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Calculating Potential Difference on A Capacitor, RC Circuits

  1. Oct 6, 2009 #1
    The circuit of Figure P.72 has been turned on for a long time, so that you can ignore the transient behavior of the capacitor. Voltage of Battery V = 31 V p19-72alt.gif

    (e) What is the potential difference across the capacitor C (C = 9 µF)?

    I thought about using Kirchoff Voltage Law, but I also though that when a capacitor is fully charged the potential difference should be equal to that on the battery, but the computer said that was wrong. It also is not zero. Any ideas?
    Last edited: Oct 6, 2009
  2. jcsd
  3. Oct 6, 2009 #2


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    Initially the uncharged capacitor will have very voltage, so a lot of current will flow that way. But as it becomes charged its voltage rises until no current at all flows into it. The resistance of that part of the circuit is effectively infinite, so you just ignore it. The circuit you have left is the battery driving current into the 2 and 3 ohm resistors in series. Calculate that current and the voltage across the 3 ohm resistor. Since there is no current through the 1 ohm resistor, it will have no voltage drop V=IR across it and so you know the voltage on the capacitor.
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