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Vibrational Decay

  1. Nov 4, 2009 #1
    If you have a steel container with steam in it and put it in the freezer the steam will condense and then form ice will it not?
    And if you put the same container in space in heavy shadow the steam will condense and then form ice also?

    The first example is explained by slower outside particles taking energy from the container and the inside particles mainly isn't it?
    The second example is explained by heat radiation mainly isn't it?

    So why, in this second example, do the vibrations between the steam atoms decay to produce heat radiation where there are no outside atoms to steal their energy?
  2. jcsd
  3. Nov 4, 2009 #2

    Vanadium 50

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    Objects don't radiate because they "know" something is out there to absorb it.

    Objects lose heat via conduction, convection and radiation. In your freezer, all three apply, although I imagine conduction probably dominates. In space, there is nothing to conduct or convect, so you have only radiation.
  4. Nov 4, 2009 #3
    Does conduction, as an extra form of heat loss, actually make a body cool faster than if it were in a vacuum though? I understand black-body is independent of the surroundings, but find it hard to believe that a body will cool slower in a vacuum at 2.7K than in a pressurised container at, say, 200K.
  5. Nov 4, 2009 #4
    "Objects don't radiate because they "know" something is out there to absorb it."
    I agree.

    My question is why does the intra-molecule vibration (specifically in a heavily shadowed vacuum) decay into radiation?
  6. Nov 4, 2009 #5

    Vanadium 50

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    Why wouldn't it? Hot objects radiate.
  7. Nov 4, 2009 #6
    Surely there is a more mechanical explanation than that?
  8. Nov 4, 2009 #7
    Ever heard of a vacuum thermos?

    Indeed it would be absorbing more radiation back again from a 200K heat bath than the CMB.

    But, in a large vacuum, the lack of vapour pressure will obviously amplify evaporation (our favourite cooling process, read: the only one more important* than convection).
    *maybe :wink:

    Isn't it natural that energy is transferred from the fundamental to higher modes of oscillation (I know of quantitative exact models which show the equivalent for Brownian motion, if not vibration)? And so ultimately, you even have the electron orbital clouds slightly vibrating with respect to the nuclear lattice; you have a changing charge distribution, ergo: EM waves.
    Last edited: Nov 4, 2009
  9. Nov 4, 2009 #8
    It's natural because it occurs, but what actually makes a molecule decide its going to bounce a little slower this time and decay that energy to emr? There is no outside agent acting on the molecule to tell it to bounce slower so why does it decide to do so this time?
  10. Nov 5, 2009 #9

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    Gonegahgah, molecules don't "decide" anything or "know" anything.

    You clearly have some sort of question, but posing it in anthropomorphic terms is more confusing than helpful. Can you try and rephrase it?
  11. Nov 5, 2009 #10


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    I think he is asking how thermal radiation is created on the molecular level, or why the vibration of molecules creates EM-waves. Can it be explained with accelerated charges?
    Last edited: Nov 5, 2009
  12. Nov 5, 2009 #11


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    Radiation cooling is VERY inefficient at low temperature. If you keep an object suspended in a chamber with a good vacuum and cool the walls of the chamber down to 4K it can take days for the object to cool down to anywhere near 4K. Exactly how long it takes will of course depends on heat capacity, surface area etc.

    If you instead e.g. let a small amount of gas (lets say a pressure of a few mBar or so) the same object will cool in a matter of minutes.

    Also, remember that the radiation goes as T^4. meaning the cooling rate goes down as the object gets colder.

    Convection and conduction are usually much, much more important than radiation.
  13. Nov 6, 2009 #12
    Thanks f95toli. I was wondering if surface area might have some relation to heat loss.

    The cooling due to surrounding gas is also evident where you have wind blowing so that this gives us a chill factor which is greater than the cooling effect of still air; despite wind having a greater velocity than still air. Moving air drags heat away better than stationary air; which to me is another curiosity. If you had a bunch of billiard balls bouncing against each other and travelling like wind they would tend to generate greater vibration in things they blow past; not less?
    But again that is what we observe to happen...

    AT, you've pretty much got it on the head what I'm asking.
    How do gases of bouncing molecules generate radiation?
    They do so by bouncing less and transferring that energy to radiation.
    But why do they bounce less?
    Why don't they just keep bouncing off each other forever?

    Certainly when we look at atoms the electrons want to return to lower energy states.
    Do the molecules want to return to a lower energy state as well?
    By 'want' I don't mean they have feelings or desires.
    I just mean that 'something' drives them drop to a lower energy state if that is right.
    What is that something?
  14. Nov 7, 2009 #13
    When molecules move relative to each other will they always generate (moving closer) or destroy (retreating) some emr as part of the process?
  15. Nov 8, 2009 #14
    Yes, think of a gas sample having its internal energy stored in different modes, and this energy can be lost. For a vibrational mode (bonds stretching, for example) the nuclei will oscillate, and constantly undergo some acceleration, by Maxwell's equations the acceleration of a charge causes electromagnetic radiation. If the molecule is at a high temperature the bond will oscillate with higher frequencies, leading to greater acceleration and more radiation emitted.

    However I am a little confused about the link to black-body radiation, which is independent of the species (so it cannot take into account vibration or rotational modes, which vary between species). Or perhaps I've got it all mixed up; I never really got a great overview of the whole process of thermal and statistical physics, partition functions and black body spectrums.
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