According to Kirchhoff, I wonder about heat storage in general. When everything that absorbs radiation well, emits it also well, so what is the point of using certain materials for specific purposes then? I know this is quite vague but I lack applied examples at the moment. What I'm trying to express is: For example a lake. The sun is heating it up and it is definitely great at storing this heat. But does Kirchhoff mean that a single atom(?) would emit the received energy immediately/soon and in a lake we don't experience this because there are so many atoms picking up this emitted energy from another atom?
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1$\begingroup$ You can't store heat. Heat is a way of transferring energy, beside work of a force. Then, you can think at an energy storage, of energy coming from heat transfer or from radiation, if I need to translate your question. $\endgroup$– basicsNov 10, 2022 at 8:38
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2$\begingroup$ @basics it depends on your definition of the word. Some quite serious physicists starting with Black and Carnot called heat or chaleur the $TS$ part of $U$, a part we now usually use unnamed but not uniformly as some recent publications especially from Germany try to do, and there is nothing wrong with that usage if you know what it means. See for example the recent pedagogic efforts of Herrmann and Pohling doi.org/10.3390/e23081078 and the many references therein., especially [9] from La Mer and many others I can give you not listed if interested. $\endgroup$– hyportnexNov 10, 2022 at 12:41
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$\begingroup$ to be honest, that publication looks like a not required and undesired over- or wrong- definition-based publication. I just hope those people are not teaching that way to students $\endgroup$– basicsNov 10, 2022 at 12:54
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$\begingroup$ @basics yeah, indeed and now we can all understand why KIT barely ranks anywhere in Germany, see en.wikipedia.org/wiki/Karlsruhe_Institute_of_Technology especially in physics. $\endgroup$– hyportnexNov 10, 2022 at 13:06
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$\begingroup$ I really don't trust these rankings 100%, but in my academic experience in Engineering field, I was used to work quite well (or know they work well), with TUM (Munich), Delft, and KTH (Stockholm), Cambridge and Imperial College, from Politecnico di Milano. I know someone at the Politecnico is working with KTU on fundamentals of turbulence in fluid dynamics, but I'd better say nothing about it... $\endgroup$– basicsNov 10, 2022 at 13:18
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2 Answers
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Firstly, Kirchhoff's law applies to bodies that are in thermodynamic equilibrium with their surroundings. Heat storage devices are, almost by definition, not in equilibrium with their surroundings - either they are accumulating heat or they are emitting heat.
Secondly, Kirchhoff's law is a macroscopic law that describes the bulk properties of bodies that are collections of (at least) millions of atoms. You cannot apply it to the behaviour of a single atom.
answered Nov 10, 2022 at 8:26
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$\begingroup$ where could we make use out of this law then? I mean which macroscopic object is in thermodynamic equilibrium? $\endgroup$– BenNov 10, 2022 at 8:30
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Kirchoff's law is about rate of radiation transfer at thermal equilibirium. So one can make big bodies to store more energy or lose less energy, because large body has small surface area. This is applied in where there is passive or active cooling is needed. The shape and design of air cooled engine's casing has thin metallic strips like structure to increase surface area, thus increasing rate of transfer of heat from engine. This same design used in motherboards where fan is not available.
In animals, an elephant require more cooling because its volume is much large. Stefan-boltzmann radiation law is based on this and Planck's law got inspiration from it. The definition of poverty line once deduced from Stefan-Boltzmann law by monetary institutes.
