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Having trouble with some review circuits using nodal analysis

  1. Feb 6, 2013 #1
    Having trouble with some review circuits using nodal analysis :(

    1. The problem statement, all variables and given/known data
    I'm trying to find node voltages for v1, v2, and v3.
    I = 3.1A

    2. Relevant equations
    V=IR


    3. The attempt at a solution
    My method of going about and solving this was fairly straight forward I thought. The way the question is set up, I believe nodal analysis is already ready to go. So I started with
    KCL(v1): 2 + 3.1 + (v1-v2)/3Ω = 0
    KCL(v2): (v2-v1)/3 + v2/2 + (v2-v3)/4 = 0
    KCL(v3): (v3-v2)/4 + v3/3 - 3.1 = 0

    Now it's been a while since I've done this (nodal analysis), but I now have 3 eqns and 3 unknown variables. The answer I have is -21.2V -5.87V and 2.8V respectively.

    It's one of those, "I've tried all my guesses, need some reassurance before I commit to my last chance" scenarios.

    Any helpful on this would be great! Thanks :D

    Note that I haven't tried the brute force method of just solving for everything, or the mesh current method.
     

    Attached Files:

  2. jcsd
  3. Feb 6, 2013 #2

    NascentOxygen

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    Re: Having trouble with some review circuits using nodal analysis :(

    Hi mitchapalooza, http://img96.imageshack.us/img96/5725/red5e5etimes5e5e45e5e25.gif [Broken]

    Have you substituted those back into the equations to verify they are the solution? The equations look right.
     
    Last edited by a moderator: May 6, 2017
  4. Feb 7, 2013 #3

    rude man

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    Re: Having trouble with some review circuits using nodal analysis :(

    Have they taught you the superposition theorem? That way you could handle the problem one current source at a time ...
     
  5. Feb 9, 2013 #4
    Re: Having trouble with some review circuits using nodal analysis :(

    I solved the system and found v1 = -20.78V, v2 = -5.78V and v3 = 2.67V
     
  6. Feb 9, 2013 #5

    rude man

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    Re: Having trouble with some review circuits using nodal analysis :(

    I got
    V1 = - 21.17
    V2 = -5.87
    V3= +2.80
    so we're close.
     
  7. Feb 9, 2013 #6
    Re: Having trouble with some review circuits using nodal analysis :(

    Well, I got:
    V1 = -127/6
    V2 = -88/15
    V3 = 14/5

    which is the exact solution. :cool:

    The 12 digit floating point approximation would be:

    V1 = -21.1666666667
    V2 = -5.86666666667
    V3 = 2.80000000000
     
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