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Possible lecture error

  1. Sep 18, 2012 #1
    here is a screenshot from one of my lectures talking about how to find active power (P) and reactive power (Q)

    He calculates P using 100*10 Cos (30-(-30), which I understand, though the answer he gets is 200W, but whenever I calculate it I get 500 because cos60 = 0.5 and 0.5*1000 is 500.

    Just wanting to know what I'm doing wrong.

    Attached Files:

  2. jcsd
  3. Sep 18, 2012 #2


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    Staff: Mentor

    It does look like it should be 500.
  4. Sep 18, 2012 #3
    Yes, its obviously 500.
    Also, VA = Sqrt(Watts^2+Var^2)
    1000 ~= sqrt(500^2+866^2)
  5. Sep 18, 2012 #4
    I think it's 500

    I use complex number. From the picture of the board:

    V=100∠+30 deg = 100(0.866+j0.5)= 86.6+50j
    I=10∠-30 deg = 10(0.866-j0.5)= 8.66-j5.

    Power =Re[IV]= Re[(86.6+j50)(8.66-j5)]= 749.956-250=500.
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