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Passive sign convention (negative watts, and negative current confusion)

  1. Jan 21, 2007 #1
    This is my (error full?) understanding from my readings:

    1. positive current flowing into a positive terminal of lets say a rechargeable battery is power being absorbed by the battery...so positive watts

    2. negative current flowing into a positive terminal encounters the higher electrical potential at the positive terminal and the battery has to do work to carry it through to the negative terminal, so the battery is discharging...so negative watts

    3. positive current flowing into negative terminal... charges the battery?

    4. negative current flowing into the negative terminal... discharges some watts?

    so i'm a bit unsure with the last two and want to know what, if anything, is wrong with my observations.
     
  2. jcsd
  3. Jan 21, 2007 #2

    mjsd

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    ok, sign convention can be a pain but as long as you are consistent, it doesn't really matter whether it is +/-... after all it is just a convention. Now, in calculations, usually common sense can help you out a lot in determining polarities and/or whether a device is active or passive.

    In cases where that is not so clear, ie. just a black box, then extra care should be taken. Some general rules:-

    negative current is just positive current flowing in the opposite direction

    Assuming a 2-terminal device
    so -10A into -ve terminal which is the same as 10A out of -ve terminal; which is same as 10A into +ve terminal; which is same as -10A out of +ve terminal!!

    defining +/- terminals? you may say what if one delibrately defines the polarity so that +ve terminal actually has a lower potential than the -ve terminal (relative to the same ground/ref point). In that case, the voltage across the given device is simply negative .

    In the examples you listed, since you know that it is a battery, so your know that when the battery is giving out energy your +ve terminal is the end with higher potential. Of course if it is just a black box, you will probably need more info from the rest of the circuit to be sure. But once everything is worked out (conservation of charge/energy etc...) , if the polarity/direction was set incorrectly, then the voltage/current will simply get a -ve sign. when working on power remember you have two conventions:

    P=VI for passive convention (+ve current into +ve terminal)

    P=-VI for active device (+ve current into -ve terminal)

    NB +/-ve here are AS DEFINED by you in the diagram.

    if you are consistent, everything will work out in the end. :smile:
     
  4. Jan 21, 2007 #3
    I think i figured it out if the attached picture shows it correctly :P
     

    Attached Files:

  5. Jan 21, 2007 #4
    Hmm no, still confused.

    In attached picture, problem (b), if i flip the current arrow so its 1.75A entering the plus side... isn't that passive sign convention satisfied then? and p = (-3.8V) * (1.75A) = -6.65W? book says answer is 6.65W.
     

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  6. Jan 21, 2007 #5

    mjsd

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    can't view your picture, so can't help you. If you are sure that you have been consistent, then you will prevail over the book :smile:
     
  7. Jan 21, 2007 #6
    Last edited: Jan 21, 2007
  8. Jan 21, 2007 #7

    mjsd

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    ok, i think you have just read the question wrong :smile:

    Q 2.7 asks for power being generated... you probably have assumed the wording for 2.6 when answering it ..but the questions clearly refer to which diagram they are talking about....
     
  9. Jan 21, 2007 #8
    DOOOOH! AAAAHAHAHAHA!!! :rofl:
     
    Last edited: Jan 22, 2007
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