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Whether capacitors generally have net charge

  1. Jan 30, 2012 #1
    Trying to learn the basics of capacitors, but I'm hung up on a conceptual issue. It seems that capacitors must be charged up, generally with the help of a current from outside the capacitor, such as a flow of electrons that pile up on one of the plates. So in that sense, the capacitor must be storing, in this case negative, charge, at least on one of the plates. Yet it also seems that capacitors, at least once charged, are described in terms of two conductors with equal opposite charge, thus having no net charge. My question is essentially: does the capacitor, taken as a closed system, have net charge? If not, then how is charge conserved when so much negative charge was added to the capacitor to charge it up?

    Any help on this would be greatly appreciated, as I'm sure there is a flaw somewhere in my conception of a capacitor and how it works.
     
  2. jcsd
  3. Jan 30, 2012 #2
    There is no net charge in the capacitor. From a circuit theory point of view the current is continuous though it so whatever charge enters the top also leaves the bottom.

    Turning to physics, if you force electrons onto the upper electrode, they will repel an equal number of electrons from the opposing electrode thus satisfying the current continuity found in circuit theory.
     
  4. Jan 30, 2012 #3
    Not generally, but obviously materials can store excess charge.

    Hair+Comb+Pieces of Paper=A common science experiment you can do yourself

    Yet, a capacitor can still work without receiving additional charge. For example, you can have stored potential in a capacitor by forcing charge through a membrane (as is the case with a plasma membrane), but that charge doesn't have to leave the capacitor to make a potential difference.

    Ultimately it is based on increased separation of opposite charges (such as by forcing them through a membrane) or reduced separation of like charges (such as by shoving in more electrons from the outside).
     
    Last edited: Jan 30, 2012
  5. Jan 30, 2012 #4
    http://en.wikipedia.org/wiki/Electrets

     
  6. Jan 31, 2012 #5
    Thanks Antiphon. Thanks kmarinas86. I'll have a look at electrets. Overall, I'm starting to get the sense that there are few cases in which very high concentrations of net charge are stored. Van De Graaff generators, for example, seem to store a surprisingly small amount of charge, yet discharge it quickly at a high voltage. Is there a law governing maximum net charge density related to voltage breakdown and dialectric strength? In the case of a Van De Graaff globe, for example, would covering the aluminum globe in a material with a dialectric strength greater than the surrounding air increase its capacitance or net charge storage capacity?
     
  7. Jan 31, 2012 #6

    sophiecentaur

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    A Van der Graff ball holds only a small charge because it is only 'half' of the sort of capacitor you'd find in a circuit. The capacity is between it and the Earth - only a few tens of picoFarads. (Q = CV, remember?) Hence, you need a massive voltage in order to store even a small charge - which is equal and opposite to the charge you have transferred to the Earth.
    Your question about voltage breakdown: What counts is the electric field strength. If it's high enough, it can ionise the air molecules and allow conduction (a spark). The field strength depends upon the Potential and also the radius of the object in question. Pointed objects have high field strength around the points and will form 'corona' very easily. The VDGG uses a 20 to 30 cm ball with no sharp edges so it can reach 100kV or so without discharging. I imagine a coating of insulating material could increase the maximum voltage it could reach.
     
  8. Jan 31, 2012 #7
    Thanks sophiecentaur. Your description of the Van der Graaff as half of a capacitor, the other half being the Earth, is illuminating, and gives me a lot to ponder. Thanks again.
     
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