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Why is this voltage negative?

  1. Oct 6, 2008 #1
    Why is this voltage negative??

    http://qaboard.cramster.com/answer-board/image/96937de22e44b1524804f2cf580462fe.jpg [Broken]
    Using superposition theorem
    With voltage source only,
    Vth1 = (3)(5000) / (2000+5000) = 2.14V
    With current source only
    Current through 5kΩ resistor
    I = (0.007)(2000) / (2000+5000) = 2mA
    The voltage across it will be
    Vth2 = - (0.002)(5000) = -10V why is this voltage negative??
    The resultant voltage is
    Vth = Vth1 + Vth2
    Vth = 2.14 - 10 = - 7.86V
    Last edited by a moderator: May 3, 2017
  2. jcsd
  3. Oct 8, 2008 #2


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    Science Advisor

    Re: Why is this voltage negative??

    The direction of current flow through the resistor (due to the current source) is the opposite of that due to the voltage source. As the voltage measurement is (implicitly) with the "-" terminal on the shared node on the bottom, this makes the voltage due to the current source negative.

    EDIT: Not implicit measurement, I missed the ground symbol on the bottom node. The more intuitive way of realizing that there is a negative voltage is that current flows from higher potential to lower. And then realize that if the higher potential is zero (ground) then the lower potential must be negative.
    Last edited by a moderator: May 3, 2017
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