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Capacitor Question

  1. Feb 19, 2009 #1
    This is not really a homework question; I simply don't understand a property of capacitors.

    How come when some emf is attached to a simple circuit with a capacitor in it, the positive charge all gathers on one plate while the negative charge all gathers on the opposite plate? Why is there not an equal distribution of charge? Additionally, why are the positive charges flowing in the wires? When we discussed charged objects, it was only the electrons that were free to move... I need a giant explanation of all of this. i think i lost something along the way.

    EDIT: Another quesrion:
    I just read this excerpt from the text explaining dielectrics: "Less extreme fields can polarize an atom, in essence stretching it so that its electrons tend to be on one side and its positive nucleus is on the other. This turns the atom into a dipole, a body with positively and negatively charged regions."

    If both ends of the capacitor are equally charged, how are we supposed to determine which side the electrons will veer towards and which side the protons will veer towards? Doesn't the dielectric need to be arranged in such a way that the net field is zero?
  2. jcsd
  3. Feb 19, 2009 #2


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    Homework Helper

    A capacitor does not allow charge to flow through. The emf pulls electrons away from one side, and pushes electrons into the other side. So + on one side (shortage of electrons) and - on the other side. Remember, the positive charges in the atomic nuclei are always present and usually canceled out by orbiting electrons. The + charge is attracted to the - charge so the capacitor will remain charged when the emf is taken away.

    We often speak of positive or conventional charges flowing because historically for a long time there was no way to tell which charge was flowing and someone guessed it was positive. It hardly ever makes a difference which you use.

    The charged capacitor has + on one side, - on the other so there is a strong electric field in the space in between. This field pulls one way on the atomic nucleus and the other way on the electrons, separating their centers slightly. You don't really need the field in the explanation - the electric force due to the positive charge causes this, and the negative charge on the other side doubles it.
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