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Transformer open/short circuit test

  1. Sep 5, 2015 #1
    Hi guys,
    The problem isn't with the question itself its more to do with the fact about how they got Z and R. Just need some clarification.
    confused.png
    In every other AC power question. I have always worked out impedance with whatever parameters I had be it P/I^2 or V/I. I'm confused as to what the difference here is? Now I'm being thrown off and don't know what I'm calculating in other questions :S
    Both of those formulas work out 'R' as can be seen here..so why are they getting two different numbers?
    sds.png

    Thank you in advance guys
     
  2. jcsd
  3. Sep 5, 2015 #2
    you have posted ohms law for DC not AC.

    is that your confusion?

    you know to replace R in your circular crib sheet with

    Z = R + (Xl - Xc)j

    ?
     
  4. Sep 5, 2015 #3
    I'm confused as to why Z = V/I and not R...but P/I^2 does
     
  5. Sep 5, 2015 #4
    You confused why Z does not equal R?

    Z = V/I is ohms law for AC circuits

    Z equals R when there is no reactive component.

    Z is the total opposition to current flow defined as I wrote previously.


    are you happy with complex numbers (both using j notation and polar notation, and swapping between the two)?

    Do you know the power triangle
    http://sub.allaboutcircuits.com/images/02218.png
     
    Last edited: Sep 5, 2015
  6. Sep 5, 2015 #5
    Thats perfect, thank you! and yes I'm good with converting between the two :)
     
  7. Sep 7, 2015 #6
    Just a note, if you draw a power triangle. You'll notice that the 'real' Power is real (Re) and reactive power (Q) is Imaginary (Im), whereas apparent power (S) is a vector of both.

    Considering that resistors only have the real component and no imaginary part, this is why you can apply the DC power equation to the AC circuit for this resistor.
     
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