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Electrically charged plates

  1. Aug 4, 2008 #1
    If two conductive plates are placed close together separated by a suitable medium to act as a dielectric. It has an amount of capacitance that can accept a charge. ie a capacitor.
    one plate has an excess of electrons and is negatively charged the other is lacking in electrons being positively charged.

    pretty basic stuff I know.

    If the terminals of that capacitor are shorted the capacitor is discharged.

    But can someone tells me what would happen when if after 'charging' the capacitor the two plates are then removed from each others proximity. assume perfect electrical insulation.

    One would think that because one plate still had an excess of electrons and the opposite for the other they would still discharge if shorted together by a long wire, even if the plates were separated by a large distance.

    Somehow that doesn't seem correct though.

    Would they discharge?
    If not why not? and if not would bringing them back into close proximity restore the previous charge?

  2. jcsd
  3. Aug 5, 2008 #2


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    One would think that in the absence of an electrostatic force which attracts the oppositely charged plates together, the electrons in the setup would flow in a way to discharge the capacitor. And as for your last question, do you think bringing two uncharged plates or previously-charged-but-now-discharged plates togther would induce a separation of charges as before?
  4. Aug 5, 2008 #3
    Assuming the plates were perfectly insulated how would they discharge. rearrange yes. but discharge. Where to?

    If the plates did discharge then its obvious returning them back together would not return them to their previous state. but I was asking that only if they didn't discharge.

  5. Aug 5, 2008 #4


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    Well if they're insulated, then your problem reduces to one in which the charges redistribute themselves over the surface of the conductor. But to really insulate them means having to cut the connecting wires, so is that what you're asking?
  6. Aug 5, 2008 #5

    The plates are charged. the charging leads are disconnected and then the plates are moved apart.

    Then say a suitable voltmeter was placed one lead on each plate. Would a potential difference be shown even though the plates are perhaps metres apart? and therefore if a wire was then used to connect the two plates still metres apart would current flow. which one would expect if there was a potential difference.

    If the potential difference declined as the plates moved apart, why?

    Surely given perfect electrical insulation the potential difference should remain even though the electrostatic force is no longer present (or vastly weakened).

    Though the latter statement seems logical. It does not feel correct.

  7. Aug 5, 2008 #6


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    Yes there'll still be a potential difference even if the plates are not close together. That's because the potential difference is a measure of the difference in potential of two points. Current will flow if you connect the two plates, now far apart from each other. The potential difference isn't a function of distance, if you do the line integral from of E from one plate to the other, the potential difference obtained is the same. Though the electric field is weakened due to separation of charges, the work done in moving a charge from one plate to the other is still the same, that's because the distance traveled is further, though the field is weaker.
  8. Aug 5, 2008 #7


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    Ok I think I got confused over some things. Apparently it's not true that the distance between the charged plates doesn't matter, at least not in the case where the electric potential depends on their separation distance would factor into the calculation of potential difference. I'll have to think about this some more, so kindly ignore the above post.
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