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Homework Help: Questions about current source and voltage

  1. Aug 8, 2011 #1
    hey guys, do ideal current sources have polarity? When I measure the voltage of a current source, it gave me a negative voltage. What does it mean? Also, can we use KVL in a loop containing a current source without converting it to equivalent voltage source?
    Last edited: Aug 8, 2011
  2. jcsd
  3. Aug 8, 2011 #2


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    Staff: Mentor

    Ideal current sources have a polarity of sorts -- they produce a unidirectional current of fixed magnitude. They will also, however, produce any voltage whatsoever that is required to enforce that current magnitude and direction, regardless of the voltage polarity. It's not a good idea to have an ideal current source unconnected to a load of some sort :smile:

    You can write KVL equations for circuits with a current source, but you'll have to introduce a new variable to represent the voltage across the current source, and the current source is going to cause you to add another constraint equation for the mesh currents (of course, since you're introducing a new variable, you'll need another simultaneous equation anyways...).
  4. Aug 8, 2011 #3


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    Staff: Mentor

    Imagine connecting a current source to a rechargeable battery. It will either charge the battery, or discharge it, depending on the connection. Reverse the connecting leads, and it will do the opposite. The voltage you measure across the terminals of the current source will be the voltage of the battery. Or if reversed, the reversed voltage.

    The same will happen if you connect a current source to a charged capacitor. The initial polarity (of the voltage across the terminals of the current source) will be determined by the initial polarity of the capacitor. Eventually the polarity will reverse if the current flow is in a direction so as to discharge the capacitor (as it then charges it up with the opposite polarity).
  5. Aug 14, 2011 #4
    Thanks everyone!
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