Energy in a human body.

  1. I only have a basic knowledge biology, but I've wondered about this before.

    From a "fully charged" adult male human to dying of lack of nourishment; in case of chemical energy from food, this is often said to be three weeks.

    If it takes that long for a human to lose energy, how much is held chemically, in joules? It's hard to believe that a human can keep moving for that long with the reserves we have. There's also the energy we gain with each breath, so that's a factor too.

    Also, if this chemical reserve was used up all at once in a single movement, how much more powerfully would our muscles fire (ignoring inhibition which stops the body from tearing it's own muscles, but then this a hypothetical of the force produced.) If an Olympic Athlete was able to use ALL of the energy to high jump, how far would he or she fly? (taking the destruction of the bone and muscle out of the equation).
  2. jcsd
  3. DaveC426913

    DaveC426913 16,532
    Gold Member

    We gain no energy by breathing.
    What we gain is oxygen, which allows us to burn the fuel that we eat.
  4. negitron

    negitron 840
    Science Advisor

    You can get a very rough first order approximation by starting with the average power output of an average human, which is roughly 200 watts. Given three weeks = 1,814,400 seconds, you get ~363 MJ.
  5. That sounds insane. That's equivalent to 80 kilos of TNT, apparently.
  6. DaveC426913

    DaveC426913 16,532
    Gold Member

    I had thought it was more like 60W. Admittedly, that's indirect.
    In electronic component clean-rooms, minute temperature changes can affect deposition rates of chemicals. The temperature of the room is so carefully controlled that, when employees enter and exit the room, they turn off or on a 60W light bulb to compensate.
  7. Moonbear

    Moonbear 11,955
    Staff Emeritus
    Science Advisor
    Gold Member

    Something important to factor into this is that as one is dying of starvation, they are NOT moving until they suddenly drop dead. I don't know exactly how long it would take for an otherwise healthy person who is deprived of food (assuming they still have access to water) to become so lethargic that they can't do anything but lie still, but I suspect it might only be a few days, if that long, and then all energy usage would be reserved to just vital functions of organs.
  8. russ_watters

    Staff: Mentor

    I generally use 70w. 200w would be during some reasonably strenuous exercise.
  9. Another way to make a very rough estimate is to assume, say, 40 kg weight loss before death. If you divide 40 kg by the average atomic mass we get the number of atoms. If we assume that the atomic mass is 12 u, then we arrive at the estimate of 2*10^27 atoms. If the energy yield per atom in the various chemical reactions is 1 eV, then the total energy is 320 MJ, which is quite close to the estimate by Negitron :smile:
  10. I think there is a factor 4 efficiency factor between energy use and power. If you generate 200 Watt useful power on a home trainer, you are actually burning energy at a rate of 800 Watt.
  11. negitron

    negitron 840
    Science Advisor

    Another order-of-magnitude estimate, based on calorie consumption. If an average person requires 2,000 calories per day, then he uses 2,000 C x 4,184 J/C x 15 days = 126 MJ. That's probably closer to the mark than my previous estimate, which is based on the derating value of a human's power output for HVAC system sizing.
  12. russ_watters

    Staff: Mentor

    The wattage might be output, but the calories it gives you are certainly input.

    The American Society of Heating, Refrigeration, and Air conditioning Engineers publishes energy consumption data for various activities for use by engineers. Thinking about it more, 70 watts is actually just the sensible heat - there's an additional somewhat lesser amount of latent heat (moisture) generated. For an office worker, it is something like 250 btu sensible, 200 latent. Anyway, sorry, 200 w wasn't far off the mark and is reasonable for someone doing some active housework or something like that.
  13. negitron

    negitron 840
    Science Advisor

    Yes, some machines do display watts.
  14. russ_watters

    Staff: Mentor

    Yeah, that's a good way to estimate it. Converting directly from calories to watts, that's 96 watts, average.
  15. russ_watters

    Staff: Mentor

    I know they display it, I just wasn't sure (until I checked) if the displayed watts was output or input.
  16. negitron

    negitron 840
    Science Advisor

    Oh, Oh, okay. I misunderstood what you meant.
  17. Hi,

    I hope it's okay to bump this older thread.

    I'm a sci-fi writer. And I've been googling for information about the amount of energy in the human body so that I can flesh out a plot detail in my current novel. (Thus I landed here.)

    As background to my questions, here is an older science news article from the Sydney Morning Herald which points toward what I am writing about in my novel.

    Anyway, in my novel, my Main Character is sort of an Alfred Hitchcock/Frank Capra "everyman" who gets unwittingly sucked into a dark power struggle of international intrigue. Part of the mystery being unraveled in the story is that he discovers (much to his horror) that he has a small implant in him. He has a doctor take it out, and then he brings it to a techy-geeky friend who identifies it as a GPS tracker --so my Main Character was unknowingly "chipped" much like a dog. But (his techy-geeky freind explains), the difference between the kind of RFID chip you would put into a dog versus an actual GPS tracker is that an RFID chip is a passive device that needs no power supply to function. But a GPS is an active transmitter which needs a hefty and continuous amount of energy to transmit all day long. Thus GPS trackers need batteries, and those batteries need to be replaced rather frequently (much like the frequency needed to recharge a cell phone). Therefore it's currently impossible to surgically place a GPS tracker inside of a dog or a horse or a human because the tracker's internal battery pack would run out of juice in a matter of days and then be useless. However ...... the device that my Main Character had in him is a very advance little gadget which astonishes the techy-geeky friend because --so far as he knew-- such a device was merely theoretical and didn't actually exist yet. This device draws upon the power generated by the human body in order to transmit. So it can theoretically transmit for years.

    So ... my questions are:

    1) How much energy does a GPS tracker need PER HOUR to transmit all day long?
    2) How much energy does the human body generate PER HOUR via normal digestion of food?
    3) How much of an energy deficit would that human suffer PER HOUR as a result of the GPS tracker stealing energy away from him? And would he need a daily allotment of extra food and more sleep to try and make up for that deficit?
    4) I am very poorly versed in the technical jargon used when discussing electricity, wattage, amps, hertz, joules, etc, and I am equally inept at trying to translate back and forth between watts and amps and volts and hertz and joules, etc, so please give me the for-dummies treatment if you launch into that sort of explanation.

    Thanks for all your help!! :)
    Last edited: Jan 27, 2011
  18. Power is energy per unit time. Watts are units of power, and 1 Watt = 1 Joule per second. Volts multiplied by Amps will give you Watts because the terms for electric charge in each unit will cancel each other out. Calories are units of energy, not power. If you convert 2000 Calories into Joules, then divide that number by the number of seconds in a day, the result will be in Watts. Remember that a lot of the energy is lost as heat (also Joules), so only a fraction of the 2000 Calories could be tapped into for "useful" work.
  19. Andy Resnick

    Andy Resnick 6,141
    Science Advisor
    Education Advisor

    1) A GPS tracker could be as simple as a receiver (to get a position) and a small tramsmitter to transmit the information. Both of those devices are available now, for real, that run on AAA batteries: it's possible to imagine the package would only draw a few milliWatts (mW) or so of power.

    2) the human body basically generates 100W of power from normal resting metabolism (normal recommended daily calorie intake).

    3) The GPS transceiver draws a negligible amount of power when compared to this. You would not notice.
  20. Okay. All very helpful information. Thanks for all yor help! :)
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