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Medical Humans give off 100W of power?

  1. Jul 24, 2008 #1
    I've heard this figure thrown around a bit and I'm having trouble wrapping my mind around it. The idea is that a human just sitting around like I am now gives off about 100W of power.

    I'm eating a pack of fruit snacks now, worth about 190 Calories. So, a quick calculation...

    4.12J/s = 1 cal/s; 100J/s ~= 25cal/s; 1000 cal = 1 Cal => 1 Cal/40s; Under a minute to burn a Calorie. A day has over 1200 minutes, so you end up burning let's say 1800 Calories.

    Is this where the "2000 Calorie Diet" that you see on food packaging comes from? They just assume you're a lazy *** and don't move at all? Or is 100W just way too simplified?
     
  2. jcsd
  3. Jul 24, 2008 #2

    CRGreathouse

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    Yep.
     
  4. Jul 24, 2008 #3

    russ_watters

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    Yes, 100W is oversimplified, but it is also a bit high. Average is more like 70W. Remember, not all of what you burn ends up as heat - some provides energy for other chemical reactions.
     
  5. Jul 25, 2008 #4

    Gokul43201

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    Somewhere between 60W and 100W is common for low to medium activity levels.

    In some chip fabrication cleanrooms, each person entering the cleanroom switches off a lightbulb while entering (and switches it back on when leaving). This special set of lightbulbs does not serve any illumination purpose, but simply helps to minimize fluctuations in the cleanroom temperature.
     
  6. Jul 25, 2008 #5

    mgb_phys

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    Remember that food calories are really Kcal, ie 1Kcal = 4200J
     
  7. Jul 25, 2008 #6

    russ_watters

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    I looked for it online and couldn't find it, but here are the values ASHRAE uses (in btu, sensible and latent, with watt converstions):

    Seated at theater: 225S, 105L = 66W, 31W
    Moderately active office work: 250S, 200L = 73W, 59W
    Walking, standing: 250S, 250L = 73W, 73W
    Light bench work: 275S, 275 L = 81W, 81W
    Moderate dancing: 305S, 545L = 89W, 159W
    Heavy work: 580S, 870L = 170W, 255W
    Athletics: 710S, 1090L = 208W, 319W

    Obviously, my 70W figure was sensible heat only. Evaporation from sweat and breathing adds a lot to the total heat dissipation, so it may be better to say the total heat generated by a person nearly at rest is more like 130W.
     
  8. Aug 5, 2008 #7
    is this counting the brain or not
     
  9. Aug 5, 2008 #8
    I remembered to make that distinction...
     
  10. Aug 5, 2008 #9

    mgb_phys

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    I know, I reread your calcs after the edit window, sorry.
     
  11. Aug 5, 2008 #10

    DaveC426913

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    Wow.
     
  12. Aug 5, 2008 #11
    That's interesting. So there is about 600 billion watts of power being wasted by the earth's population.
     
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