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Homework Help: Find the voltage Vm (mesh / nodal analysis)

  1. Feb 16, 2013 #1
    1. The problem statement, all variables and given/known data

    http://img541.imageshack.us/img541/5039/homeworkprobsg27.jpg [Broken]

    Find the value of the voltage Vm

    2. Relevant equations

    V = IR,
    mesh equations,
    nodal equations

    3. The attempt at a solution

    http://imageshack.us/a/img507/5556/homeworkprobsg27edit.jpg [Broken]

    Doing it this way, and doing mesh, I came up with:

    I1 + 5ia = ia;

    I1 = -4ia

    then for the left loop:

    24V - 32Ω*I1 - Vx = 0

    Vx = 24V - 32Ω*I1

    then for the right loop:

    Vx - 3Ω*5ia - Vm = 0

    Vx = - 3Ω*5ia - Vm

    So my equations are:

    1.) I1 = -4ia
    2.) Vx = 24V - 32Ω*I1
    3.) Vx = - 3Ω*5ia - Vm

    combining the last two above:

    - 3Ω*5ia - Vm = 24V - 32Ω*I1

    - 3Ω*5ia - Vm = 24V - 32Ω*(-4ia)


    24 V + 32Ω*(4ia) + 3Ω*(5ia) = Vm

    24V + 128Ωia + 15Ωia = Vm


    24V + 143Ωia = Vm but not sure what to do now, or if I approached this the wrong way.

    Any suggestions? Thank you
    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. Feb 16, 2013 #2

    The Electrician

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    Gold Member

    This isn't going to work because the current Ia is passing downward through a wire, which has a resistance of zero ohms. There can't be any voltage across this wire; Vx is zero in other words.

    Superposition seems like the appropriate method here.

    The 24 volt source causes 3/4 amp downward through the wire. The 3 ohm resistor has no effect on the current the dependent current source delivers to Ia.

    So, we have Ia = the sum of 3/4 amp plus whatever the dependent source delivers. Can you write an expression for that? Then you can solve for Ia.

    Once you have Ia, you also have 5*Ia, which is the current through the 3 ohm resistor. Since the left end of the 3 ohm resistor is grounded, the voltage across the dependent current source will be the same as the voltage across the 3 ohm resistor. Just be sure you get the sign of the voltage across the dependent source right.
    Last edited by a moderator: May 6, 2017
  4. Feb 16, 2013 #3


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    Staff: Mentor

    That branch shown carrying ia is just a wire, it's not a current source.
  5. Feb 16, 2013 #4
    So far as I understand the superposition method, it goes finding each current though (in this case) ia with each voltage source short-circuited and each current source open circuited,and then summing those currents through it to get ia.. ? :

    5ia becomes open circuited leaving only the 32 ohm resistor with current flowing through and with V = IR i got 0.75A like you said, Electrician.


    say ia = i (current through)

    this i(current through) = I' + I''

    so I' = 0.75A

    Then I'' would be from short circuiting the 24V

    But then I made a short circuit out of the 24V getting this

    http://img600.imageshack.us/img600/2826/homeworkprobsg27edit2.jpg [Broken]

    stuck again; I know I would have to solve for I'' in the new diagram now though, right?
    Last edited by a moderator: May 6, 2017
  6. Feb 16, 2013 #5


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    Staff: Mentor

    If you have a dependent current source in your circuit then you need at least one independent source active in order to analyze the circuit. Otherwise there's nothing for the dependent source to 'sample' and react to to get started.

    In this circuit you've got one dependent source and one independent source, so you'll need to leave both active to analyze. Why not just write KCL at the top node and solve for the current of interest?
  7. Feb 16, 2013 #6

    The Electrician

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    The 32 ohm resistor is shorted by the wire containing I''.

    It looks to me like I'' = 5Ia

    Proceed from there.
    Last edited by a moderator: May 6, 2017
  8. Feb 16, 2013 #7
    let me check that,

    so ia = 0.75A + 5ia

    -4ia = 0.75A

    ia = -0.1875

    5ia = -0.9375A

    so Vm = (0.9375A)(3 ohm) = -2.81V, but not sure, and no answer given.

    Trying what gneill said:

    With this:

    http://imageshack.us/a/img507/5556/homeworkprobsg27edit.jpg [Broken]

    then 5ia + I1 = ia

    I1 = -4ia

    I think I1 = 24V / 32ohm

    I1 = 0.75A

    -4ia = 0.75A

    ia = -0.1875 A

    5ia = -0.9375 A

    Vm = (-0.9375 A)(3 ohm)

    Vm = -2.81V again, surprising if it's really just KVL too.

    Would this be correct?
    Last edited by a moderator: May 6, 2017
  9. Feb 16, 2013 #8


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    Staff: Mentor

    Looks fine.
  10. Feb 16, 2013 #9

    The Electrician

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    Gold Member

    It seems we can't do a nodal analysis because the place where we would want to have a node (the junction of the 32 ohm and the 3 ohm resistors) is shorted to ground.

    A technique I've used in cases like this is to replace the wire carrying Ia with a resistor R. Then perform the nodal analysis and take the limit as R goes to zero.

    Let the aforementioned junction be V1 and the right end of the 3 ohm resistor be V2 (also Vm). Then we can form two nodal equations and solve them with matrix algebra:


    Vm is -2.8125 volts.

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