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Homework Help: Use Joules to find Energy

  1. Feb 27, 2006 #1
    Please Help...

    Given: A person walks about 3.6 calories/minute.
    How much energy is in a 40 Watt light bulb, that is lit for 5.6 hours?

    Answer: I think it is 144000 J/hr. I am not sure what to do with the 5.6
    hours. I think it should be used to caluculate the total J which
    would be 806400.

    Question: I need to find the amount of time it takes to walk that equals the energy of the light bulb (in hours) using the calorie amount given.:confused:

    I have taken the total J (806400) and divided by the calorie/hr (I converted the 3.6 calorie/min to calorie/hour). Then I tried to solve for the total number of hours. I keep getting the wrong answer.
    Last edited: Feb 27, 2006
  2. jcsd
  3. Feb 27, 2006 #2


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    Well, "how much energy is in a 40 watt lightbulb" doesn't make sense. It's correct to say "How much energy is used by a 40 watt lightbulb".

    What answer are you getting and did you convert from calories to joules?
  4. Feb 27, 2006 #3
    Thanks for the reply...

    When converting calories to J: 3.6 calories =0.251 J/s

    0.251 J/s = 903.6 J/hour

    Since I already have the total Joules: 806400 J for 5.6 hours, I used it by dividing the 806400 by 903.6. I am getting 892.43 hours.
  5. Feb 27, 2006 #4


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    First, notice the units. 1 watt is defined as one joule per second. A 40 watt bulb consumes 40 joules per second. 5.6 hours is 20,160 seconds. 40 joules per second * 20,160 seconds = 806,400 joules. (Not joules/hr[/b].)

    - Warren
  6. Feb 27, 2006 #5


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    Uhm, 1 calorie = 4.186J. Calorie is not a rate.

    You need to determine how many Joules of energy per second are being used if theres 3.6 calories/minute being used. I think they maybe using the biological use of "calorie" which is usually Kilocalorie but still denoted as calorie. If this is the case, 1 Calorie = 4,186J
  7. Feb 27, 2006 #6
    I converted 3.6 calories/minute to Joules of energy per second. I got 0.251 J/s. Then I divided the total energy (806400) by 0.251. This gave the number of seconds which is 3,212,749. I coverted this to hours and got 892.43 hours. It is wrong. I am not sure what I am doing wrong.
  8. Feb 27, 2006 #7


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    If those calculations are correct, I suspect they are using the nutritional concept of "calorie" which is actually kilocalorie which is 4186J per Calorie.
  9. Feb 27, 2006 #8


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    I think that's 4.187 to 4 significant digits (from 4.1868...) - not trying to be picky! :)
  10. Feb 27, 2006 #9


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    Tell my textbook that! :P
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