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Check this Circuit and answer a question?

  1. Mar 5, 2014 #1
    vMBG1v2.jpg

    I believe I have solved this circuit correctly. Each loop and node satisfies KVL and KCL, respectively. All I was given was Vg=528V.

    The question is as follows: "Find the power delivered by the dependent voltage source."

    Currently my answer is 80V*21.6A=1728W, but I want to be sure because I need to get it right.

    Here's the thing! My teacher said 'if the dependent voltage comes out to be negative (opposite in polarity to what is shown in the circuit), in that case you must include a negative sign in your answer for the power delivered by the voltage source'.

    This note from my teacher is making me doubt myself. If I flip the direction of every current then the circuit still works, right? That would cause the dependent source to have a negative value and the power in my final answer would become negative too.

    I don't know which one it is or how to be sure.
     
  2. jcsd
  3. Mar 5, 2014 #2

    gneill

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    Staff: Mentor

    [STRIKE]Can you show your work for finding your circuit current values? I'm not seeing the same results.[/STRIKE]

    EDIT: Hang on, I'm checking my work! May have mucked up a loop...
    EDIT: Never mind. I botched a sign in a loop equation (AURGH! I hat when that happens). Your values look okay to me now.

    Your teachers note is implying that if it turns out that current is actually flowing INTO the defined "+" terminal of a voltage source rather than OUT of it, then the source is sinking (absorbing) power rather than sourcing (injecting) power into the circuit.
     
    Last edited: Mar 5, 2014
  4. Mar 5, 2014 #3
    Thanks a lot for the reply. I guess my question is, how do I know which way the current is actually flowing? If I reverse all the values then the circuit still seems like it works.

    Do I just start the current flow out of the positive independent voltage? Do you agree that "The power delivered by the dependent voltage source is 1728W"?
     
  5. Mar 5, 2014 #4

    gneill

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    You always begin by making assumptions about current directions, then write the equations accordingly and solve. If a current turns out to be negative then your assumption for the current direction was incorrect. No problem! The math will always tell you if your assumptions were wrong. In fact, it really means that there are no bad assumptions for current direction -- the math will lead you to the correct answer no matter what.
     
  6. Mar 5, 2014 #5
    So assuming all those values in the picture are good, it's correct to say the power delivered by the independent source is 80V*21.6A=1728W?

    I really don't get why my teacher would go out of their way to essentially say "watch your sign!" when it's all positive and straightforward. Makes me think I'm getting it wrong.
     
  7. Mar 5, 2014 #6

    gneill

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    Staff: Mentor

    Yup. Looks okay to me.

    Well, it's certainly important to always beware of the signs and check them every time when figuring the power contribution of a source.
     
  8. Mar 5, 2014 #7
    Thanks very much gneill
     
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