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I Heat is infrared radiation...

  1. Jun 8, 2016 #1
    Hello Forum,

    Thermal energy (heat) is associated to the infrared portion of the electromagnetic spectrum. Why? I know the human body emits radiation in the infrared (10 micron). Heat is associated to warming. Are infrared wavelength the resonant wavelengths at which molecules in most objects rotate and vibrate producing an increase in temperature in the object itself?
    Clearly, even visible light can produce warming and an increase in temperature. But I am not clear why thermal energy is specifically associated with heat. I have a laser thermometer that is sensitive to radiation between 8-14micron only to determine the object temperature...

    Thanks,
    fog37
     
  2. jcsd
  3. Jun 8, 2016 #2

    Drakkith

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    Heat is the transfer of energy between two systems by means other than work. This can include the entirety of the EM spectrum, not just the infrared part.

    The human body emits radiation across a broad range of wavelengths, it simply peaks around 10 microns. The image below is a plot of the power radiated by a black body at 310 kelvin, of which the human body is a good approximation. Note that the curve doesn't actually stop at either end of the graph. It actually continues to fall as you move away from the peak in both directions. The graph peaks at 10 microns because of the details of how the internal energy is partitioned into various states. Put simply, the faster the molecules vibrate, rotate, and move, the more radiation they emit and the higher the average frequency of the emitted radiation. The higher the temperature, the faster the average motion of these molecules. You can visit the site listed on the graph to play around and make your own graphs.

    http://www.spectralcalc.com/blackbody_calculator/plots/guest1190043401.png [Broken]
    Thermal energy is part of the internal energy an object possesses. When an object heats up or cools down it is the thermal energy that increases or decreases. Thermal energy is associated with heat because the transfer of energy from one system to another via heat usually causes a temperature change in both systems.

    That's because the radiation emitted by everyday objects peaks within the that range of wavelengths. You will almost certainly lose accuracy if the emitted radiation peaks outside of this range.
     
    Last edited by a moderator: May 7, 2017
  4. Jun 9, 2016 #3
    Does that mean that molecules in most ordinary objects start rotating and vibrating at the infrared frequencies when incident infrared radition is shined on them?
    Once set in motion, the vibrating and rotating molecules emit that same incident infrared energy as radiated infrared energy. This mechanism should be called non-resonant elastic scattering. The nonresonant part means that electrons remain in their ground state (there are no electronic transitions taking place).

    In dissipative absorption, the incident photons have the right frequency and energy to excite electrons to higher energy states (resonance). But the electrons do not return to the lower energy states by emitting a new photon (that may happen in low density materials, like gases). The energy that should be released back going to the lower states is instead converted into thermal energy ( infrared radiation) because of the random chaotic collisions between the molecules. In a sense, there is a a resonant absorption and a conversion of the incident photons to infrared photons (which is what thermal energy is). So I would call this process resonant nonelestic scattering (inelastic because the the incident and radiate photon frequencies are not the same).

    A red substance has a red color because of dissipative resonant absorption of all the photons but the red photons. The red photons suffer nonresonant elastic scattering and reach our eyes giving us the red color perception...

    Is that correct?
     
  5. Jun 9, 2016 #4

    Drakkith

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    No, the molecules simply absorb the energy of the radiation. They could be moving very little, if the object is very cold, or they could be moving very rapidly, if the object is very hot.

    The object emits radiation based on its temperature, not on the radiation it absorbs. A white-hot tungsten filament in a light bulb still absorbs much of the long-wave infrared radiation impinging on it, even though the emitted radiation peaks close to the visible part of the spectrum.
     
  6. Jun 9, 2016 #5

    russ_watters

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    I would consider calling the association between infrared and "heat" to be a colloquialism based on the associated temperatures being near ambient or otherwise associated with everyday experienced temperatures.
     
  7. Jun 9, 2016 #6

    davenn

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    They are already in motion
    I suspect you don't realise that EVERYTHING above absolute zero emits thermal energy ( IR radiation).
    It doesn't need to be irradiated by an outside source to do that.


    Dave
     
  8. Jun 9, 2016 #7
    Hi Dave,

    I do know that everything that has a nonzero T emits radiation. I am just wondering why infrared radiation is associated to thermal heat and not other type of radiation.
    When we are in front of a fire, do we feel warm solely because of the infrared radiation portion of the electromagnetic radiation that is incident on us?
     
  9. Jun 9, 2016 #8

    Drakkith

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    No, it includes all EM radiation being emitted from the fire that your body absorbs.
     
  10. Jun 9, 2016 #9

    russ_watters

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    By whom? Please provide a source. As I said: it appears to me to be a nonscientific colloquialism.
    Infrared radiation is electromagnetic radiation -- so, yes.
     
  11. Jun 9, 2016 #10

    Delta²

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    Not sure, I believe that the EM radiation due to the vibration of atoms/molecules as a whole is the infrared radiation. Other types of EM radiation from the atoms are due to the electrons changing energy states.
     
  12. Jun 9, 2016 #11
    thanks. I see your points russ_watters. Why does my non contact thermometer, which can measure temperature between -76 and 932 F but only focuses on the 8-14 micron range (infrared)?

    Fog37
     
  13. Jun 9, 2016 #12

    Drakkith

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    If that were true, you would see individual emission lines in the spectrum of an incandescent light bulb.
     
  14. Jun 10, 2016 #13

    davenn

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    Drakkith has already answered that in post #2


    Dave
     
  15. Jun 10, 2016 #14

    Delta²

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    Ok hypothetical question cause I understand it cant be done in real world, if we could keep the nucleus of the atoms in the blackbody perfectly still ( I guess this means the temperature would be 0K?) but the electrons could move around em and change energy states, wouldn't the blackbody radiation curve be a zero line?
     
  16. Jun 10, 2016 #15

    davenn

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    not if the electrons are still in motion as they will be the source for your EM radiation IR or otherwise
     
  17. Jun 10, 2016 #16

    OmCheeto

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    I believe the 8-14 micron range is determined by the sensor material. It's not really focusing on that range. It's sensitive in that range.
    According to a table at wiki, it means your sensor is most likely Mercury cadmium telluride (MCT)
    Spectral band: Long wavelength infrared
    Range: 8-14 µm
    Detector material: HgCdTe
    Applications: Ambient temperature(outdoor, industrial inspection)

    [ref]​

    If you plug in the high and low temperatures that your thermometer can read, and also the range (8-14µm) , into the tool Drakkith mentioned in post #2, it will give you "Band Radiance" figures:
    Temp(°F)_________Band Radiance(W/m2/sr)
    -76________________8.1
    932______________1023​
     
  18. Jul 30, 2016 #17

    OmCheeto

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    appears to have been linked to a temporary file, as it is now gone.
    A permanent link is: http://www.spectralcalc.com/blackbody_calculator

    I'm bringing this up, as today is the second time I've wanted to reference the calculator, and it takes some finagling to extract the URL.
     
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