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P=VxIxt is energy?

  1. Feb 5, 2016 #1

    GSK

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    The question:
    Calculate the energy used by the lamp in 6 hours of use.

    The answer:
    P=V.I.t
    P= 240v X 0.2A X 6hrs X 3600s

    P= 1036800W!!?

    I thought energy was Joules??? Why has the answer been given in watts? Please help!
     
  2. jcsd
  3. Feb 5, 2016 #2

    berkeman

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    Staff: Mentor

    Welcome to the PF.

    Please be sure to use the Homework Help Template that you are provided when starting schoolwork threads here at the PF. It makes it much easier for us to help you.

    On your question, yes, energy should be in units of Joules. Can you post the full question?
     
  4. Feb 5, 2016 #3
    The answer is indeed in joules
    You have done a mistake
    Power does not equal the energy applied okay??
    It's power ×time which is equal to the energy used up
    The expression which you gave gives the amount of energy used by the lamp in 6hours
    If I were you I would convert 1 volt
    As 1 joule/1 coulomb, 1 ampere as 1 coulomb/1 second
    And then multiply these two with the total no.of seconds
    As you can then clearly see
    You are only left with the unit joules in your answer
    You can perform dimensional analysis to further verify your answer!:)



    UchihaClan13
     
  5. Feb 5, 2016 #4

    berkeman

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    Staff: Mentor

    BTW, when doing unit conversions, it's best to just multiply by "1" and cancel units. So using 1 = 3600s/hr, you should have written:

    E= 240V X 0.2A X 6hrs X 3600s/hr

    And your answer is in units of V * A * s = Joules. :smile:
     
  6. Feb 5, 2016 #5

    GSK

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    I'm really sorry, but I don't understand what you mean?

    Why does 's/hr' then mean it converts to joules from watts?

    Apologies, u have a challenge on ur hands with me.

    My problem isn't physics, but maths. You're gonna tell me they're the same huh?!


    m really
     
  7. Feb 5, 2016 #6

    cnh1995

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    Homework Helper

    s/hr means seconds/hour. You are just converting hr into s so that the unit of energy will be joule. In SI system, joule=V×A×s.
     
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