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.
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.
> 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.
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.
> 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.
Yes, you can explain refrigeration with ideal gas. But...
You need non-ideal gas (attraction) to get temperature inversion. Then you just need compressor and voila, look at PT charts to find what temperature range you need. With ideal gas reverse carnot refrigerator your refrigeration effect is bounded on low temperature side by the available low temperature source.
So yes, you can refrigerate with ideal gas, but it's not very helpful in warm areas or if you need to get something super cold.
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.
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.
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...
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?
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.)
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).
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.
What sounds like nonsense? The fact that atoms are not actually infinitely solid billiard balls that just bounce off each other instantaneously should not be controversial. I think what's given is actually a pretty good layman's explanation of why a certain amount of the total internal energy of a gas is stored as potential rather than kinetic energy.
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.