Can there be different IR emissivity & IR absorptivity

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Discussion Overview

The discussion revolves around the relationship between infrared (IR) emissivity and absorptivity of surfaces or coatings, particularly whether it is possible for a material to have different values for these properties at a specific wavelength and temperature. The scope includes theoretical considerations and practical applications in thermal radiation and energy conservation.

Discussion Character

  • Debate/contested
  • Technical explanation

Main Points Raised

  • Some participants assert that there can be surfaces or coatings with different IR emissivity and absorptivity, citing examples like selective absorbers in flat plate collectors that have high absorptivity and low emissivity.
  • Others question the possibility of differing values, referencing Kirchhoff's law of thermal radiation, which states that at a specific wavelength, absorptivity equals emissivity, although this may not hold in practical scenarios with a range of frequencies.
  • A participant argues that if absorptivity were greater than emissivity, it would lead to spontaneous warming of the object, while the opposite would cause spontaneous cooling, suggesting a violation of energy conservation principles.
  • Another participant mentions IR bulbs used for curing powder coatings as a potential example of surfaces with high IR emissivity and low IR absorptivity.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between IR emissivity and absorptivity, with some supporting the possibility of different values and others arguing against it based on established physical laws. The discussion remains unresolved with multiple competing perspectives.

Contextual Notes

Participants reference Kirchhoff's law, but there is an acknowledgment that practical applications may involve complexities not fully addressed by this law. The discussion also touches on the implications of energy conservation in relation to the properties of materials.

shivadevrao
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At a particular wavelength (In IR range), & at particular temperature, can anybody or coating has different IR emissivity & IR absorptivity?
 
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yes there can be bodies or surfaces that may have different absorptivity and emissivity, like in flat plate collectors they use selective absorbers which have high absorptivity and low emissivity in order to absorb maximum solar energy. the case about which u r talking is a gray surface having equal emissivity and absorptivity.
 
Thanks for the reply,

I understand that certain coatings have high solar absorptivity & low IR emissivity, my question is are there any surfaces or coatings which have high IR emissivity & low IR absorptivity.
 
u should read about ir bulbs used for curing of powder coating
 
shivadevrao said:
At a particular wavelength (In IR range), & at particular temperature, can anybody or coating has different IR emissivity & IR absorptivity?

engnr_arsalan said:
yes there can be bodies or surfaces that may have different absorptivity and emissivity, like in flat plate collectors they use selective absorbers which have high absorptivity and low emissivity in order to absorb maximum solar energy. the case about which u r talking is a gray surface having equal emissivity and absorptivity.

The above answer does not strictly address the above question. The wavelength specific absorptivity is the same as the emissivity (this is Kirchoff's law of thermal radiation). However, Kirchoff does not address what happens with a wide range of incident and emitted radiation frequencies (i.e. a practical case).
 
shivadevrao said:
At a particular wavelength (In IR range), & at particular temperature, can anybody or coating has different IR emissivity & IR absorptivity?

No, otherwise energy would not be conserved and you could violate the second law of thermodynamics.
 
if the 'absorptivity' were greater than the 'emissivity' the object would warm up spontaneously.
if the 'absorptivity' were less than the'emissivity' the object would cool down spontaneously.
 

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