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Simple Power Equations Gone Wrong!

  1. Jan 16, 2005 #1
    I like to think of myself as pretty smart and on the ball with the electrical stuff. But I am having trouble figuring out this one concept with power and it seems really simple. I am probably just over thinking the whole thing as usual. Hopefully someone can give me some insight.

    In my Circuit Analysis (Elecrical Networks) class, we just covered power, P=IV, and all the stuff about how to figure out if an element is absorbing or delivering power. What we have been told is the following:

    When current goes from + to -, Pabs = +VI and Pdel = -VI
    When current goes from - to +, Pabs = -VI and Pdel = +VI

    This seems pretty simple... you have 2A going into a element from + to - and the voltage on the terminals is 5V. (5)(2) = 10. Since the answer is positive, there is 10W power being absorbed.

    Then we say that in the same case it is -2A. So, (-2)(5) = -10W. Here is where things start to get crazy. Current is going from + to - still... but then you can say that -2A from + to - is like 2A from - to +. This would then be correct with 10W Absorbed.

    But when things really hit the fan is when we get a problem like this:

    You have -2A going from + to - and the absorbed power is 6W. What is the voltage across the terminals?

    Well, from P=IV we know that V=P/I. So, 6W/-2A = -3V. But if you have -2A going from + to -, then you have 2A going from - to +. So power abs needs to be -VI in this case....

    I think I just confused myself again. Do you see my plight? I think I know it... it is just getting jumbled up in over thinking.

    If you can answer the following questions, I would be grateful.

    What is the convention to use to determine the power in an element and whether it is being absorbed or delivered?

    How would you use the same convention to determine either Voltage or Current given the power and one or the other quanitity listed.

    Thanks.
     
  2. jcsd
  3. Jan 16, 2005 #2
    You CANNOT have negative power. When you have a negative voltage you will also have a negative current.
     
  4. Jan 16, 2005 #3
    There are questions in my textbooks and notes that show only one of those values as being negative, leading to a negative answer for P=IV. It is my understanding that the sign on P tells you whether or not the power is absorbed or supplied.... yet the answer to that continues to escape me depending on which direction the current is in and such.
     
  5. Jan 18, 2005 #4

    SGT

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    The + and - signs are an arbitrary reference, and so is the direction of the current. When you state that the positive reference of the current is from + to -, you have associate references. In your example of a -2 A going from + to -, what you are really saying is that your current goes in the opposite sense of your reference, so you have a +2 A going from - to + in your chosen reference. Since power is being absorbed, your voltage must be -3 V. This means that the potential in your terminal labelled - is greater than the potential labelled +.
     
  6. Jan 22, 2005 #5
    For the person who said there cannot be negative power--- I like to actually use negative power when finding out what sources PROVIDE power to the circuit, and positive power for power absorbed by resistors (considering P = iiR is always positive). Just my way of using convention... but you are correct when in a physical sense there cannot be negative power.
     
  7. Jan 23, 2005 #6
    I considered that but decided not to confuse the OP any farther than he already was.
     
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