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Capacitor charging

  1. Apr 28, 2009 #1
    How does current flow across a capacitor?
    When a cap is charging up, the current is high initially and then gradually decreases as the charges start to build up on the plate of the capacitor.
    I can't figure out how current flows from one plate to the other?
    Its not like a resistor, where electrons flow thru the resistor.
     
  2. jcsd
  3. Apr 28, 2009 #2

    Born2bwire

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    It doesn't, under ideal situations. If we are talking about a DC current, then the initial current during the transient is the result of building up the charges on opposite plates. There is no current going between the plates, but we are shifting around charges to the plates via the wires. Once we have enough charges built up such that we have the opposite potential across the plates that we are trying to apply, then the total voltage is zero and we cannot build up any more charges, making the current zero.

    If we have an AC current, then we are just moving charges back and forth on and off the plates.

    Current only flows between the plates (not via the wires) when we have dielectric breakdown. If the voltage across the plates gets high enough, the dielectric "breaks down" and allows a spark or current to run through the dielectric. This is rather undesirable since it circumvents the desired behavior of the capacitor and usually destroys the capacitor in the process. A simple example of dielectric breakdown is lightning (although I won't go as far as to say that the dielectric in capacitors turn to a plasma when they breakdown).
     
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