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Current source and capacitor in parallel

  1. Oct 27, 2011 #1
    I don't know if this is the right place to post this question, but here goes.

    Whilst solving a circuit in class from a homework assignment, we arrived at a situation where a capacitor was in parallel with just a single independent DC current source. The question is, would current flow through the capacitor? The cap was charged (sequential switching situation) to 9.9 [V] beforehand. Many people in the class felt that the capacitor would allow current to flow even with no other circuit elements present. The source is ideal, by the way.
  2. jcsd
  3. Oct 27, 2011 #2


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    Staff: Mentor

    An ideal current source in parallel with a capacitor is a one of those situations that's best avoided unless there's something else in the circuitry to limit the duration that the connection is made or bypass the current! The reason is, the current source will continue to force current through the capacitor indefinitely, adding more and more charge to the plates. As a result, the voltage across the plates will continue to rise indefinitely also.

    For ideal components the voltage would continue to rise linearly forever. For more realistic components the capacitor would shortly evolve into a ball of plasma and molten plastic and metal flying by your left ear when the manufacturer's stated maximum voltage limit is exceeded by egregious margins :smile:
  4. Oct 30, 2011 #3


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    Staff: Mentor

    It would. The current through the capacitor would be exactly what came from the DC source. For this reason, I would prefer to say they are in series, rather than parallel. On paper they might be drawn using neatly parallel pictorials, but electrically I'd call it a series connection of a capacitor and a current source.

    Oh, BTW, I award you an extra half-mark for using "whilst". :smile:
    Last edited: Oct 30, 2011
  5. Oct 30, 2011 #4
    I agree. I said parallel because in the circuit, they are in parallel. With the switches in the circuit, there exists a moment of time when they are the only two elements in the circuit.
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