# Thermal radiation, emissivity and Kirchhoff

Hi Guys
Studying thermal radiation here and im 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)

## Answers and Replies

Bystander
Science Advisor
Homework Helper
Gold Member
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.

Carlos de Meo
?

f95toli
Science Advisor
Gold Member
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.

Carlos de Meo
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?