Clarification about Kirchhoff's law of thermal radiation

In summary, Kirchhoff's law of thermal radiation states that at thermal equilibrium, the ratio of absorptivity to emissivity is equal. However, this can be confusing when considering absorption and emission rates. Absorptivity and emissivity are intrinsic properties, while absorption and emission rates are extrinsic. Intrinsic properties are those that belong to the object itself and do not change with the size or temperature of the object, while extrinsic properties do change with these factors. It is important to understand the difference between intrinsic and extrinsic quantities when studying thermal radiation.
  • #1
carter7gindenv
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Hello, I'd like some clarification with the exact wording of Kirchhoff's law of thermal radiation.
The law states that at thermal equilibrium αλ=ελ.
However sometimes I read that absorptivity= emissivity and sometimes I read that rate of A. = rate of E.
Which one is correct? I'm not sure but I get an itch in the back of my head that it means the same thing.

Thanks for the help!
 
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  • #2
Basically you are correct, but you are using four terms, not two: absorptivity, emissivity, absorption (A), emission (E). Absoptivity and emissivity are intrinsic quantities. The other two are extrinsic.
 
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  • #3
That was what I feared. I feel like online documentation often confuses those terms. And I believe it is also the source of my confusion in my other thread here.

I will try to find more information about those 4 terms thank you very much!
 
  • #4
Do you know the difference between an intrinsic and extrinsic quantity? If I have some object, like a can of soda, it has some mass and temperature. If I grab a second can of soda, I now have twice as much mass, but not twice as temperature. The temperature doesn't change with the amount of the substance, so we call this an intrinsic quantity. Mass is an extrinsic quantity.
 
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This is something that has always been a bit badly explained to me but you broke it down very nicely thank you!
 
  • #6
Dr_Nate said:
Do you know the difference between an intrinsic and extrinsic quantity? If I have some object, like a can of soda, it has some mass and temperature. If I grab a second can of soda, I now have twice as much mass, but not twice as temperature. The temperature doesn't change with the amount of the substance, so we call this an intrinsic quantity. Mass is an extrinsic quantity.
You are confounding intrinsic/extrinsic with intensive/extensive.

Properties that change with the size of the system are extensive; the that don't are intensive.

Intrinsic properties are those that belong to the object itself. For example, emissivity is intrinsic because it is a property of the object itself. It doesn't change with the size of the object, so it is also intensive, but it doesn't change with its temperature either, i.e., it doesn't depend on external factors.
 
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  • #7
DrClaude said:
You are confounding intrinsic/extrinsic with intensive/extensive.
Yes. Yes, I did. Thanks for the correction!
 

1. What is Kirchhoff's law of thermal radiation?

Kirchhoff's law of thermal radiation states that the emissivity of a material is equal to its absorptivity at a given wavelength and temperature. In other words, materials that are good at absorbing radiation at a certain wavelength are also good at emitting radiation at that same wavelength.

2. How does Kirchhoff's law apply to thermal radiation?

Kirchhoff's law applies to thermal radiation by stating that the ratio of the emissivity to the absorptivity of a material is equal to the ratio of its emissive power to its absorptive power at a given wavelength and temperature. This means that the emissivity and absorptivity of a material are directly related to its ability to emit and absorb thermal radiation.

3. Does Kirchhoff's law only apply to specific wavelengths?

No, Kirchhoff's law applies to all wavelengths of thermal radiation. However, it is most commonly used in the infrared region, as this is where most thermal radiation occurs.

4. How is Kirchhoff's law related to the Stefan-Boltzmann law?

Kirchhoff's law is related to the Stefan-Boltzmann law, which states that the total emissive power of a blackbody is proportional to the fourth power of its absolute temperature. This means that a material with a high emissivity (according to Kirchhoff's law) will also have a high emissive power (according to the Stefan-Boltzmann law).

5. Can Kirchhoff's law be applied to real-world materials?

Yes, Kirchhoff's law can be applied to real-world materials. However, it is important to note that the emissivity and absorptivity of a material may vary depending on factors such as surface roughness, temperature, and wavelength. Therefore, it is often necessary to use average values or specific measurements for accurate calculations.

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