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What do we radiate?

  1. Jul 15, 2007 #1
    When you are in a closed room with other people is the heat produced simply from the the co2 we exhale or do we radiate something from our skin? :redface:
     
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  3. Jul 15, 2007 #2

    ranger

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  4. Jul 15, 2007 #3
    ok but what is it that we radiate specifically? i've already read about radiation and it was stated that its the emmision of electromagnetic waves... so what is happening, we have excess energy and so the electromagnetic waves become more intense?
     
    Last edited: Jul 15, 2007
  5. Jul 15, 2007 #4

    ranger

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    The radiation can be said to be of specific frequencies. When you look at the human body through an infrared camera, what do yo see? Don't you see all sorts of cool color distortions? This is radiated energy in the form of infrared.

    The human body needs to be in temperature equilibrium (steady state); therefore, when the usual ways of getting rid of excess energy (radiation and convection) are not sufficient, we rely on perspiration . If you are interested in finding the net power, you'll have to apply Stefan-Boltzmann law.
     
  6. Jul 15, 2007 #5
    I'd figure that most of the heat removed from your body leaves by convection or through the latent heat needed to vaporize water.

    If you treat a human body as a blackbody you'll emit in the infrared. The sun emits quite a bit in the visible, which is why wearing a black shirt on a sunny day is warmer than wearing a white shirt on a sunny day. But if you're indoors then the colour of your shirt doesn't make a difference to how hot or cold you are - your body emits radiation in the infrared (which your shirt doesn't absorb) and fluorescent lights emit more in the high-end of the visible (which doesn't help to warm you up).

    So if you have a choice between a black shirt and a white shirt in a cold room, you're probably better off just wearing BOTH shirts because your body is mostly losing heat through convection rather than radiation.
     
  7. Jul 15, 2007 #6
    So help me understand this then.. I agree the heat moving around the room is do to convection but when the air molecules touch the skin and absorb the heat is that conduction?
     
  8. Jul 16, 2007 #7

    Integral

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    No, it is convection.
     
  9. Jul 16, 2007 #8
    oky doky :) danke
     
  10. Jul 16, 2007 #9
    The air next to your skin is heated through conduction - but then that air moves away from your skin and cooler air replaces it. That's convection.
     
  11. Jul 17, 2007 #10

    xez

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    Yes, the radiation of heat from the body is from
    conduction, convection, heat of vaporization,
    wien law blackbody radiation of electromagnetic waves
    with high power concentration being around 10um
    wavelength corresponding to a temperature of ~ 300K.

    People radiate especially strongly on hot humid days
    when they've not bathed recently, though that's another
    sort of radiation....
     
  12. Jul 17, 2007 #11

    russ_watters

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    You guys are mixing words together that don't belong together.

    First, Integral was correct and the reason is that there is no coherent layer of air next to your skin to say there is conduction going on: individual molecules in constant near-random motion bounce off your skin and that is convection.

    Second, radiation is radiation is radiation. Radiation is not conduction and radiation is not convection.
     
  13. Feb 7, 2009 #12
    May i know why do we radiate? As in what mechanism actually happening that caused us to radiate em wave?
     
  14. Feb 7, 2009 #13

    Dale

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  15. Feb 7, 2009 #14
    people give that link often on this forum. i dont see how it answers his question though. i think he wants to know if a bio-chemical reaction is causing us to radiate em waves and if not, what is? if thats not his question, i would like to know ;)

    im sure this is more of a chemistry/biology topic but hopefully someone can tell us here.
     
  16. Feb 7, 2009 #15

    russ_watters

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    It does answer the question. For your question: no, there is no biochemical reaction causing us to radiate. We radiate (as the link describes) via the same mechanism all black bodies do.
     
  17. Feb 8, 2009 #16

    DaveC426913

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    The biochemical reaction that contributes to heat loss is perspiration. We perspire even when we're not actually "sweating". Live at a comfy room temperature for a whole day and you will still lose a fair amouint of moisture from your skin. Evaporation is an endothermic reaction - it takes away heat from the body.

    (On an anecdotal note: once, when I was bored waiting in the car in winter, I was touching the window with my finger. A halo of condensing dew danced around on the window next to the tip of my finger, out to about a half inch. I was astonished to realize the implication: at all times, I am surrounded by a thin layer of off-gassing water vapour continually evaporating from my body. I'd always known it, but I'd never seen it so graphically portrayed. After 24 hours, over the entire surface of my vbody, I must lose some fraction of a litre of water.)

    And the tie-in here is that all this water vapour has absorbed heat from my body to evaporate, and is taking it with it as it off-gasses.
     
  18. Feb 8, 2009 #17

    Dale

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    Yes, that is true, but that is evaporation, not radiation. I am sure you already know, but just so that the OP is aware there are three primary mechanisms of heat transfer: conduction, convection, and radiation. Conduction is when two (usually solid) objects of different temperatures are touching and exchange heat through that contact. Convection is when heat is transfered through contact between an object and a flowing fluid. Radiation is when heat is transfered through EM radiation without any contact.

    In addition to those three mechanisms of pure heat transfer there are also mechanisms where both heat and mass are transfered, specifically evaporation and condensation. In evaporation some of the liquid phase converts to gas phase which results in heat and mass being transfered to the gas phase. Condensation is the reverse. I am sure there are other heat and mass transfer mechanisms, but evaporation is the most important biologically.
     
  19. Feb 8, 2009 #18

    russ_watters

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    Well I would say that perspiration is not a biochemical reaction and the question was about radiation, not evaporation...
     
  20. Feb 8, 2009 #19

    HallsofIvy

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    loss of heat is from conduction, convection and vaporization, not "radiation". That word only applies to black body radiation.

    [/quote]People radiate especially strongly on hot humid days
    when they've not bathed recently, though that's another
    sort of radiation....[/QUOTE]
     
  21. Feb 8, 2009 #20

    Dale

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