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Kirchoffs law confusion

  1. Aug 22, 2011 #1
    1. The problem statement, all variables and given/known data
    I have this circuit diagram (see attached), and the question is: Which of the following equations is Kirchhoff's first rule as applied to the above circuit?
    With the answers:
    I1 + I2 + I3 = 0
    I1 - I2 + I3 = 0
    I1 + I2 - I3 = 0
    I1 - I2 - I3 = 0
    (it's multiple choice)

    2. Relevant equations
    sum of Iin = sum of I out

    3. The attempt at a solution
    I can't even begin. I understand Kirchoff's laws pretty well, but it just appears to me that the circuit is drawn wrong.

    What am I doing wrong?

    Attached Files:

    Last edited: Aug 22, 2011
  2. jcsd
  3. Aug 22, 2011 #2


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    Staff: Mentor

    It seems that your attached diagram isn't attached :smile:
  4. Aug 22, 2011 #3
    Now it is :smile:
  5. Aug 22, 2011 #4


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    Science Advisor
    Homework Helper
    Gold Member

    Apply the equation that you posted. What is the sum of currents going in? What is the sum of currents coming out? Look at your drawing.
  6. Aug 22, 2011 #5


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    Staff: Mentor

    I see that the diagram has the currents labeled on each branch. Why don't you add some more such arrows nearer the nodes where the currents come together or diverge? Note that the current in any given series connected branch is always the same current!
  7. Aug 22, 2011 #6
    I see part of it, but shouldn't the current in I3 flow the other direction?
  8. Aug 22, 2011 #7


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    Staff: Mentor

    Whether the actual current flows in the other direction or not does not matter; it simply means that when the equations are solved and calculations are completed with the assumed direction, it will yield a negative value for that current.

    Take the assumed directions as given and write the equation from them.
  9. Aug 22, 2011 #8
    okay, thanks
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