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Homework Help: Determine node voltages in circuit

  1. Feb 10, 2013 #1
    1. The problem statement, all variables and given/known data

    http://imageshack.us/a/img713/9389/homeworkprobsg23.jpg [Broken]

    Determine node voltages for the circuit

    2. Relevant equations

    V = IR,


    Supernode equations

    3. The attempt at a solution

    I am sorry to say that There is a LOT of stuff in this problem I still need help with, like how/ when to apply supernodes.

    Is it true that where you see a ground node, then they do NOT want you to put supernode there? Or can you really put it anywhere with a voltage / current source?

    The only equation for this problem I know for sure to make out at this point is

    Va - Vc = 10V

    Putting the supernodes like this:

    http://imageshack.us/a/img94/5332/homeworkprobsg23edit.jpg [Broken]

    I think I would get (with both currents in the middle going towards B, if I can set it that way):

    5A - 2A = (Va - Vb)/10Ω

    and then

    5A - 2A = (Vc - Vb)/40Ω

    I do not know if this is right at all though.

    Sorry if I should know this well already too, but can a current from another source (ie from the 5A and 2A) pass freely through a wire with a voltage source (ie 10V and 12V)?

    Thank you.
    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. Feb 10, 2013 #2


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    Gold Member

    The voltage on one node is obvious. Look carefully at node b and what's it's connected to.

    yes. A voltage source only regulates it's voltage.
  4. Feb 10, 2013 #3


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    Staff: Mentor

    Denote the current through the 10Ω as i₁
    and current through the 40Ω as i₂

    That gives you 2 unknowns, so you must find two equations involving i₁ and i₂.

    Does the textbook provide the answers?
  5. Feb 10, 2013 #4
    My professor gave me this problem, but yeah he gave the correct answers too and that's it.

    But I have little clue on how to get the correct answer. He says they're based on textbook problems though.

    Can I say that I#1 and I#2 both flow TO node b?

    I'm thinking if they do, then

    I#1 = (Va - Vb)/10Ω and

    I#2 = (Vc - Va)/40Ω

    then if both currents flow towards node b then they would have to go out through the 12V node path so then

    I#1 + I#2 = 3 amps

    Would this be right / the right idea so far?
  6. Feb 10, 2013 #5


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    Staff: Mentor

    You are free to mark each in which ever direction you like. But once chosen, mark the direction with an arrow and stick to that direction rigidly.
    Using the notation of the diagram, what is the voltage across the 40Ω?

    On what basis exactly do you claim this?
    You don't explain your thinking, so I can't say whether you are on the right track. Maybe you are.
  7. Feb 10, 2013 #6
    Sorry, I made a dumb typo for the second equation:

    I meant:

    I#2 = (Vc - Vb)/40Ω

    along with

    I#1 = (Va - Vb)/10Ω

    now would it be right that they would both be going out through the 12V wire?


    I#1 + I#2 = 3 amps, because then it's the last current for the ground and so 2A + 3A = 5A

    Would that be along correct now?
  8. Feb 10, 2013 #7


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    Staff: Mentor

    Yes, 3A flows to ground via the 12V source. So can you now determine the node voltages?
  9. Feb 10, 2013 #8
    Okay so the starting equations (simplified to my needs) are:

    Va = 10V + Vc

    i1 = (Va - Vb)/10Ω

    i2 = (Vc - Vb)/40Ω

    i1 + i2 = 3A

    So then:

    Plugging Va in to I1:

    i1 = (10V + Vc - Vb) / 10Ω

    Now plugging this i1 and i2 in to i1 + i2 = 3A, then:

    (10V + Vc - Vb)/10Ω + (Vc - Vb)/40Ω = 3A

    Multiplying 10Ω and everything above it by 4 to get 40Ω as the denominator:

    (40V + 4Vc - 4Vb)/40Ω + (Vc - Vb)/40Ω = 3A

    (40V + 5Vc - 5Vb)/40Ω = 3A

    40V + 5Vc - 5Vb = 120V

    5Vc - 5Vb = 80V

    0Va - 5Vb + 5Vc = 80V

    And from here it looks like I need to use Cramer's rule. Looks like I get stuck again here, if I was going correct.

    Would I need another equation to do Cramer's rule? If so, how would I get it? Or was there another way?

    Thanks for all your help and patience so far, by the way.
    Last edited: Feb 10, 2013
  10. Feb 10, 2013 #9


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    Staff: Mentor

    Doesn't Vb have a numerical value?

    Cramer's Rule?! I don't think that will be necessary. http://img41.imageshack.us/img41/7325/winkingw.gif [Broken]
    Last edited by a moderator: May 6, 2017
  11. Feb 10, 2013 #10
    Wait... so Vb was just -12V all along?

    That's because it's the only source of potential touching it right, and the other node it touches is the ground?

    Okay so then.

    5Vc + 60V = 80V

    5Vc = 20V

    Vc = 4V


    Va - Vc = 10V

    Va = 10V + 4V

    Va = 14V

    Thanks. Can't believe I didn't see Vb = -12V like that...
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