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The validity of the interconnection of ideal sources

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  1. Aug 12, 2017 at 4:04 AM #1
    1. The problem statement, all variables and given/known data
    Sorry in advance for such a basic question, just starting my first electrical engineering course. The question asks if the interconnection of ideal sources is valid.

    The next part of the question asks us to verify that the total power developed in the circuit equals the total power absorbed.

    The diagram given is, from the left going clockwise: a -30V ideal voltage source, +10V ideal voltage source, -8V ideal current source.

    edit: image added as requested:

    [​IMG] https://ibb.co/eGkEev
    [​IMG]

    2. Relevant equations

    p = vi

    3. The attempt at a solution

    I gave the sign of each voltage above in lieu of a diagram (negative sign a voltage rise, positive a voltage drop). The 30V and 8V source are developing power, the 10V consuming it. I am not sure how this is a valid circuit given that we have -38V + 10V -> According to KVL shouldn't that equal zero?

    However, it confuses me as to how this could be valid. According to the answer to a similar question, a circuit is valid when the voltage sources can carry the Amp of current supplied, and the current source can carry the voltage drop required by the interconnection. I don't understand how to calculate whether this is true of a circuit. In the similar question I referenced, which was valid, you first had one 50V source, then a 10V and 40V source, so I suppose you could equate 50 and 40+10 (there was also a 5A source) (the directions in at question were -50, -10, +40, so I wasn't adding them up, just trying to work out why it was considered valid (and I don't know if that is how you go about it anyway).

    Regarding power absorbed, we are not given the current, so I am unsure how to proceed there.
     
    Last edited: Aug 12, 2017 at 4:25 AM
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  3. Aug 12, 2017 at 4:08 AM #2

    Nidum

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    Can you just show us the circuit diagram ?
     
  4. Aug 12, 2017 at 4:26 AM #3
    I added a link to the diagram. It's not very clear, sorry, best I could do.
     
  5. Aug 12, 2017 at 4:33 AM #4

    Nidum

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    circuit.png

    That's the best that the image enhancer could do . It's good enough to see what the problem is about anyway .
     
  6. Aug 12, 2017 at 5:17 AM #5

    CWatters

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    Hint: What kind of source is the one marked "8V" ?
     
  7. Aug 12, 2017 at 5:40 AM #6
    It's an ideal current source, but I'm not sure how that helps.
     
  8. Aug 12, 2017 at 6:02 AM #7

    cnh1995

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    Yes, and how is it lebelled? What is the unit of current?
     
  9. Aug 12, 2017 at 6:10 AM #8
    It's labelled in volts. So 8V goes into the 10V source.
     
  10. Aug 12, 2017 at 6:15 AM #9

    cnh1995

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    That dosn't make sense. How can 8V go into 10V source? It should be 8A. Current sources provide constant current.
    It is either a typo or they did it intentionally and want you to spot it.
     
  11. Aug 12, 2017 at 6:23 AM #10
    That's what I thought (that they provide constant current), but they also supply some voltage too don't they? That's why it was confusing.Okay, so assuming it is a typo, how can 30V go into a 10V source?
     
  12. Aug 12, 2017 at 6:26 AM #11

    cnh1995

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    The two voltage sources are in series. I don't understand what you mean by "go into" 10V source.
     
  13. Aug 12, 2017 at 6:30 AM #12
    Sorry, I just thought we started from the left. So do we start from the ideal current source and go round the circuit in that direction? In which case the 10V is developing power and the 30V absorbing it. And to make it balance the 8A current would have to develop 20V.
     
  14. Aug 12, 2017 at 7:10 AM #13

    cnh1995

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    You can start from anywhere, it doesn't matter. I think what you want to find is who is supplying power and who is absorbing power.
    Yes. So what is the 8A source doing? Supplying or absorbing?
     
    Last edited: Aug 12, 2017 at 7:34 AM
  15. Aug 12, 2017 at 11:53 AM #14

    CWatters

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    They produce whatever voltage is necessary to make the specified current flow. However no current has been specified. Specifying a voltage 8V is nonsense so the circuit has an error or is invalid. As a result further analysis of the circuit is rather pointless.
     
  16. Aug 13, 2017 at 12:25 AM #15
    It would be supplying power. Thank you so much for your help, it makes sense now.
     
  17. Aug 13, 2017 at 12:30 AM #16
    Thank you. I think given the rest of the question it was an error. Going through it helped clear up some of my misconceptions and helped with my general understanding though, so I am grateful for the responses. I really appreciate this forum!
     
  18. Aug 13, 2017 at 12:35 AM #17

    cnh1995

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    You're welcome!
     
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