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How does a capacitor get charged?

  1. Jul 9, 2012 #1
    [I have attached a diagram of the setup below].

    I am trying to understand how exactly a parallel-plate capacitor [the one with two metal plates separated by a dielectric] gets charged. This is what I have gotten so far.

    1) When the capacitor is in its uncharged state, both metal plates have both positive and negative charges in equal amounts.

    2) When the battery is connected, the electrons in the top plate move towards the positive terminal of the battery.

    3) ??? Where does the electron go now?

    4) The electrons in the top plate keep moving moving towards the positive terminal, making the top plate positively charged.

    5) How does the bottom plate become negatively charged?

    Attached Files:

  2. jcsd
  3. Jul 9, 2012 #2
    The electrons all along the wire are set into motion when the battery is connected. Those close to the top plate tend to move away from it thus the plate is charged positive. Respectively the electrons on the bottom part of the wire are repelled by the negative pole and move in the direction of the bottom plate where they stack, charging it negatively.
  4. Jul 9, 2012 #3
    What is the source of the negative pole? Further, if the bottom plate is neutral, why would electrons move towards it. Wouldn't they get repelled, since the neutral plate contains electrons as well?
    Last edited: Jul 9, 2012
  5. Jul 9, 2012 #4
    How does the battary work is another topic... Just consider that the two poles function in a reverse manner, the positive attracts and the negative repells. Thus the electrons on the top are moving away from the plate while those in the bottom are moving towards it, cause they are being repelled by the negative pole.
  6. Jul 9, 2012 #5
    3) The electron from the top plate when it reaches the positive electrode of the battery combines with a positive ion (i.e., cation) in the electrolyte that surrounds that electrode. What happens next is described by the electrochemistry of that battery. However, it really doesn't matter to the capacitor. The top plate now has more protons than electrons, so it is positively charged.

    5) Electrons leave the negative ions (i.e., anions) in the electrolytic solution near the negative electrode of the battery. The details again have to do with the electrochemistry of the battery. However, electrochemistry does not directly influence the capacitor.
    The electrons move into the metal on the bottom plate. The bottom plate now has more electrons than protons, so that it is negatively charged.

    There is an electric field in the dielectric of the capacitor that points from the top plate to the bottom plate. Therefore, the extra protons in the top plate attract the extra electrons in the bottom plate. This attraction helps keep the capacitor charged.
    There is also an electric field from the positive terminal to the negative terminal of the battery. This also helps keep the capacitor charged.

    A more detailed description would require a solid state description of both the metal and dielectric of the capacitor, in addition to the electrochemistry of the battery.
  7. Jul 9, 2012 #6
    The electrons then exit the negative battery terminal and migrate to the negative plate.
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