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Homework Help: Find voltage across resistor using mesh current analysis

  1. Mar 17, 2016 #1
    • Thread moved from the technical forums, so no Homework Help Template is shown

    I'd like to find Vth by mesh current analysis.

    Mesh A and B share a current source, so they're treated as one supermesh.

    What's the equation for this supermesh? Also, do I ignore the loop with the open circuit? Since there's no current flowing there...

    2A = ib - ia

    Anyway, is this the right equation:

    +12V + ia(6Ω) + ib(6Ω + 4Ω) = 0?

    if yes, then my ia ends up being -32V/(4Ω) = - 8A

    ib = 2A + 8A = 10A

    And the Voltage across 4Ω (Vth) = ib * 4Ω = 40 V!

    But that's not correct, Vth should be 6V!
  2. jcsd
  3. Mar 17, 2016 #2


    User Avatar

    Staff: Mentor

    With currents marked clockwise, the voltages across all resistors here sum clockwise as voltage falls, and you could pencil in a small + and - sign on each resistor as a reminder.

    So summing in a clockwise path the voltage rises around the loop:
    +12 - ia(6) - ib(6+4) = 0
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