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Kirchoff's Current Law - Negative Current Value?

  1. Sep 7, 2005 #1
    First of all, sorry for the poorly drawn figure.

    In this figure, given are several conducting elements connecting by conducting wires, with the direction and amount of current flow indicated. I have to find the current across elements I1 through I4. When I apply Kirchoff's Current Law at node A (pointed to by the red arrow), I get:

    Current entering node = Current leaving node
    0 = I2 + 7A + 3A
    I2 = -10A

    Similarly, when I apply the law at node B (blue arrow), I get:

    Current entering node = Current leaving node
    2A = I4 + 4A
    I4 = -2A

    Am I doing something wrong, or is it possible for current to have a negative value depending on direction? In other words, can current be a vector quantity?

    Thanks for regarding my dumb question.

    Attached Files:

  2. jcsd
  3. Sep 7, 2005 #2


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    Science Advisor
    Homework Helper

    Sure, this means the current I4 is actually flowing towards node B. Current is always a vector quantity although in electronic circuits you don't need to bother with three-dimensional vectors, one dimension will do. The important thing is that is has a direction which is shown in the sign of the current. YOU have to specify the positive direction though, that's why there are arrows in the diagram. These arrows do not show the direction the current is flowing, but the positively oriented direction.
    Current is exactly like velocity in this respect. (Current IS charge density times velocity).
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