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Kirchoff's circuit problem

  1. Apr 12, 2007 #1
    1. The problem statement, all variables and given/known data
    Determine the magnitude and directions of the currents through r1 and r2 in the diagram below.


    2. Relevant equations
    V=IR

    here is the schematic, i hope you can get the idea of the circuit from it:
    [​IMG]



    3. The attempt at a solution

    so i found the current through r2 to be -.4 Amps, which was the answer given using:

    V=IR
    6.0=15I

    but i dont know if i just got lucky.

    the answers given were I1=.68 amps and I2= -.4 amps. i am completely stuck on all of these kirchoff's circuit problems, so any help will be much apreciated!!
     
  2. jcsd
  3. Apr 12, 2007 #2

    marcusl

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    Science Advisor
    Gold Member

    Kirchoff's Laws require you to sum the currents into each node or sum the voltages around each loop. To do the voltage law, call I1 the current circulating, say, CW around the top loop (9V battery, R1, R2, back to battery) and I2 the current circulating around the bottom loop. The voltages around the top loop are

    -9 + 22*I1 + 15*(I1-I2) =0

    Do you see how that works? Now you do the bottom loop. Then it is a simple matter to solve for the currents I1 and I2.

    BTW, are you at CU? Just wondering from your screen name...
     
  4. Apr 12, 2007 #3
    thanks so much! that makes a lot more sense now. i couldnt figure out the paths, and tried almost everything.

    i am at boulder high, just down the street.
     
  5. Apr 12, 2007 #4

    marcusl

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    Gold Member

    Good, glad to help :biggrin:

    I'm south nearer Fairview.
     
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