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Simple Junction Rule question

  1. May 3, 2010 #1
    1. The problem statement, all variables and given/known data
    I need to find the current equation for the junction in node a in the following circuit:
    http://i749.photobucket.com/albums/xx137/abcdmichelle/gjgjhg.jpg


    2. Relevant equations

    Current in = Current out so I(in)=I(out)

    3. The attempt at a solution
    The arrows in the diagram represent the direction of the current.
    At node a I thought the junction rule would be

    I(3)=I(2)+I(1)

    Is this wrong?
    I think it is but I don't understand why!?
    Please help!
     
  2. jcsd
  3. May 3, 2010 #2

    diazona

    User Avatar
    Homework Helper

    Yes, that is wrong... how did you come up with that?

    Anyway, here's how you do the junction rule:
    1. List all currents flowing in to the junction. This means, for each piece of wire connected to that junction for which the current arrow points into the junction, write down the associated current.
    2. List all currents flowing out of the junction. This means, for each piece of wire connected to that junction for which the current arrow points away from junction, write down the associated current.
    3. Write the junction equation, which is
    (sum of list #1) = (sum of list #2)
     
  4. May 3, 2010 #3
    Thank you so much!

    oh ok, so it would just be
    I(3)+I(2)=I(1)

    right?
     
  5. May 3, 2010 #4

    diazona

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    Homework Helper

    Yep, that's it.
     
  6. May 3, 2010 #5
    Thanks again! :) :) :)
     
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