The Joule-Thomson Effect and Models We Know
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The Joule-Thomson Effect and Models We Know
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Re: The Joule-Thomson Effect and Models We Know
#2One of my favorite things about the JT effect is how gases not being “perfect” is actually “better” than if they were.
What I mean is, if gases all behaved as some kind of perfect, platonic* ideal of a gas and followed the ideal game law exactly, there would be no temperature change. But because they don’t, the Joules-Thomson effect is what allows for refrigeration.
*Helium is probably the closest to some platonic ideal of a gas.
Re: The Joule-Thomson Effect and Models We Know
#3One of my favorite things about the JT effect is how gases not being “perfect” is actually “better” than if they were. What I mean is, if gases all behaved as some kind of perfect, platonic* ideal of a gas and followed the ideal game law exactly, there would be no temperature change. But because they don’t, the Joules-Thomson effect is what allows for refrigeration. *Helium is probably the closest to some platonic id…
Could you explain this a bit more? If every gas blob in every situation, system, vessel, pipe were to behave according to the ideal gas law a heat pump that cools down the inside of a box and radiates the heat away would be impossible? :o
Re: The Joule-Thomson Effect and Models We Know
#4But of course the clue is in the name: Ideal gas law.
All models are wrong, but some models are useful. What students of science are really practicing is the application of just enough additional constraints on a model to explain some observation.
Re: The Joule-Thomson Effect and Models We Know
#5Good article.
Certain tempting things left unexplained though :) . Like, why properties of H, He and Ne are such that they are at this part of their diagram at normal conditions? Or where dispersion bonding stores the kinetic energy - when two atoms bond this way, both momentum and energy should be preserved, so some places to put excess of energy should be present, otherwise the pair should be unstable. Like, requiring another external collision to dissipate that energy.
High school physics also teaches adiabatic processes, which can suggest a temperature change. But those are all minor comments to the idea that there are models, and they are imprecise, but still could be useful.
Re: The Joule-Thomson Effect and Models We Know
#6One of my favorite things about the JT effect is how gases not being “perfect” is actually “better” than if they were. What I mean is, if gases all behaved as some kind of perfect, platonic* ideal of a gas and followed the ideal game law exactly, there would be no temperature change. But because they don’t, the Joules-Thomson effect is what allows for refrigeration. *Helium is probably the closest to some platonic id…
If you use fugacity can’t you rescale and collapse all gas properties into basically universal curves?
Re: The Joule-Thomson Effect and Models We Know
#7This article ought to appear as a wallchart on all highschool physics labs!
No mind the fact that the more complicated bits might not be taught at school - they are there to show students that with progress comes a better understanding.
Highschool students now learn Newtonian physics without Relativity - or Lagrangian, Hamiltonian mechanics but they know that a better understanding of mechanics requires a more in depth approach if they're to get the full picture. With mechanics, they'll pick up that fact from popular culture alone.
That learning thermodynamics is absolutely crucial to having a proper understanding of physics is not so well understood - nor in my experience was the fact taught with the necessary conviction when I was learning physics - much to my later chagrin.
Initially, I found thermodynamics somewhat boring and it came as a shock when it eventually dawned on me that it's at the very central heart of physics - and very interesting at that. For years, I've thought that one of the main problems is the somewhat lack of direction many textbooks take to teaching the subject. Why that's so is too big to cover here except to say the article demonstrates the reason - as Einstein said, 'make everything as simple as possible but not simpler'. If not taught carefully, thermodynamics suffers the problem of getting early concepts across in preconceived ways that are at the risk of having to be 'unlearned' later.
Re: The Joule-Thomson Effect and Models We Know
#8One of my favorite things about the JT effect is how gases not being “perfect” is actually “better” than if they were. What I mean is, if gases all behaved as some kind of perfect, platonic* ideal of a gas and followed the ideal game law exactly, there would be no temperature change. But because they don’t, the Joules-Thomson effect is what allows for refrigeration. *Helium is probably the closest to some platonic id…
Could you explain this a bit more? If every gas blob in every situation, system, vessel, pipe were to behave according to the ideal gas law a heat pump that cools down the inside of a box and radiates the heat away would be impossible? :o
Yes. One of the assumptions of the Ideal Gas Law is that particles don’t interact with each other or even collide with each other. They only collide with the walls of the container.
So if the ideal gas law was true then a heat pump wouldn’t be able to refrigerate anything. When the gas would expand, the volume would go up, pressure would go down, and temperature would remain the same because there wouldn’t be any reason for the gas particles to slow down (because under the ideal gas law the particles don’t interact with eachother).
Re: The Joule-Thomson Effect and Models We Know
#9One of my favorite things about the JT effect is how gases not being “perfect” is actually “better” than if they were. What I mean is, if gases all behaved as some kind of perfect, platonic* ideal of a gas and followed the ideal game law exactly, there would be no temperature change. But because they don’t, the Joules-Thomson effect is what allows for refrigeration. *Helium is probably the closest to some platonic id…
Why wouldn't there be any temperature change in ideal gases? When I compress an ideal gas in a bike pump, it heats up; when I expand it through a nozzle, it cools down. Often I can approximate these processes as isentropic, and then the temperatures are uniquely determined by the expansion as either a function of pressure ratio T2 = T1 (p2/p1)^(g-1/g) or volume ratio; T2 = T1 (V2/V1)^g, where g is gamma, the ratio of specific heats.
https://en.wikipedia.org/wiki/Isentropic_process
[edit: typo]
Re: The Joule-Thomson Effect and Models We Know
#10One of my favorite things about the JT effect is how gases not being “perfect” is actually “better” than if they were. What I mean is, if gases all behaved as some kind of perfect, platonic* ideal of a gas and followed the ideal game law exactly, there would be no temperature change. But because they don’t, the Joules-Thomson effect is what allows for refrigeration. *Helium is probably the closest to some platonic id…
If you use fugacity can’t you rescale and collapse all gas properties into basically universal curves? https://en.wikipedia.org/wiki/Compressibility_factor
Yeah, one of the kind of early hints of universality https://en.m.wikipedia.org/wiki/Theorem_of_corresponding_sta.... Weird, I've never seen it called the "theorem" of corresponding states, always the "law". You get it straight from VDW EoS
Edit-- I guess there's multiple laws of corresponding state, duh. My last sentence above should say you can a law of corresponding states from VDW.