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Homework Help: Power Calculation in a Circuit

  1. Jun 8, 2010 #1
    1. The problem statement, all variables and given/known data

    001453a8ba9edc1b4b0e34f120b7ae262g.jpg


    vT = 10.3[V], vW = -7.6[V], vX = -8.4[V], vY = -9.5[V], vZ = 1.9[V]
    iM = 3.3[mA], iN = 24.1[mA], iP = 15.2[mA], iQ = -18.5[mA], iR = 5.6[mA], iS = -8.9[mA]

    2. Relevant equations

    P = vi or P = -vi

    mA = miliamps

    3. The attempt at a solution
    Well I used the passive convention and so when the current flow was going from the positive terminals to the negative I used P = vi. And when current flow was going from the negative terminal to the positive I used P = -vi.

    I can't seem to get the right answer..

    Apparently the right answer is

    Summation of Power Delivered = 315.xx [mW]
    Summation of Power Absorbed = 315.xx [mW]

    I am not getting these answers when I add up my positive and negative power values...

    Could someone please explain how I would work my way around the circuit schematic to solve this problem and if possible the solution as well

    Thanks
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Jun 8, 2010 #2

    vela

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    First thing I did was flip the directions of any currents and voltages so that they're all positive. For example, the voltage across A would be +8.4 V with the plus sign on the right end and the minus sign on the left end. Similarly, I'd flip the direction of the current through B so that it flows to the right and is equal to +8.9 mA. That'll get rid of the problem of trying to keep track of all the signs.

    After you do that, you should find only two elements are sources, where the current flows from lower potential to higher potential. The rest are sinks, where current flows from higher potential to lower potential.
     
  4. Jun 8, 2010 #3
    I did what you asked Vela. I flipped the Polarities of the Voltages that were given negative values and I switched the direction of current which was given negative values.

    Now I have another problem.

    You said that Developing Elements are where current goes from - >> + and Absorbing Element are ones where current goes vice versa.

    In my modified diagram,

    iR goes from negative to positive
    iS goes from negative into positive
    iQ goes from negative to positive
    iP goes from negative to positive
    iM goes from negative [ Vw] to positive

    So that would mean that there are 5 sourcing elements and 1 sink element. The sink element being the F element with vT & incoming current iN. Going from Positive to Negative.

    What am I doing wrong?

    Help please +(
     
  5. Jun 8, 2010 #4
    Actually I need one more thing clarified. Which current is flowing through Element C?
     
  6. Jun 8, 2010 #5
    Ignore my last two responses.

    I did get the right answer but I have an issue with a statement you made.

    You said that, "find only two elements are sources, where the current flows from lower potential to higher potential. The rest are sinks, where current flows from higher potential to lower potential".

    But when I modified the polarities and current directions like you said. You get 4 sourcing Elements where current is going through a voltage rise. - >> +

    And 2 sinking elements where current is going through a voltage drop. + >> -

    Did you perhaps make a mistake? Or am I missing something?
     
  7. Jun 8, 2010 #6

    vela

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    I just rechecked a few elements, and it looks like I indeed got it backwards.
     
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