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B Power P=IV

  1. Mar 16, 2016 #1
    There are three Formulas for Power, ( P=IV, P=I^2R, P=V^2/R). Can someone tell me when to use the right forumlas?
     
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  3. Mar 16, 2016 #2

    ProfuselyQuarky

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    How has your textbook explained it?
     
  4. Mar 16, 2016 #3
    It is quite confusing in the book as they have used different formula for different question. But I know that P=I^2R is used to find the heat
     
  5. Mar 16, 2016 #4

    Dale

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    What are your thoughts? What is different about the three equations and when might those differences be important?
     
  6. Mar 16, 2016 #5

    ProfuselyQuarky

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    Okay, well, do you know what each variable stands for?
     
  7. Mar 16, 2016 #6
    I think that P=IV is mainly used to find the power of the whole circuit whereas P=I^2R is used to find how much energy is given out by the component per second.
    But not sure if I am right.
     
  8. Mar 16, 2016 #7

    I for current
    R resistance

    I understand the fact that it is simple substitution and the formulas are literally same. But when it come to practice questions it is used differently in different context
     
  9. Mar 16, 2016 #8

    Dale

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    OK, that is a good start.

    What is different about P=I^2 R that makes it better for individual components than P=IV? Or what do components have that might lead you to use P=I^2R.
     
  10. Mar 16, 2016 #9
    Well we are looking at difficulty of current going through due to resistivity.
     
  11. Mar 16, 2016 #10

    Dale

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    Right. So would it be possible to use P=I^2 R if you don't know the resistance?
     
  12. Mar 16, 2016 #11

    ProfuselyQuarky

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    Not really :smile: It just depends on what you're looking for--and the only way to know what you're looking for would be to know what each part means. Perhaps this picture will help: click here
     
  13. Mar 16, 2016 #12
    Well it is possible if you know the voltage
     
  14. Mar 16, 2016 #13

    ProfuselyQuarky

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    That's correct! See, you can use different equations (in this case, Ohm's Law) to find the missing parts you need. So where's the trouble?
     
  15. Mar 16, 2016 #14

    Dale

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    Yes, exactly. There are only two formulas here: P=IV and V=IR. You have two equations in four variables. You can always substitute one equation into the other to eliminate any one variable.

    That is all those equations do. You are then left with one equation in three variables. Use the one that fits.
     
  16. Mar 16, 2016 #15
    If you look ag question 3c you can not use all the formulas. Only one will give you the right answer.
    1458172754881.jpg

     
  17. Mar 16, 2016 #16

    Dale

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    What are the knowns and what are the unknowns? Which formula fits?
     
  18. Mar 16, 2016 #17

    ProfuselyQuarky

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    And based on that, which formula for power matches what you know?
     
  19. Mar 16, 2016 #18
    P=v`2/r
     
  20. Mar 16, 2016 #19
    This gives me the wrong answer
     
  21. Mar 16, 2016 #20

    ProfuselyQuarky

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    What numbers did you use?
     
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