Radiation vs. conduction in thermal equilibrium

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As I understand it, thermodynamics indicates that heat will dissipate from hotter parts of a system to cooler parts. This is why, for example, it is impossible to harness latent heat to create energy if there is no heat-sink to move the heat from relative warmth to cold.

What has me puzzled is whether radiation also somehow responds to thermal differential/equilibrium. With conduction/convection, it is logical that the kinetic energy of particle motion would dissipate insofar as particles collide with other particles and transfer momentum. But why would radiation cease to be emitted because thermal equilibrium between emitter and absorber was reached?

Don't particles keep emitting radiation even when thermal equilibrium within the system is reached? If so, wouldn't it be possible to somehow capture this latent radiation? Admittedly, this sounds impossible to me, but I'm trying to understand exactly why instead of going with my intuition.
 

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  • #2
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Don't particles keep emitting radiation even when thermal equilibrium within the system is reached?
Yes. See thermal radiation, black-body radiation.

If so, wouldn't it be possible to somehow capture this latent radiation?
Not if your capturing device is in thermal equilibrium with the system(is also radiating).
 
  • #3
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Yes. See thermal radiation, black-body radiation.


Not if your capturing device is in thermal equilibrium with the system(is also radiating).

This requirement isnt a prohibitive barrier - it can be overcome
 
  • #4
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Ok, so consider the following scenario:

A fan is placed in a closed room with no windows. The fan is powered by a photovoltaic cell that is capable of running on ambient room radiation (I realize that this would be very little power).

As the fan converts room radiation into mechanical motion, would the temperature of the room decrease? Assume that there is a perfect energy barrier that prevents any new heat entering the room from outside.

Would the fan continue to run until the room reached absolute zero, provided the photovoltaic cell was sensitive to low enough levels of radiation?
 
  • #5
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photovoltaic cell that is capable of running on ambient room radiation
Such a cell is impossible. It would be able to run on its own thermal radiation and cool itself to absolute zero.
 
  • #6
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Such a cell is impossible. It would be able to run on its own thermal radiation and cool itself to absolute zero.

Good point. Interesting. So could one say that for radiation to be dissipated from a system, their has to be disequilibrium between the source-level of radiation and that of the receptor cell? In other words, there is such a thing as radiation entropy?
 
  • #7
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Dissipation can only occur if the system is in a disequilibrium. It will move the system towards equilibrium, increase entropy. In order for work to be performed/extracted there has to be some kind of disequilibrium(gradient).
 
  • #8
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Dissipation can only occur if the system is in a disequilibrium. It will move the system towards equilibrium, increase entropy. In order for work to be performed/extracted there has to be some kind of disequilibrium(gradient).

So what drives an endothermic reaction? Shouldn't there be some way to produce an "endothermic machine" that absorbs radiation by dissipating energy from a photovoltaic cell that is then primed to receive new ambient radiation to replace it? I know this was already explained, but I don't see why the device couldn't create the radiation disequilibrium required to draw energy out of the photocell, if it drew ambient radiation from the cell itself leaving the particles in the cell in disequilibrium to receive new photons from outside the circuit. Isn't there such a thing as a one-way valve for electric current?
 
  • #9
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Exothermic and endothermic reactions are methods to create a disequilibrium. But they are not perpetual. You are sharing heat for chemical bond energy. You'll still increase the entropy of the system.
 
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  • #10
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Exothermic and endothermic reactions are methods to create a disequilibrium. But they are not perpetual. You are sharing heat for chemical bond energy. You'll still increase the entropy of the system.

The reason I mentioned endothermic reactions was that these reaction somehow seem to be able to draw ambient heat out of a system otherwise in equilibrium. I was positing that if chemical potential is somehow able to draw energy out of a system in equilibrium, so could a mechanical system of some sort. But are you saying that the ability to draw heat out of a system is the result of potential energy stored in the chemical structure of the ingredients of the reaction? Is an endothermic reaction somehow actually expending more energy than it is drawing out of its environment?
 
  • #11
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No. The endothermic reaction will draw thermal energy(heat) from the system to increase potential energy(chemical bonds). But there has to be an activation energy well for the endothermic reaction to happen. The difference between exothermic and endothermic reactions is that the latter will release less thermal energy than the activation energy of the reaction.
 
  • #12
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No. The endothermic reaction will draw thermal energy(heat) from the system to increase potential energy(chemical bonds). But there has to be an activation energy well for the endothermic reaction to happen. The difference between exothermic and endothermic reactions is that the latter will release less thermal energy than the activation energy of the reaction.
Ok, so then why wouldn't it be possible to have a mechanical system that works similarly; i.e. using a small activation energy to setting an endothermic or radiation-consuming circuit into motion that draws energy out of a system in equilibrium and converts it into mechanical motion or something else?
 
  • #13
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Once again, reactions are not perpetual. They will stop after reaching a new equilibrium. It doesn't matter if they are exo or endothermic.

Edit: With equilibrium here I mean chemical(reaction) equilibrium not the thermal one.
 
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  • #14
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Once again, reactions are not perpetual. They will stop after reaching a new equilibrium. It doesn't matter if they are exo or endothermic.

Edit: With equilibrium here I mean chemical(reaction) equilibrium not the thermal one.

Ok, nothing is perpetual. Now I'm trying to figure out what the details of the process are and what the limiting factors are and how they work.
 

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