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Power and Efficiency Question

  1. Nov 7, 2013 #1
    1. The problem statement, all variables and given/known data
    An incandescent bulb is only 5.0% efficient. How much waste thermal energy is porudced by a 100.0W bulb in 1.0h? (Anwer=95J)

    2. Relevant equations
    P = E/T
    Efficiency = Eout/Ein x 100%

    3. The attempt at a solution
    P = E/T
    E = PT
    = (100)(1)
    = 100J

    Eout= 95J

    This is the correct answer, but why is time in hours? Shouldn't it be in seconds (1h=3600s) so the equation becomes:

    E = PT
    = (100)(3600)
    = 360000

    This is the wrong answer, but I don't understand why time isn't measured in seconds as the SI unit is Seconds for time.
    Last edited: Nov 7, 2013
  2. jcsd
  3. Nov 7, 2013 #2


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    Gold Member

    Yes, 100 W = 100 J/s, so in an hour, the total energy used by the bulb should be

    E = P*t = (100 J/s)*3600 s = 360,000 J
  4. Nov 8, 2013 #3
    I did this but the answer comes out to be:
    95 = Eout / 360000 x 100
    0.95 = Eout / 360000
    Eout = 342000

    This would mean 342000J of thermal energy is produced by a 100W bulb
    However, this answer is 95J which you only get when you use time in hours. So maybe the answer is wrong at the back of the book.
  5. Nov 8, 2013 #4


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    Yeah, maybe. All I know is that a watt is a joule per second.
  6. Nov 8, 2013 #5


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    I agree the book answer is wrong.

    As you say..

    A 100W bulb consumes 100J/S or 360,000 J/H

    If 95% is wasted then the answer is 0.95 * 360,000 = 342,000 Joules
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