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Applying linearity in a circuit

  1. Feb 28, 2014 #1
    1. The problem statement, all variables and given/known data
    Find Io in the network in the figure below using linearity and the assumption that Io = 1 mA.
    Figure:http://i.imgur.com/Xtu0VmG.jpg


    2. Relevant equations
    KCL, KVL, basic analysis techniques.


    3. The attempt at a solution
    The following values I have calculated correctly:
    VR1=9V
    VR2=61V
    VR3=70V
    I1=1mA
    I2=2mA
    VR4=138V
    VS=208v
    I3=??
    IS=??
    IoActual=??

    So, my trouble lies in finding I3, as once I find I3, I can easily find IS using KCL, though, the current source is doing something to the voltage drop across I3 that I'm not understanding.

    How I tried to solve for I3 was the same way I solved for VS. Since the voltage drop across the source is 208v, then I assumed that the voltage drop across the two resistors on the left summed to 208v as well.

    So, using Ohms Law: V/R=I, 208v/(42000Ω+26000Ω)=3mA, which is incorrect. Where did I go wrong?

    Thank you
     
  2. jcsd
  3. Feb 28, 2014 #2

    vela

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    Rounding errors probably. Your analysis is fine. Just use fractions instead of calculating decimals and rounding.
     
  4. Feb 28, 2014 #3
    Thank you very much Vela. You were correct. 3mA was incorrect, but 3.05mA was correct. (My online homework has a +/- 2% tolerance, so 0.5 was enough to fall outside of the threshold!

    Thanks for verifying my methodology.
     
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