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Confusion in thermal radiation concepts - infrared, microwave, etc.

  1. May 22, 2012 #1
    Microwaves heating is often referred to as dielectric heating, but I'm not sure why this seems so special to specify of all the different thermal radiation frequencies. Doesn't infrared heat the dielectric material as well in a similar process? Isn't radiation heat transfer common for a wide range of EM frequencies (radio to ultraviolet)? I already know the answer to that last question - that heat transfer is not specific to just the infrared or microwave, but then that's why I am confused of our references to thermal radiation as infrared only.

    So, why is infrared considered "thermal radiation" while others really are not? I know that some frequencies can actually bump electrons to higher energy states, and this might be considered different from dielectric heating as a form of energy transfer, but there are still many frequencies that are absorbed by a material and heated besides just infrared.

    Does anyone see why I'm confused, or can you clarify this some what?
    Last edited: May 22, 2012
  2. jcsd
  3. May 22, 2012 #2
    Infrared is occasionally called thermal radiation because objects at room temperature have the peak in their thermal spectrum in the infrared. Similarly 'thermal' neutrons have kinetic energies similar to an ideal gas at room temperature.
    Microwave ovens don't actually necessarily use microwaves anyway, they tend to use radio frequencies instead.
  4. May 22, 2012 #3
    So the heating mechanism is the same for all EM radiation energy transfer, regardless of wavelength?
  5. May 22, 2012 #4
    In terms of dielectric heating, the frequency has to be greater than about 10MHz, but any frequency over that should work by the same principle, but you don't want the frequency to be too high otherwise it might ionise the electrons.
  6. May 22, 2012 #5
    What about lower frequencies? The black body radiation intensity curves are low but non-zero at frequencies below 10MHz, so what is the mechanism there?
  7. May 22, 2012 #6


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    As far as I know, they all use a Magnetron and a frequency of 2.45GHz. There is no other amplifier inside them that could generate any other frequencies. What device could they use and how would they avoid interference? The screening of a microwave oven door involves a quarter wave slot and is specific for just one frequency.

    As for the terminology that is used, I don't think it's worth while getting too excited about broad brush categories. Classification of most thing involves fuzzy edges of the classes. Think about gamma rays and X rays, for instance.
  8. May 22, 2012 #7


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    AHHHH ... for your information .... Microwaves ARE radio frequencies
    generally anything 1000MHz (1GHz) and up are referred to as microwaves :smile:

  9. May 23, 2012 #8
    So is all heating from EM waves a form of dielectric heating?
  10. May 23, 2012 #9


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    Dielectric heating applies to dielectrics. Conductors heat up because of current flowing through a resistance. You would call that Resistive Heating, perhaps.
    Just applying terms to a process doesn't do much for ones understanding of that process.
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