate regarding stored charge on a capacitor.

    In have been told that "a 15 pF capacitor charged to 4 volts stores 60 picocoulombs." I'd like to clarify whether physicists, rather than electrical engineers, would consider that to be a false statement.

    To my mind, the net charge on a capacitor is zero (same current goes in as comes out, there is no net charge stored). However, an electrical engineer uses Q=CV routinely and describes that capacitor as having charge. What that really means of course is the plates thave +Q and -Q respectively.

    So, my question is whether a true physicist would automatically assume that convention was being followed and assume, therefore, that the plates have +/- 60 picocoulombs on them (i.e. no net charge), or would they consider the statement to be false without knowing the context in which it was stated (in other words whether the statement, which is is standalone and made outside of any context, refers to the abolute value of charge on the two plates, or whehter it refers to net charge)?

    This comes down to a question whether context is important in making statements like that, and whether that context is assumed by a physicist from its content.

  2. jcsd
  3. Aug 7, 2010 #2
    There is no net charge stored in the capacitor (by the phenomenon of total induction). When we say that a capacitor stores a certain charge, we probably mean that it is capable, when short circuited with a resistance or other passive elements, to establish the flow of that amount of charge.
  4. Aug 7, 2010 #3


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    I'm not really a physicist, but I am a grad student in physics (so, close to being a physicist I guess) and I would interpret "a capacitor stores 60 pC" to mean that the magnitude of the charge on each plate is 60 pC. I would also assume, unless told otherwise, that one plate has positive charge and the other has negative charge and those charges are of equal magnitude. So basically, I'd make the same interpretation as they do in EE. I think most people I know would do the same.
  5. Aug 7, 2010 #4
    I agree, no net stored charge. However, you said "When we say that a capacitor stores a certain charge", and give that charge as a value in Coulombs, you are contradicting your first statement. So, presumably someone who says this, from a physicist's point of view, is making a false statement (?)
  6. Aug 7, 2010 #5
    OK, so you would infer context from content and make assumptions accordingly. The capacitor BTW could actually have net electrostatic stored charge equal in magnitude and sign on each plate prior to charging, but I think Q=CV wouldn't hold for that.
  7. Aug 7, 2010 #6
    I think the context is set with the word "capacitor".
  8. Aug 8, 2010 #7
    Agree with diazona.

    But I've always thought saying "a capacitor is storing x Coulombs of charge" is quite misleading. This phrasing should be changed.
  9. Aug 8, 2010 #8


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    Reality check please. It doesn't matter if it's a physicist charging a hollow metal sphere or an EE charging a capacitor we only every separate charge, never create it. It's pointless really to make the distinction of charging capacitors versus charging of other objects, when the reality is that it’s always only charge separation in every case.
  10. Aug 8, 2010 #9
    @uart: Good point.

    I'm afraid, however, this is not always true. Some photons can produce an electron-positron pair. In this case, you've created charges (no net charge, of course, but you've created charges).

    Language can't always convey the details required by the rigour of physics.
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