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B Shorting of a capacitor

  1. Jan 14, 2017 #1
    hey, so I learned about capacitors and one particular detail about capacitors that is stressed upon is that the field of a capacitor is contained between the plates of it( assuming the field from the edge of plate to be very small). So my question was that when we short a capacitor, why does the charge from one plate flow to the other to balance it out when there is no electric field in the wire?if there is, what creates the field?
    Please do not use the potential argument as my understanding of physics tells me that potential follows field and not vice versa.
    I had thoughts that the existing charges on the plate pushes the charge 'out of the plate' due to repulsive forces but I don't think this explanation is correct as charge doesn't flow when just one plate of a capacitor is earthed.
    Thank you

    Edit 1.
    Consider the diagram. When a wire is connected on the outside of the capacitor ie it lies in region A or C, how is an electric field set up in the wire which drives a current to neutralise the charge on the plates?
     

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    Last edited: Jan 14, 2017
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  3. Jan 14, 2017 #2

    BvU

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    Then there must be a lot of field at the 220 V (or 110 V) sockets in your house ?
     
  4. Jan 14, 2017 #3
    Is there a way of creating a potential difference without a field?i should think that a strong field Is created when I connect an appliance to a socket which causes a potential difference
     
  5. Jan 14, 2017 #4

    Svein

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    Before the short there is a positive net charge on one of the plates and a negative net charge on the other. When you connect the plates, you have effectively one plate (which by the way is a conductor). And any difference in charge along a conductor will equalize (difference in charge will drive a current).
     
  6. Jan 14, 2017 #5
    Implicit in the idea of PD is that both plates have a PD relationship to everything including earth. PD is relative. Er, I'll say that again, PD is relative. So each plate has a PD to earth and a PD between each other. Start your thought experiment by welding one of your plates (A) to earth with really thick conductor that's connected to a massive array of conductor buried meters deep in the soil, and charge up. A is at earth potential. The other plate (B) now has the same PD-to-earth as it has PD-to-A. Shorting the plates will set the the B's PD-to-earth to be the same as A's PD-to-earth. In this case the current flows to earth and no current flows into A.

    Now cut that thick conductor, bundle your experiment up and launch it into space or levitate the whole thing in an upflow of say Nitrogen gas. There is no obvious earth. There will be a current into A when the short is made.

    As to field, the purpose of the field concept is to describe how current flows in a space where a PD is maintained. If you have a sqare flat plate of conductor and you put a voltage across it at opposite corners then current will flow. But at the two corners that are not wired up you will see very little current. The plate might heat up and glow in the middle, and the connected corners might be white hot, but those unconnected corners will be relatively cool. The intellectual device that we use to deal with this phenomenon is field. Field describes how PD varies in space. In a wire the field is linear. In a 2-D plate it is not. In a cube is sort of like the 2-D plate rotated, but not exactly.
     
  7. Jan 14, 2017 #6

    BvU

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    You are seriously mistaken here. When plate B is charged, there will be a current to (or from) plate A such that A and B carry opposite charges of the same magnitude. When plate B is discharged, that current will go in the other direction. All the time plate A is at zero potential.
     
  8. Jan 14, 2017 #7

    cnh1995

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    In a practical capacitor, there exists a fringe field which causes the capacitor to discharge. See the detailed explanation here..
    https://books.google.co.in/books?id...ved=0ahUKEwjTwImTssLRAhVJvY8KHRWmC1gQ6AEIITAB
     
  9. Jan 14, 2017 #8

    BvU

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    Just so nobody starts thinking a fringe field enables a capacitor to discharge: in the picture in post 1 there is a field from the charges. Shortening the plates forces the charges to move until the potentials are equal: there can be no potential difference over an ideal conductor.
     
  10. Jan 14, 2017 #9

    sophiecentaur

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    This is chicken and egg. To produce a field, work needs to be done in the first place.

    It isn't the potential wrt Earth that counts, it's the Potential Difference.

    The two quantities go together and are of equal significance. Why would one be more important than the other?
    This is a bizarre scenario and it requires your conductor to have finite resistance and the plates will not be equipotential. The equivalent circuit is no longer ideal or there will be no heat generated anywhere. Thought experiments with perfect conductors can often lead to nonsense conclusions. You seem to be looking for something 'wrong' with the standard analysis.
     
  11. Jan 14, 2017 #10

    Dale

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    Neither follows the other. They are just two different ways of writing the same thing.

    Regarding your original question, there are surface charges on whatever wire does the shorting. The current flows due to the fields produced by those surface charges.
     
  12. Jan 14, 2017 #11

    fresh_42

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    At school a friend of mine had his fun by shorting pretty big ones by arcing.
     
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