Does Kirchhoff's law mean absorptivity equals emissivity or rates equal?

[carter7gindenv]

Hello, I'd like some clarification with the exact wording of Kirchhoff's law of thermal radiation.
The law states that at thermal equilibrium α_λ=ε_λ.
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!

 

[Dr_Nate]

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.

 

[carter7gindenv]

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!

 

[Dr_Nate]

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.

 

[carter7gindenv]

This is something that has always been a bit badly explained to me but you broke it down very nicely thank you!

 

[DrClaude]

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.

 

[Dr_Nate]

You are confounding intrinsic/extrinsic with intensive/extensive.

Yes. Yes, I did. Thanks for the correction!