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Can electrons "fly" between capacitor plates?

  1. Feb 15, 2015 #1
    How exactly do the electrons move from the capacitor to the rest of the circuit? Do they fly between the capacitors plates, or can they only travel where there is a wire connection? (I.e., if a small chunk of our circuit looks like this: A-------| |-------B can an electron start from A, go to our plate, then fly across the plate and go to B? Or can the electron only move backwards from the plate back to A?)
     
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  3. Feb 15, 2015 #2
    From what I understand, if you were to track an electrons journey, it would never cross the gap (provided a perfect dielectric between the plates). If A were connected to the positive side of a battery, and B to the negative side, a negative charge would build on plate B, repelling the electrons from plate A resulting in a net positive charge on plate A.
     
  4. Feb 15, 2015 #3

    NascentOxygen

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    Electrons don't cross the gap. If they did it would represent dielectric breakdown and a resistive path, and the capacitor would not be able to store charge for very long.

    The plates in a capacitor are so close together that any build-up of charge on one plate causes field lines to reach across and influence an equal number of charges on the other plate. So if Q electrons are added to one plate, they will repel that same number from the other plate, sending them into the circuit that second plate is connected to. To an outside observer it may therefore seem that current is flowing "through" the capacitor, but the non-conducting dielectric in the path means it is not.
     
  5. Feb 16, 2015 #4

    LvW

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    The above description is based on the effect called "electrical influence".
    More than that, in case the capacitor is part of a closed current loop we observe a current within the loop:
    * a decaying current if a dc voltage is switched-on, or
    * a continuous ac current in case of an ac voltage source.

    Because this observation needs an explanation in the "current domain", we have invited the term "displacement current" (through the capacitor).
     
  6. Feb 16, 2015 #5

    jim hardy

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  7. Feb 16, 2015 #6
    Well, they can fly between capacitor plates, but it is not recommended they do that too much... It is way much better they just dance on plates.
     
  8. Feb 16, 2015 #7
    But then isn't the circuit not closed? For instance, if we have a simple capacitor connected to a battery, then electrons wouldn't be able to flow across the capacitor. (Although this now negative plate of the capacitor will repel electrons from the opposite plate, making it positive). What happens when eventually tons of electrons are crammed at the negative plate, and there are no more electrons to repel on the opposite plate?

    Like, eventually there will be so many electrons crammed at one of the capacitor plates, and there will be no more electrons to repel on the opposite plate?? Therefore it seems like there will be no more current in the circuit, even though it is hooked up to a battery?
     
  9. Feb 16, 2015 #8

    jim hardy

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    The secret to understanding capacitors is to realize that the energy is stored not on the plates but in the dielectric.

    Good dielectrics have polar molecules that the electric field 'twists' out of their preferred alignment.

    diel.gif
    http://hyperphysics.phy-astr.gsu.edu/hbase/electric/dielec.html

    Water for instance is a very polar molecule. That's why water has a dielectric constant 80X that of free space.
    0016.jpg

    Why free space has a dielectric constant i do not know - there's nothing there to deform.



    old jim
     
  10. Feb 16, 2015 #9

    davenn

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    that's correct

    then the capacitor fully charged equal to the voltage potential across it. Increasing the voltage applied to the plates will increase the charge
    Eventually, depending on the construction of the capacitor, the increasing voltage will get to a point where the electric field between the plates will cause a
    breakdown of the dielectric and it will cause a current to flow. At that point, the capacitor has failed.

    Dave
     
  11. Feb 17, 2015 #10

    LvW

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    How do you define "closed"? What do you observe in the above described case?
    s there a flow of charges (not "electrons")? Yes, there is.

    We have a so called charging current (or loading curent) until the capacitor is charged up and its voltage is equal to the battery voltage.
    This charging process assumes (at least!) a certain wiring resistance and/or a finite source resistance (internal to the battery), otherwise the connecting of the capacitor to the battery would cause something like a "Dirac impuls" which is unrealistic.

    As mentioned in my reply#4 during this charging process we define a so-called "depletion current" to fill the "optical gap" caused by the capacitor.
    Please note that, therefore, we consider a circuit as "closed" if such a depletion current can flow. This is true for all ac waveforms as well as for dc circuits during the switch-on period (charging period).
     
  12. Feb 17, 2015 #11

    sophiecentaur

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    Here is yet another example where electrons just add complication. The fact that they don't actually flow across the gap in a Capacitor is no more relevant to the electrical situation than the fact that they may not even reach the end of the wire between the switch being turned on and then off.
    What have electrons actually got to do with electricity? (A Joke but not totally a joke)
     
  13. Feb 17, 2015 #12

    LvW

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    Yes - this illustrates the fact that "current" is NOT identical to "flow of electrons".
    Instead, current is "movement of charges" - caused by electrons, but the "body" of the electrons is moving much slower than the charges.
     
  14. Feb 17, 2015 #13
    I'm sorry but I don't understand this point at all. How can current not be identical to the flow of electrons when current = coulombs/second? Yes it is a flow of charge but that charge can only flow with the flow of electrons, which not carry, but ARE that charge. One coulomb has a defined number of electrons associated with it. So if charge flows, so do electrons.
     
  15. Feb 17, 2015 #14

    LvW

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    It is really necessary to distinguish between the "drift velocity" of electrons (see here: http://en.wikipedia.org/wiki/Drift_velocity) and the "charge velocity" which is app. identical to the speed of light. This is because the charge of an electron is a property of an electron (and not the electron itself).
     
  16. Feb 17, 2015 #15
    I think you're confusing charge of charge carriers and the propagation of voltage across a circuit, which is what travels near to the speed of light. Even if charge is a property of an electron, you can't transfer that charge any faster than the movement of the actual electrons in a circuit.
     
  17. Feb 17, 2015 #16

    LvW

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    No - I am not mixing things, but you are right: I have expressed myself rather unclear.
    Let me try agian: The drift velocity of one single electron is rather low.
    However, the thing we call "current" consists of charge movement (charge per time unit) - and because we have a huge number of electrons in a wire, we have a rather large amount of charges crossing a certain point within or at the end of the wire.
     
  18. Feb 17, 2015 #17

    NascentOxygen

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    I think you needn't worry about running out of free electrons, the free electron density in metals is so high. http://hyperphysics.phy-astr.gsu.edu/hbase/tables/fermi.html#c2
     
  19. Feb 18, 2015 #18
    I suppose we were probably chasing each others tails and going round in circles. Glad to have cleared that up.
     
  20. Feb 18, 2015 #19

    nsaspook

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    Maybe this will help with the confusion.
    sefton.pdf
     
  21. Feb 19, 2015 #20
    This article looks amazing. I'm reading it now. Thank you.
     
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