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What is heat?

  1. Sep 29, 2004 #1
    I am familiar with the usual concept that heat is kinetic energy (the vibration of matter particles) but I also read that heat is one of the faces of electromagnetic radiation, such as visible light, radiowaves or x rays.

    Is it correct?
    Is it actually both? (e.g. EM radiation just a side effect of the kinetical movement ? and probably able to cause kinetical movement again on the matter absorbing it?) if really EM radiation, in which lenght of the spectrum?
     
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  3. Sep 29, 2004 #2

    Integral

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    Infrared is the EM radiation associated with "heat". This is includes wavelengths from around 10-2m to 10-6m.

    I will speculate that molecular collisions with energy of this wavelength induces translational vibrations as much as it does electronic level changes.
     
  4. Sep 29, 2004 #3
    Thanks Integral.
    Do I get some strange feeling by the fact that you wrote "heat" between quotes ? :-)
    As you can guess from my original question, it was about understanding clearly what heat is.
    The fact that you write "heat" between quotes makes me feel that even for you there's something not so clearly defined about what is meant by "heat" ?
     
  5. Sep 29, 2004 #4
    Heat is the energy that flows between two systems (as a result of temperature differences).
     
  6. Sep 29, 2004 #5

    Chronos

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    As Integral explained, when molecules vibrate [which they always do] they emit infrared radiation [IR]. If you rapidly drag your finger across a smooth board, the molecules of your finger collide with molecules of the board imparting kinetic energy to the molecules of both surfaces. This causes them to vibrate more rapidly and emit higher frequency IR [and your finger feels hot]. If you took a picture using IR film, the hot spot on your finger would look bright and you would see a bright streak on the surface where your finger crossed it [although not as bright as your finger].
     
  7. Sep 29, 2004 #6
    Are you saying that ALL molecules at ALL temperatures emit IR, or is this just a generalisation? I've often wondered.....
     
  8. Sep 29, 2004 #7
    "heat" is in quotation, perhaps because what we typically call heat is when our own temperature nerves are stimulated up to our brain.

    Heat transfer theory will say that heat flows three ways : conduction, convection and radiation. If it is strictly radiation, there is now doubt that we speak of IR radiation. (other radiations can indirectly cause heat by causing IR.)

    Inside matter (a metal or plastic), "heat" is a combination of IR radiation, vibrating molecules, and/or flowing electrons. Generally speaking, you might define heat as a flow of energy within the IR-range of oscillations (whether it is particle kinetic energy or radiation). IR radiation is always in the picture, but energetic particle cannot be ignored.
     
  9. Sep 29, 2004 #8

    Integral

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    I quoted heat because it is easily misused and wanted to make clear that I was using the term loosely and not in the formal sense. Thus I was referring to that sensation we have in the palms of our hands when we hold them in front of the fire. Notice that in the formally correct definition given by Sirus there is no mention of wavelength, so heat is more general then infrared radiation. Infrared is the band of radiation which produces a sense of warmth to our nervous system. The heat would be the energy which is transfered from the source to our hands.

    Heat will flow when any 2 systems in thermal contact are at different temperatures. It is not necessarily infrared in nature.
     
  10. Sep 30, 2004 #9

    Chronos

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    Yes. I may, however, be mistaken and need a refresher course. I would, however, pose this question Can heat [energy] be exchanged between molecules without photons being emitted? If this is true, the black body spectrum of a mass must not be reliable indicator of its temperature.
     
  11. Sep 30, 2004 #10
    It is possible to talk about heat (and even create an entire heat conduction theory) without referring to IR radiation, but at the atomic level, I have not been able to think of a case where no IR would be involved in the last 5 minutes.
     
  12. Sep 30, 2004 #11

    Integral

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    Even in a system of only heat conduction?

    How about a very low temperature system where the total system energy is comparable to the energy of infrared,..... is this even possible?
     
  13. Sep 30, 2004 #12

    Chronos

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    Having completed refresher training, Integral is correct. Heat conduction is achieved without any photon exchanges, it is a kinetic transfer of energy through molecular collisions. IR is still emitted, but, that is not the heat transfer mechanism for conduction.
     
  14. Sep 30, 2004 #13
    If a system is so cold as to not radiate IR at all... well the cosmic background is I believe microwaves, and this is associated to 2-3 K. Yah, I guess, but that's pretty cold. Perhaps people use radio and microwave to "heat up" superfluids in small increments. Do they call it "heat" though?

    I'll agree with Chronos, and thank him for confirming that IR is at least emitted. (if temperature is sufficiently high I suppose, i.e. above a few Kelvins)
     
    Last edited by a moderator: Sep 30, 2004
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