1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Capacitor charge

  1. Nov 14, 2009 #1
    this may seem stupid but i cant remember is the charge in a capacitor stored within the plates or on top of the plates?
  2. jcsd
  3. Nov 14, 2009 #2


    User Avatar
    Homework Helper

    If you imagine a parallel-plate capacitor, the charge is stored on the inside surface of the plates. The charge inside a metal must always be 0, and if you draw a Gaussian cylinder extending from inside one of the plates to outside the capacitor, you'll see the outside surface of the plate has no charge either.
  4. Nov 14, 2009 #3
    So all the stored electrons in this capacitor are attracted to the inside of the opposite +ve plate. Then how come we can discharge this capacitor with a conductor connected to the middle of both the outside surfaces?
  5. Nov 16, 2009 #4
    How come there's (apparently) a charge flowing against the direction of the electrical force?
  6. Nov 17, 2009 #5


    User Avatar
    Science Advisor
    Gold Member

    Because the attractive force between the oppositely charged particles of the plate through the conductor is larger than the force from the applied field. Of course this only occurs if we have reduced or removed the applied potential after charging to steady-state. In your example of shorting the plates with a wire, then you have removed the potential that induced the separation of charges in the first place.

    When a capacitor is uncharged and you initially apply a voltage across the plates, the voltage induces an applied electric field between the plates. The charges migrate in response to this field onto the plates and in doing so create a secondary field that opposes the applied field. At steady state, the secondary field perfectly cancels out the applied field. When you remove the applied field and short the plates, you now still have the secondary field that applies an effective reverse potential and a means of migration between the plates for the charges. Thus, the charges migrate due to their own attraction to the opposite (and oppositely charged) plates. This continues until the plates are neutral (if you just shorted the plate) or there is no net field between the plates (the same condition if you shorted the plates but this also works if you simply changed the applied potential across the plates).
  7. Nov 17, 2009 #6
    I would say both surface has electrons stored because of electrostatic induction, do you agree?
  8. Nov 17, 2009 #7


    User Avatar
    Science Advisor
    Gold Member

    I do not think so, I think that the connection between the plates prevents this from occuring. If we had a single plate that was subjected to a constant electric field normal to the plate's plane, then we would have charges on both surfaces, say positive on the upper and negative on the lower. If we had two plates that were parallel to each other then it would be the same (ignoring the secondary fields produced by the induced charge distribution). Now, we have surfaces, running from top to bottom, of positive, negative, positive and negative charges. But with a capacitor, the outer surfaces of the plates are connected, which allows free movement of the charges between the plates. Now, the applied electric field is replicated by a potential source applied between the plates. But since the plates are connected, the positive charges on the top-most surface and negative charges on the bottom-most surface can move freely to the opposite plates and join the charges building up on the interior faces.
    Last edited: Nov 17, 2009
  9. Nov 17, 2009 #8
    Born2bwire is talking about a secondary field something I have not come across.

    There’s much more to a capacitor then just 2 inside surfaces, although these provide the bulk of the stored charges.

    It’s like this: any small tiny surface you can think about has a certain capacity with respect of any other such surface of this capacitor. All these tiny capacitances add up in parallel to form the actual value. Therefore there’s a value between the rim and the inside surface, outside – inside, outside – outside, rim – connecting stud, stud – stud, and so on and so on. In fact you could even include wires and objects around the capacitor not necessarily even connected. However fortunately for dc purposes we can add up a lot of these smaller value’s and end up with 3 main parts. Inside, outside and rim. Therefore when a capacitor gets charged we put a charge on the outside, rim and inside. There’s no charge inside the metal. Therefore it follows that both plates are completely surrounded with an electric field, and charges on the outside surface are feeling a force to the outside and not to the inside. Hence when a wire is connected to both outside surfaces, first of all, we discharge some charges pointing in the direction of this wire.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook