[HN Gopher] The Joule-Thomson Effect and Models We Know
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The Joule-Thomson Effect and Models We Know
Author : mferraro89
Score : 65 points
Date : 2021-12-17 15:35 UTC (7 hours ago)
(HTM) web link (mattferraro.dev)
(TXT) w3m dump (mattferraro.dev)
| hilbert42 wrote:
| This 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._
| lordnacho wrote:
| But 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.
| Pulcinella wrote:
| One 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.
| selimthegrim wrote:
| If you use fugacity can't you rescale and collapse all gas
| properties into basically universal curves?
|
| https://en.wikipedia.org/wiki/Compressibility_factor
| chermi wrote:
| 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.
| bernulli wrote:
| 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]
| hashimotonomora wrote:
| JT is isoenthalpic not isoentropic. For an ideal gas, h = u +
| pv = CT + RT = f(T). So an isoenthalpic process is
| necessarily isothermic for an ideal gas.
|
| Expansion through a nozzle is extremely chaotic and generates
| entropy. You can't model JT that way.
|
| When you compress air in an air pump you are doing work
| against the system and increasing its internal energy u = q -
| w, which can be explained using ideal gases by knowing that u
| = u(T). But this is not because the pressure increases but
| because of your work.
| chermi wrote:
| The emphasis on the entropy generation kind of confuses the
| point from my POV. What's important about the nozzle setup
| is that, by construction, it generates the JT throttling
| process. That is, the procedure is isenthalpic. Focus on
| the thermodynamic consequences of that.
|
| Sorry for butting in. It took me a long time to get
| comfortable with throttling. Non-equilibrium stat mech
| stuff can really throw you (well, at least me) off if you
| come at it too microscopically at first.
|
| Edit -- H. Callen's thermo book has a great little section
| on it. Best book on thermo out there if you're into a real
| postulate-and-construct approach. One of my favorite books
| of all time. https://en.m.wikipedia.org/wiki/Thermodynamics
| _and_an_Introd...
| bernulli wrote:
| You can very much model expansion in a _nozzle_ as
| isentropic, but I did miss the part where the original
| comment was restricted to JT. Thanks!
| [deleted]
| BlueTemplar wrote:
| But the isentropic part is what is being violated here... (I
| was surprised that the article didn't mention that even
| though it managed to demonstrate the real behaviour in
| another way.)
| Pulcinella wrote:
| I didn't mean that there would be no temperature changes. But
| that in the example where you allow the gas to expand into a
| new volume there would be no temperature change. Remember,
| temperature relates to the speed the gas particles are
| moving. If the barrier between the two sides of the container
| was removed and the gas allowed to expand into that new
| volume, why would the ideal gas particles slow down and
| decrease the temperature?
| [deleted]
| bernulli wrote:
| I did miss the part where the original comment was
| restricted to JT. Thanks!
| mattb314 wrote:
| > Joules-Thomson effect is what allows for refrigeration
|
| I don't think this is true. The "simple" model of refrigeration
| taught in highschool is just a carnot cycle running backwards,
| and this can be modeled with an ideal gas. The author of the
| post covers this the section on "the Thermodynamics 101
| Answer"[1], where all you need to drop the temperature of a gas
| is to let it do work on the piston.
|
| That's not to say that JT is not useful, just that we can
| explain a theoretical refrigerator without it.
|
| [1] https://mattferraro.dev/posts/joule-thomson#the-
| thermodynami...
| pas wrote:
| 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
| Pulcinella wrote:
| 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).
| avmich wrote:
| Good 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.
| wizzwizz4 wrote:
| > _Like, why properties of H, He and Ne are such that they are
| at this part of their diagram at normal conditions?_
|
| This is the wrong question. You should, rather, ask: "under
| what conditions are things at various parts of the diagram?"
| "Normal conditions" are really just happenstance, if you're
| considering physics; stars and the immense void of space are
| both much more common than room temperature, room pressure.
| BlueTemplar wrote:
| - H and He (and Ne ?) are small.
|
| - The energy is stored in the bond ?
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