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Radiation of heat - how?

  1. Aug 26, 2008 #1

    rcgldr

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    I assume electrons emit photons in the infra-red range.

    Here's my dilemma, heat is a way of stating the average speed of atoms or molecules, for example 27C corresponds to air molecules moving at about 500m/s. Ok, so the molecules are moving fast, is it the collisions with other molecules that tranfers some of this energy into the electrons which then emit infra-red photons (and why not photons of other frequencies)?

    Also is there spin energy (spinning nucleus of atom, not "spin") in molecules, and how does this affect ineraction?
     
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  3. Aug 26, 2008 #2

    uart

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    Hi Jeff , the classical view is that electons accelerating (& decelerating) give off EM radiation.

    Assuming we're talking about conduction electron in solids, the result is "black body" radiation and it occurs over a broad range of frequencies, with the frequency of the peak output increasing with temperature.
     
    Last edited: Aug 26, 2008
  4. Aug 27, 2008 #3

    Andy Resnick

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    Temperature is more than simply a measure of how fast molecules move- it's also a measure of how much energy is available for a system to perform work. They are related through Statistical Mechanics, but the atomic model merely compliments the continuum model, it does not supercede it.

    In terms of molecules, energy is radiated by molecules undergoing either bound-bound or bound-free transitions. Collisions are a typical mechanism to transfer energy (and is the mechanism for heat transfer in materials), and the photon wavelength corresponds to the particular transition- ionization corresponds to x-rays and ultraviolet, electronic transitions to visible, vibronic to infrared, and rotational to millimeter and microwave. Bound-bound transitions produce sharp spectral lines, while bound-free transitions produce broadband radiation.

    The nuclear spin does have an effect (spin orbit coupling), and is manifested by hyperfine splitting of spectral features.

    In the continuum model, a material is endowed with certain properties (emissivity, for example), and the radiation emitted by a meterial at temperature T is the product of the blackbody curve at temperature T and the spectral emissivity. Room temperature objects typically emit in the 8-12 micron range, the deep sky is in the millimeter/microwave region, and the sun primarily emits in the yellow.
     
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