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Radiating heat and vacuums

  1. Sep 23, 2005 #1
    A very non specific question about heat loss in a vacuum.

    If you threw a human into space (with out worrying about his/her lungs bursting out) is there a rule of thumb about how fast he/she would cool down.

    As there is molecules heat transfer can only be through radiation, and assuming he/she is not in the sun's path, there is no radiation adding heat and no molecules hitting him/her to add heat through kinetic energy.

    So would they lose heat very quickly or would they be like coffee in a thermos flask and be perfectly insulated in space and only lose heat very slowly.

    (This is probably very basic)
  2. jcsd
  3. Sep 23, 2005 #2


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    Heat can be radiated via emission of IR (infrared) radiation. That's how the heat from the sun gets here. IR is an EM radiation, just like visible light, so it requires no medium for transmission.

  4. Sep 23, 2005 #3
    Yes, I was aware of that and hoped I made that clear when I (mis)wrote
    "As there [are no] molecules heat transfer can only be through radiation"

    [] represents my poor original writing corrected.

    So as an analogy would it be correct to say that the rate of heat loss of coffee in thermos vacuum flask and the rate of heat loss of the same coffee in a sealed container outside of the sun's rays would be roughly the same?

    ie, the heat would radiate away very slowly (comparitively) and that this rate would be determined by.....? (the propensity of that particular material to produce IR radiation? Is that a fixed physical property?)

  5. Sep 23, 2005 #4
    I guess in the thermos flask on earth you would also have the coffee being heated by IR radiation from the outside.

    Is it possible to estimate the rate of those two heat exchanges?
  6. Sep 23, 2005 #5


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    The rate at which heat is radiated away is given by the Stephan-Boltzmann Law. If the poor soul floating in space dies, and there is no heat being produced by the metabolism in his/her cells, then the temperature of the person will fall away exponentially, leveling off at about 3K.

    The properties that determine the rate of heat radiation are primarily (i) the total surface area, (ii) the emissivity, and to a lesser extent, (iii) the heat capacity, (iv) the geometry of the radiator, and (v) the mean thermal conductivity of the body.
  7. Sep 27, 2005 #6

    Claude Bile

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    Thermos flasks are insulated via are vacuum to prevent heat loss due to conduction and convection. Heat can still be lost through radiation, and this is why thermos flasks have a highly reflective interior.

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