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.
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.
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
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.
hilbert42|4 years ago
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.
avmich|4 years ago
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|4 years ago
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|4 years ago
- The energy is stored in the bond ?
Pulcinella|4 years ago
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.
mattb314|4 years ago
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...
bernulli|4 years ago
https://en.wikipedia.org/wiki/Isentropic_process
[edit: typo]
selimthegrim|4 years ago
https://en.wikipedia.org/wiki/Compressibility_factor
pas|4 years ago
lordnacho|4 years ago
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.
onos|4 years ago
pdonis|4 years ago