Can there be different IR emissivity & IR absorptivity

In summary, at a specific wavelength in the IR range and temperature, there can be bodies or surfaces with different IR emissivity and absorptivity. While Kirchoff's law states that these values should be equal, in practical cases there can be variations. However, if the absorptivity is greater than the emissivity, the object will warm up spontaneously, and if the absorptivity is less than the emissivity, the object will cool down spontaneously. This is important to consider in applications such as solar energy collection and curing of powder coating.
  • #1
shivadevrao
7
0
At a particular wavelength (In IR range), & at particular temperature, can anybody or coating has different IR emissivity & IR absorptivity?
 
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  • #2
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.
 
  • #3
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.
 
  • #4
u should read about ir bulbs used for curing of powder coating
 
  • #5
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).
 
  • #6
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.
 
  • #7
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.
 

Related to Can there be different IR emissivity & IR absorptivity

1. What is IR emissivity and IR absorptivity?

IR emissivity and IR absorptivity are both properties that describe how well a material can emit and absorb infrared radiation, respectively. They are measured on a scale from 0 to 1, where a value of 0 represents a material that does not emit or absorb any infrared radiation, and a value of 1 represents a material that emits or absorbs all incoming infrared radiation.

2. Can there be a difference between IR emissivity and IR absorptivity?

Yes, there can be a difference between IR emissivity and IR absorptivity. While both properties measure the interaction of a material with infrared radiation, they measure different aspects of this interaction. IR emissivity describes how well a material can emit infrared radiation, while IR absorptivity describes how well a material can absorb incoming infrared radiation.

3. What factors can affect the IR emissivity and IR absorptivity of a material?

Several factors can affect the IR emissivity and IR absorptivity of a material, including its composition, surface texture, and temperature. Generally, materials that are dark in color and have rough surfaces tend to have higher IR emissivity and IR absorptivity values, while materials that are light in color and have smooth surfaces tend to have lower values.

4. Can different materials have the same IR emissivity and IR absorptivity values?

Yes, it is possible for different materials to have the same IR emissivity and IR absorptivity values. This can occur if the materials have similar compositions and surface properties that affect their interaction with infrared radiation. However, it is more common for materials to have different emissivity and absorptivity values due to variations in their properties.

5. How do IR emissivity and IR absorptivity affect an object's temperature?

IR emissivity and IR absorptivity play a crucial role in determining an object's temperature. Materials with high emissivity and absorptivity values will absorb more infrared radiation and emit more heat, leading to a higher temperature. On the other hand, materials with low emissivity and absorptivity values will reflect more infrared radiation and emit less heat, resulting in a lower temperature.

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