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Homework Help: Simple Power in a circuit problem

  1. Jan 24, 2007 #1
    1. The problem statement, all variables and given/known data

    Determine the missing quantity in the circuits in figure below.

    2. Relevant equations

    P = IV = V^2 / R
    V = IR

    3. The attempt at a solution

    From the diagram, I know the current is flowing opposite to the reference direction (because of the negative current). I used the equation, P = VI.


    -20 W = V (-2 A)... I get V is equal to 10 V. It says something is wrong, what am I doing wrong?

    I know since the power is negative, and due to Passive Sign Convention, it is supplying power. I'm kind of stuck as to why it doesn't work out the way I did it. Thanks.

    Attached Files:

  2. jcsd
  3. Jan 24, 2007 #2
    I'm sorry, I re-read the text in the mean while and I figured this out. I forgot to change the polarities and switch V_1 to -(V_1).
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