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I Why are conductors not used to store electricity?

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  1. Jul 28, 2016 #1
    I read somewhere that conductors don't really store charge? How can this be correct?

    When a conductor is charged by conduction, the electrons spread throughout the surface of the conductor. Doesn't this mean that the capacitor is storing this electricity? What prevents conductors from being used to store electrical energy?
     
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  3. Jul 28, 2016 #2
    Yes, a piece of metal acts like a capacitor and can store some energy.
    However the capacitances are too small for practical purposes. Unless the conductors are made really really big in size (see capacitance of the Earth to get some idea)
    To increase the capacitance without increasing the size to unpractical values systems of several conductors are used, as in capacitors. Capacitors can store significant amounts of energy.
    With the development of super-capacitors, this modality of energy storage has already entered the practical, commercial domain.
     
  4. Jul 28, 2016 #3
    I see. What factors determine the capacitance of a single conductor?
     
  5. Jul 28, 2016 #4
  6. Jul 28, 2016 #5

    jbriggs444

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    A conductor is not charged by conduction. It does not build up more charge when a current is passing through it than when there is no current.
     
  7. Jul 28, 2016 #6

    russ_watters

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    In other words, the electrons are already there, they just start moving when electricity flows. Like a pipe already full of water before you open the tap.
     
  8. Jul 28, 2016 #7
    So why is it that a conductor can go from a charge of 0 coulombs to a charge of -4 coulombs? Aren't electrons entering the conductor and being stored?
     
  9. Jul 28, 2016 #8

    jbriggs444

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    They are not. If you shove 4 coulombs of electrons in one end, 4 coulombs come out the other.
     
  10. Jul 28, 2016 #9

    Vanadium 50

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    They are exiting as fast as they are entering. Like a pipe.
     
  11. Jul 29, 2016 #10

    CWatters

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    That's assuming there is somewhere for them to exit to. If the conductor isn't connected to anything then charge can be stored in the small capacitance between it and earth. However the capacitance is too small to make this useful.
     
  12. Jul 29, 2016 #11

    jbriggs444

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    Indeed. We're talking about something much less than a picofarad. Storing 4 Coulombs in a picofarad capacitor... That would require billions of volts, the energy equivalent of several tons of TNT and you'd have exceeded the breakdown voltage of the air by a huge factor before even getting close.
     
  13. Jul 29, 2016 #12
    I think he means "conduction" as used in some introductory text about electrostatics. Is used as one of the modes to charge a neutral body: by touching it with another, charged body. The other modes may be by induction and by friction.
    It is not about electrical conduction with the same exact meaning used in the topics about electric current.
    A body can be charged by this "conduction".

    http://www.physicsclassroom.com/class/estatics/Lesson-2/Charging-by-Conduction
     
  14. Jul 29, 2016 #13
    If electrons exit the conductor as other electrons enter, then why is it that a conductor can have a charge of "n" coulombs?
     
  15. Jul 29, 2016 #14
    Yes. That is the definition of conduction that I was referring to.
     
  16. Jul 30, 2016 #15

    CWatters

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    The flow in and out is only equal once the capacitance of the wire is charged. However that happens very quickly because the capacitance is very small.

    You cannot put "n coulombs" of charge into the capacitance of a wire. You can put "n coulombs" through a wire. There is a difference.

    A small water pipe cannot store a billion cubic feet of water but a billion cubic feet might pass through it.
     
  17. Jul 30, 2016 #16

    anorlunda

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    There is at least one context where the capacitance to ground is not small: Long-distance high-voltage AC power transmission lines. For example, a 750 kV, 1000 kilometer line. Underground or underwater power cables are even worse.

    If you energize one end of such a line, and leave the far end open circuited, then the voltage rises km by km all along the length of the line because of that capacitance. That configuration must be prevented to protect components from overvoltage damage.
     
  18. Aug 5, 2016 #17

    David Lewis

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    The OP did not specify the size of the conductors, how close they were to the ground or to each other.
     
  19. Aug 5, 2016 #18

    ZapperZ

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    Then you've been using the wrong name! "Conduction" means the FLOW of charges. You are talking about static electricity, which, by definition, isn't a flow charges. This is why your question created such a confusion.

    A conductor cannot store energy efficiently because it has mobile charges, which means that it can easily lose or gain charges simply via contact, even with air! And contrary to our ability to cause static charges in conductors, it really is difficult to get it to store a lot of charges. Think of all the effort one needs to put in in a Van de Graaf generator. And then what happens when you turn it off? How quickly are those charges from the metallic dome dissipated?

    Zz.
     
    Last edited: Aug 5, 2016
  20. Aug 5, 2016 #19

    David Lewis

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    Thank you. You're bringing up some good points. Would it be correct under the broad definition of conduction to call the plates of a capacitor conductors?
     
  21. Aug 5, 2016 #20

    ZapperZ

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    Conductors are the material. "Conduction" is a process. It is OK to say that the plate of a capacitor is a conductor, because that is the material being used. But under static condition, there is no "conduction".

    Zz.
     
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