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Homework Help: Find the total power in the circuit

  1. Jan 25, 2009 #1
    1. The problem statement, all variables and given/known data
    Find the total power developed in the circuit (on the attached picture + table)

    2. Relevant equations
    P = IV
    P = -IV

    3. The attempt at a solution
    The answer supposed to be 770mW...
    (attempt to solve the problem - see attached spreadsheet)

    Can anybody help me to solve this? I can not get the answer at no way...

    Attached Files:

  2. jcsd
  3. Jan 30, 2009 #2


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    Homework Helper

    I really don't want to open the Excel file because of possible viral infection but the strategy you should use here is to note if the calculated power of each circuit element tell you if power is either being dissipated or generated. The positive/negative sign in front of the power should tell you that. Just check out your book on the sign conventions of how to calculated power given that the current is entering the element from the side of higher voltage or that of the lower one. Once you've done that add up all the power generated and there you have it.
  4. Jan 30, 2009 #3
    What you may want to do is convert your table so that you are only dealing with positive currents and voltages. Any current that is listed as negative, go to the circuit diagram and change the direction of the current arrow to make it positive. Any voltage that is listed as negative, go to the circuit diagram and reverse the polarity across the circuit element to make it positive. You may want to redraw your circuit diagram to match your new all-positive table to avoid confusion.

    Now go element by element and calculate the power (P = IV). If the current is flowing from the positive terminal toward the negative terminal, the element is dissipating/consuming power. If the current is flowing from the negative terminal toward the positive terminal, the element is providing/generating power. Only add up the power generating elements to obtain the answer.

    Hope that helps. It's kind of a long way of doing it, but I think it will help you understand why each element is providing or absorbing power.
  5. Jan 31, 2009 #4

    The Electrician

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    Gold Member

    Some of the blocks are absorbing power, and some are sourcing power. The law of conservation of energy tells you that if the network has no way to lose or gain energy, such as by radiation, for example, the sum of the powers from each block must be zero.

    If you work out the power from each block, taking into account the signs of current and voltage, and add them all up, you will in fact get zero.

    If you identify the blocks that are sourcing power (they are b, d and f) and add up all the power they produce, you will get a total of 770 mW. If you add up the power absorbed by blocks a, c and e, you will get a total of -770 mW.
  6. Aug 13, 2009 #5
    This problem was driving me crazy as well....

    I, as The Electrician also calculated, found the b,d,and f are producing 770 mW and a,c, and e are absorbing 770 mW. Yet, as the answer in the back of the book (Nilsson / Riedel Electric Circuits 8.e) gives 770 mw.

    Carefully reading the question, it asks for how much power is *developed,* or produced. This should not be taken to mean total power (power produced + power absorbed), because also as The Electrician pointed out, it will necessarily equal zero in the closed system.
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