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Charge of a capacitor

  1. Jan 25, 2007 #1
    A capacitor have zero net charge because there is same amount of positive and negative charges. However, if amount of +ve and -ve charge are the same, shouldn't delta V and electric field strength be zero? Why is there still an electric field?

    This may be silly, but it indeed confuses me a lot.:confused:
  2. jcsd
  3. Jan 25, 2007 #2


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    Have you ever heard about Coulomb's law? Where the electric field lines starts from the positive charge and ends at the negative charge. Because of the dielectric, the charges are separated from each other and hence a stronger electric field.


    + positive charge
    - negative charge
    = electric field lines
    > direction of electric field lines
    Last edited: Jan 25, 2007
  4. Jan 25, 2007 #3
    Imagine initially all the + and - charges are together - they attract each other - so you must apply some energy to separate them. This is called work - its equal to force times distance. The amount of work you invest in separating each positive charge from a negative charge is stored in the field -i.e., energy is conserved.
  5. Jan 25, 2007 #4

    I will try to explain the simpliest way I can.
    Imagine two metal spheres :One with diameter d1 and another one with diameter D2>d1.
    Let both be hollow metal spheres and let the smaller be located inside (in the center) of the bigger one.
    Now let some process shift certain amount of electons from the smaller sphere to the bigger one.
    The outcome is ,and what arises in the space between two spheres ,a certain electrical field created by the separation of charges (electrons).
    But ,what would you observe if you are observer in the space outside the bigger sphere ? No electrical field.
    For you,the spherical metal object stays neutral,and in this part of your thinking you are right .
    Last edited: Jan 25, 2007
  6. Jan 26, 2007 #5
    So the entire sphere maintains the same charge (let's say neutral), it is just transferred from the inner sphere to the outer surface sphere. So outside of the sphere you see no Electric Field but inside there is one between the outer and inner spheres.
    It's confusing for me because I see a capacitor as kind of similar to a dipole, but whereas a dipole has an electric field which decreases ~[tex]\frac{1}{r^3}[/tex], but in a capacitor there is no E outside.
    Can sombody explain why the capacitor shouldn't be thought of as just a big dipole?
  7. Jan 26, 2007 #6
    Look into Gauss law

    I hope that you are talking about electric field within the two plates of the capacitor.

    First you must understand that the mere presence of equal and opposite charges i.e., zero net charge, is no condition for a region to have zero electric field.

    Let us now assume a parallel plate capacitor for simplicity. Let the left plate be positively charged and the right plate be negatively charged. The field at any point between the plates because of the positively charged plate is directed towards the right. The field at any point between the plates because of the negatively charged plate is also directed towards the right. Hence, the fields only add and DO NO CANCEL.

    To determine the direction and magnitude of the electric field within a capacitor kindly use Gauss law. Try applying Gauss law to determine field at a point just outside a conducting sheet of charge having a uniform charge distribution. Begin by considering a cylindrical Gaussian surface.

    This would be covered in just about any school/under graduate level text book in electricity and magnetism.

    I hope that helps.:smile:
  8. Jan 26, 2007 #7
    It's not true that for every capacitor [itex]E_{outside}=0[/itex] !!
    Only for self-closing geometries ,like in example of 2 spheres I described ,this field equals 0.
    Therefore,you have a geometry dependent situation which vary from case to case (assuming [itex]Q_{net}=0[/itex])
    Last edited: Jan 26, 2007
  9. Jan 26, 2007 #8
    So in a parralel plate capacitor with finite plates is there an E outside?
  10. Jan 26, 2007 #9


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    When two parallel plates [+ and -], are placed next to each other, the electric fields inside add, while outside cancels out.
  11. Jan 26, 2007 #10
    Yes, there is. It decreases approximately with 1/r^3.
  12. Jan 26, 2007 #11
    Am I misunderstanding something, or are the last two responses contradictory? Do the fields cancel (I assume cancel means E=0) on the outside of the plates, or do they decrease with 1/r^3.
  13. Jan 26, 2007 #12


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    Mayb if you look at the math and some illustrations, it will become clear:
    http://www.ac.wwu.edu/~vawter/PhysicsNet/Topics/Capacitors/ParallCap.html [Broken]
    Last edited by a moderator: May 2, 2017
  14. Jan 26, 2007 #13
    Thanks, so with infinite sized parrallel plate capacitors Eoutside=0. and for finite sized parallel plate capacitors the E inside adds together and E outside is ~ 1/r^3 correct?
    Last edited by a moderator: May 2, 2017
  15. Jan 26, 2007 #14


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    Hi americanforest,

    I'm sorry, I ignored when you said finite plate. My case was for an ideal capacitor with infinite plates. Manchot reply is true where it decreases 1/r^3 becuase of the dipole field.
  16. Jan 27, 2007 #15
    I am surprised to see so much replies!

    Thank you very much for your answer. I'll try to look it at a different wayo:)
    Last edited: Jan 27, 2007
  17. Jan 27, 2007 #16
    What I described is nothing else than a "spherical capacitor".
    For that and other cases see:http://ocw.mit.edu/NR/rdonlyres/Physics/8-02TSpring-2005/7EC48A5F-B4BB-485D-BF6C-91CA3D5C448C/0/presentati_w03d1.pdf [Broken]

    Yes there is (usually very small though).
    Only ,when the plates are infinite (or they intersect in infinity :smile:),Eoutside=0
    Last edited by a moderator: May 2, 2017
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