Thermal radiation, emissivity and Kirchhoff

Click For Summary

Discussion Overview

The discussion centers on thermal radiation, specifically the implications of Kirchhoff's law in non-equilibrium thermodynamics. Participants explore how to model the energy absorption and emission of photons by a solid body before reaching thermodynamic equilibrium, considering the relationship between absorbed energy, temperature increase, and re-emission of energy.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions whether a model exists to predict the distribution of absorbed energy between temperature increase and re-emission before equilibrium is reached.
  • Another participant suggests that the problem involves a heat equation combined with Stefan-Boltzmann's law, leading to a partial differential equation (PDE) whose solution depends on the object's shape and properties.
  • A later reply indicates that there is no generic formula for predicting temperature as a function of absorbed energy, emphasizing the need for numerical solutions using software like COMSOL.
  • The original poster clarifies their scenario involving a solid body with a specific vibrational mode responding to a photon frequency, questioning if the software can provide answers by combining Stefan-Boltzmann's law and the heat equation.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the existence of a model for predicting energy distribution before equilibrium. There are competing views on the complexity of the problem and the applicability of numerical methods.

Contextual Notes

Participants note the dependence on the shape and properties of the object, as well as the potential limitations of existing models in predicting outcomes in non-equilibrium conditions.

Carlos de Meo
Messages
22
Reaction score
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)
 
Science news on Phys.org
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.
 
  • Like
Likes   Reactions: Carlos de Meo
?
 
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.
 
  • Like
Likes   Reactions: 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?
 

Similar threads

Undergrad Clarification about Kirchhoff's law of thermal radiation
  • · Replies 6 ·
Replies
6
Views
2K
Undergrad How do emissivity and absorptivity affect thermal imaging results?"
  • · Replies 3 ·
Replies
3
Views
4K
Graduate Average temperature in a greenhouse
  • · Replies 2 ·
Replies
2
Views
2K
High School Cooling effect of evaporation occurs even if the surroundings are colder?
  • · Replies 10 ·
Replies
10
Views
3K
Undergrad Paradox: Thermodynamic equilibrium does not exist in gravitational fields
  • · Replies 135 ·
5
Replies
135
Views
9K
Undergrad Contradiction with Kirchhoff's Radiation Law?
  • · Replies 4 ·
Replies
4
Views
2K
Undergrad Why is the emissivity of metals low?
  • · Replies 17 ·
Replies
17
Views
4K
Graduate Kirchhoff law for absorption and emission of a gas
  • · Replies 2 ·
Replies
2
Views
2K
Graduate Thermal radiation in transparent objects
  • · Replies 5 ·
Replies
5
Views
3K
Undergrad Questions about Jean-Rayleigh's derivation of Ultraviolet Catastrophe
  • · Replies 1 ·
Replies
1
Views
2K