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Kirchoffs Law and simple circuit problem

  1. Mar 23, 2012 #1
    1. The problem statement, all variables and given/known data
    The problem is in the attached image. I can't figure out what is going on. The circuit as given is at the top, I have circled what I am trying to find.

    2. Relevant equations

    3. The attempt at a solution
    I used the junction law to find the current through the 3 ohm resistor to be 8 amps. Then I took three loops of the system and applied kirchoffs voltage law, and solved the system of three equations and 3 unknowns, but it's inconsistent. I don't see where I am going wrong but I suspect it has something to do with my loops.

    2ptdsb8.jpg

    No matter how I do this, it always works out to be impossible???

    Thanks
     
  2. jcsd
  3. Mar 23, 2012 #2

    gneill

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    Staff: Mentor

    I think you're overthinking this :smile:

    Assign a potential of 0V to the node labeled d (make it a reference node). Then using the known currents and resistances, assign relative potentials to nodes b, h, f. Continue...
     
  4. Mar 23, 2012 #3
    That potential of 0 volts is from which voltage source though? I'm not sure I understand what you mean exactly. I have done about 20 problems like this, finding currents and resistances and emf's without any problems just doing loops and solving. It should work?

    If you mean, using node d as the "ground" if I were say, hooking up a multimeter or something, then how do you know that node d has 0 potential?
     
  5. Mar 23, 2012 #4

    gneill

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    Staff: Mentor

    It's zero because you say it is :smile: It's a point of reference, as you say, as if you were to connect the negative lead of your multimeter to it. The same thing is done when you choose a reference node for nodal analysis. The circuit as a whole has no absolute potential reference to anything external, so you're free to assign your own "zero" reference point.

    EDIT: Note that your choice of loops did not include the 3 Ω resistor in any path. That's why the system of equations you wrote would not yield a consistent result. Your equations must cover every component.
     
    Last edited: Mar 23, 2012
  6. Mar 23, 2012 #5
    I didn't know that you had to hit every component, but that makes sense since most/all of the variables are dependent on the others. To be clear, I need to make sure I cover all the components in my system, but that does not mean I need to do all possible loops right?
    (I got the right solution now, knowing that information, thanks!)
     
  7. Mar 23, 2012 #6

    gneill

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    Staff: Mentor

    Correct.
     
  8. Mar 24, 2012 #7
    Got it, thanks Gneill.
     
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