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Power Formula

  1. May 13, 2016 #1
    Hello all,
    I have a question, probably a controversial one:

    is there any difference between the Formula- P= V*I and P= I*V?
    is there any technical difference between the two expressions?
    thank you.
     
  2. jcsd
  3. May 13, 2016 #2

    ENE

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    HELLO,
    It is like 2*3=6 or 3*2=6
    both are same
     
  4. May 13, 2016 #3
    in my academic days, I was taught both are not the same in a certain sense,
    Power, P, was always V.Icosø, and never I.Vcosø.
     
    Last edited: May 13, 2016
  5. May 13, 2016 #4

    sophiecentaur

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    afaik, a Dot vector product is commutative. The cos of the angle between the vectors is the same whichever one you are measuring relative to: Cos(x) = Cos (-x).
    So you can write it either way round.
     
  6. May 14, 2016 #5

    ENE

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    I was thing P= V*I as numeric
     
  7. May 14, 2016 #6

    ENE

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    Can you explain again?
     
  8. May 14, 2016 #7

    ENE

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    Can you explain what you said?
     
  9. May 14, 2016 #8

    sophiecentaur

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    IF you are talking about DC then VI is exactly the same as IV because multiplication is commutative (2X3 is the same as 3X2). If you are talking about AC then you need a vector product between Volts and Current. The Power is given by the Dot Product (not just the RMS Volts times the RMS Current). But that is also commutative so either way round is ok there too.
     
  10. May 14, 2016 #9

    ENE

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    Thanks for explaining who to do this?
    i was not aware as i usually work with DC.
     
  11. May 14, 2016 #10

    sophiecentaur

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    Your electricity bill charges you for actual Energy delivered. That involves a Dot Product. Google Power Factor to find all about it.
     
  12. May 14, 2016 #11

    ENE

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    In AC circuits, the power factor is the ratio of the real power that is used to do work and the apparent power that is supplied to the circuit. The power factor can get values in the range from 0 to 1. When all thepower is reactive power with no real power (usually inductive load) - the power factor is 0.
     
  13. May 14, 2016 #12

    ENE

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    I know about power factor lag and lead of current or voltage due to presence of extra L or C load.
     
  14. May 14, 2016 #13
    OK thanks Ene and Sophiecentaur, but will I be right to contribute from this point of view:
    Mathematically the commutative product of Vector proves the product V.I to be equivalent to I.V
    but in AC circuits for example, the mean power consumed by a circuit is given by the product of V and that of the CURRENT COMPONENT which is in phase with V, Voltage.
    I assume that the fact of possibility that current can be resolved into two components- Isinø and IcosØ- is key in explaining the expressions V.IcosØ and I.VcosØ
    the factor to be envisioned here is, cosØ, which is the power factor of the circuit.
    current 'chases' voltage, I.e, it always tries to make up a perfect Power (constantly wants to achieve cos0°) hence IcosØ...
    so will I be right to attribute I, current, as the variable component in the expression?
     
    Last edited: May 14, 2016
  15. May 14, 2016 #14
    I have no intention to teach something, actually. However, as I said somewhere, the vector notion
    is not a clear one, today. Once, a vector was a real thing as force [F] or field [E electric or H,B magnetic] intensity, or current density [J] or else. After this a phasor-which represents on a paper a wave- it turned to be a "vector”. No vector product [cross product] it could be done with V or I or other such "vectors”. However, a scalar [dot] product it could be meaningful.
    Now, extending the notion, a column of a matrix is named "a vector”. Also, a column of a library
    stand... How this "vector" could present gradient , divergence or curl?:frown:
     
  16. May 14, 2016 #15
    hi Babadag,
    I'm sorry I don't understand what you stated. Maybe you should give a different perspective to it.
     
  17. May 14, 2016 #16

    jim hardy

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    I suspect Mr yungboss is not versed in vector calculus.

    If all three variables (V I and cosø) are just numbers then it doesn't matter in what order you write them. That's grade school algebra.
    If in your prior training ø changes sign depending on which variable you take ,V or I, as starting point for the angle then Sophie's post #4 answers your question for power. But you'll have to pay attention to sign when calculating VARs, VIsinø .

    Yes, draw your phasor triangle and apply Pythagoras.

    I dont understand. Those verbs apply to the observer not to the inanimate charge in motion.
    We cannot know what you have in mind. V, I and ∅ are all variables.
    Can you draw a phasor diagram in Paint , label the variable(s) and upload it with button in lower right ?
     
  18. May 14, 2016 #17
    please oblige my curiosity and response, not intending labor the point.
    ok, what I'm tryingvto express is stated here https://en.m.wikipedia.org/wiki/Leading_and_lagging_current
    voltage is generated and this gives rise to a resultant current that is either in phase, lead or lag. because of the above conditions (whereby current is in phase, lead or lag), the expression Icos∅ obtains.
    I understand that the three components V, I and cosØ exist as variables but I was of the opinion that cosØ represents the cosine of the angle by which Current departs or conforms with the Voltage phase
    sorry if I'm not making much meaning here...
    but does the cosine law of vectors have an impact on the expression?
    https://www.math.washington.edu/~king/coursedir/m445w04/notes/vector/dotproduct.html
    also this link, scroll downwards
    https://en.m.wikipedia.org/wiki/Law_of_cosines
     
    Last edited: May 14, 2016
  19. May 14, 2016 #18
    but you all say the expression I.VcosØ is correct and technically same (from electricity viewpoint) as v.icos∅, then I must accept it.
    many thanks
     
    Last edited: May 14, 2016
  20. May 14, 2016 #19

    jim hardy

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    Yes, this time you have stated that very clearly, i understand what you meant.

    No apology necessary; congratulations on your improved clarity.
    https://www.math.washington.edu/~king/coursedir/m445w04/notes/vector/dotproduct.html


    upload_2016-5-14_16-3-47.png
    i guess i don't understand the question.
    Trigonometry applies to all triangles, be they phasors or vectors or real geometric shapes like land surveys.


    Much of learning is just coming to believe what you already know. One reassures himself by working out with pencil and paper enough examples that the steps become automatic.
    Make up three numbers and assign them to A B and cosAOB.
    Let me call cosAOB by "C" so it's easier for me to type
    There are only two possible sequences because i can repeat the sequence and start anyplace in it.

    ABCABCABC is same sequence as BCABCABCA which is same as CABCABCAB just i started at a different place.
    Likewise
    ACBACBACB is same sequence as CBACBACBA which is same as BACBACBAC ...

    does A X B X C equal A X C X B ?

    You knew that all along .

    old jim
     
  21. May 15, 2016 #20
    got that right on old Jim, thanks...
     
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