Thermal radiation, emissivity and Kirchhoff

In summary, Kirchhoff's law states that at thermodynamical equilibrium, the amount of energy absorbed must be reemited. However, before the system reaches equilibrium, there is no model to predict how much energy of these photons absorbed will be used to increase temperature and how much will be reemited.
  • #1
Carlos de Meo
23
2
Hi Guys
Studying thermal radiation here and I am kinda stuck at one point
Kirchhoff law states that at thermodynamical equilibrium, the amount of energy absorbed must be reemited.
But before the system reaches equilibrium, is there a model to predict how much energy of these photons absorbed will be used to increase temperature and how much will be reemited?
So far couldn´t find anything (Trying to find it on some non equilibrium thermodynamics literature)
 
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  • #2
Carlos de Meo said:
But before the system reaches equilibrium, is there a model to predict how much energy of these photons absorbed will be used to increase temperature and how much will be reemited?
Just ponder that for a bit.
 
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  • #3
?
 
  • #4
No, not really.
I don't think you've been specific enough to actually define the problem, but I I think I can guess what you have in mind.

The underlying physics of this problem is not very hard (if I understood the question correctly it would be a heat equation combined with Stefan-Boltzmann's law) , but you will end up with a partial differential equation (PDE) and the solution will depend on the shape and properties of the object.
Hence, there is no generic formula that can predict e.g. the temperature as a function of temperature.

Problems like these are ideally solved numerically using software like COMSOL.
 
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  • #5
Sorry for not being specific enough. I wanted to say a problem like this. Imagine i have a solid body and i know that there is a vibrational mode that responds to a specific photon frequency (let´s say this photon is in the Infra red range). Now assuming that all the radiation described is absorbed (let´s forget scattering at the moment). So when this solid reaches thermodynamical equilibrium, kirchhoff law stats that the amount absorbed will be reemited. But before reaching equilibrium, would this software (sorry about my ignorance, never heard about it before), give me the answer by just combining Stefan Boltzmann´s law and heat equation?
 

1. What is thermal radiation?

Thermal radiation is a form of electromagnetic radiation that is emitted by all objects with a temperature above absolute zero. It is a result of the movement of charged particles within the object.

2. What is emissivity?

Emissivity is a measure of how efficiently an object emits thermal radiation compared to a perfect blackbody at the same temperature. It is represented by a value between 0 and 1, with a higher value indicating a greater ability to emit radiation.

3. How does emissivity affect thermal radiation?

Emissivity plays a crucial role in the amount of thermal radiation emitted by an object. Objects with high emissivity will emit more thermal radiation, while those with low emissivity will emit less. This is because the higher the emissivity, the more efficient an object is at emitting thermal radiation.

4. What is the relationship between emissivity and temperature?

The relationship between emissivity and temperature is a direct one. As the temperature of an object increases, its emissivity also increases. This means that at higher temperatures, objects will emit more thermal radiation, regardless of their emissivity value.

5. How does Kirchhoff's law relate to thermal radiation and emissivity?

Kirchhoff's law states that the emissivity of an object is equal to its absorptivity at a specific temperature and wavelength. In other words, an object that is a good emitter of thermal radiation is also a good absorber. This law is essential in understanding the behavior of thermal radiation and the role of emissivity in it.

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