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Finding the current on each resistor

  1. Apr 13, 2010 #1
    1. The problem statement, all variables and given/known data
    [PLAIN]http://img297.imageshack.us/img297/7053/96217079.jpg [Broken]

    given this circuit find the current on R1,R2 and R2


    2. Relevant equations
    I = V/R


    3. The attempt at a solution

    i applied kirchhof's law and I have this:

    36 - 5*I - I1*4 = 0
    36 - 5*I + 20 - 2*I2 - 14 = 0
    20 - 2*I2 - 14 + I1*4 = 0

    and i find I1 = 66/19 which is wrong
    the book says
    I1 = 1.10 A
    I2 = 5.21 A
    I = 6.32 A

    what am i doing wrong?
     
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Apr 13, 2010 #2

    tiny-tim

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    Hi kliker! :smile:

    (try using the X2 tag just above the Reply box :wink:)
    The circuit only has two independent loops, so you can only get two independent equations from it.

    The third Kirchhoff equation you need is for one of the junctions. :smile:
     
  4. Apr 13, 2010 #3
    hi, thanks for the answer

    if we say I = I1 + I2 and take the first two equations

    then we will have

    36 -5*I1 - 5*I2 - 4*I1 = 0
    36 - 5*I1 -5*I2 +20 - 2*I2 - 14 = 0

    36 - 5*I2 - 9*I1 = 0
    36 - 5*I1 - 7*I2 + 6 = 0


    36 - 5*I2 - 9*I1 = 0
    42 - 5*I1 - 7*I2 = 0

    then by solving the equations I found the desired results

    thanks a lot

    Ps: what's the x2 tag? I can't find it
     
  5. Apr 14, 2010 #4

    tiny-tim

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    Hi kliker! :smile:

    (just got up :zzz: …)
    :biggrin: Woohoo! :biggrin:
    Press the "QUOTE" button or the "Advanced Reply" button, and you get to the Reply to Thread page, with loads of useful tags to play with, above the box where you enter your text. :wink:
     
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