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Find total Power absorbed or delivered to Element B .

  1. Aug 26, 2011 #1
    Find total Power absorbed or delivered to Element "B".

    I'm supposed to find how much power was absorbed or delivered by element "B". What is the thought process/solving technique for this type of problem? This was a quiz problem, it has already came and gone; if I see a problem like this again please help me in knowing how to solve this type of problem and how I might apply that knowledge to other similar problems with different questions. In other words, I'd like to understand it all completely. Please see my attachment. Thanks for any help given or offered!
     

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  2. jcsd
  3. Aug 26, 2011 #2
    Re: Find total Power absorbed or delivered to Element "B".

    Pick a node as a ground (0 V). I would recommend the bottom of the picture. Now You know the top left node is at 20 V. You also know the current through the 6.25 ohm resistor is 4A. That means the voltage at the center node is 20 - (6.25 * 4). You know the current going through element 'B'. You know the current going through element 'C' (ix) by Kirchoff's current law (4+6). You now know the voltage at the top right node. Now you know the voltage difference over 'B'. Multiply the voltage by the current, and that gives the power you're looking for.
     
  4. Aug 26, 2011 #3
    Re: Find total Power absorbed or delivered to Element "B".

    Thanks Pligrimman,

    So with Kirchoff's current law (4+6) gives me Ix=10. Then 1.5Ix=15V. The potential difference in voltage is 15V-5V=10V across B. Then, since the current across B is 6A and the voltage is 10V, I would have 60W absorbed by B?

    Does this sound correct? I got a little confused on the 15V-5V, thinking it would be 15V-(-5V). I wish that there were something out there complied all into one drawing that gave all of the information that should be know and then develop them into memorization through using it over and over again; not really memorizing but knowing it through repetiveness...like studying for math and physics test.

    Thanks for all of your help!
     
  5. Aug 31, 2011 #4
    Re: Find total Power absorbed or delivered to Element "B".

    According to the professor, he says that the correct answer is 120watt; does anyone else get 120watt? Where did I go wrong in my solving?
     
  6. Aug 31, 2011 #5
    Re: Find total Power absorbed or delivered to Element "B".

    Did you get -5V for the center node? 15-(-5) = 20V

    20(6) = 120 Watts

    You were right in thinking 15-(-5).
     
  7. Sep 1, 2011 #6

    rude man

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    Re: Find total Power absorbed or delivered to Element "B".

    Confusing! I assumed the right-hand diamond thing is an independent current source of 1.5ix = 6A. So going into B from the left is 4 - ix = 4 - 4 = 0A. 6A in and 0A out?

    But if the current source is a depedent one, that would make more sense. And I imagine that would lead to the answer you got.
     
  8. Sep 1, 2011 #7
    Re: Find total Power absorbed or delivered to Element "B".

    No. The diamond thing is a current controlled voltage source (ccvs). That means the voltage is a function of the current through some wire, in this case ix. So the voltage difference across the ccvs is 1.5ix. It has 6 amps through it by kirchoffs current law, and ix is 10 amps, so the voltage gain is 15 volts (at the right end of 'B'). The voltage at the center is -5 volts (which you found earlier). 15-(-5) = 20. 20(6) = 120 watts.
     
  9. Sep 1, 2011 #8

    rude man

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    Re: Find total Power absorbed or delivered to Element "B".

    Thank you Pilgrimman.
     
  10. Sep 3, 2011 #9
    Re: Find total Power absorbed or delivered to Element "B".

    Thanks Pilgrimman. You are correct; this is how the professor demonstrated the solution to the problem on the board. He used KCL, showing the 6A and 4A in was 10A out. Ix was equal to 10A and then this 10A multiplied by the 1.5 gave the 15volts. Then he skipped straight to 20(6)=120watts...
     
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