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Tom,
I really don't think it is nit-picking to say that gasses NEVER contract on their own, or that the outward pressure of any gas (by definition) is greater than any inter-molecular forces, or gravitational forces, or dark energy.
Besides, surely the whole point of this discussion is to try and pick holes in established, over-simplified ideas?

MikeB wrote: ↑Fri Aug 18, 2023 5:49 am Tom,
I really don't think it is nit-picking to say that gasses NEVER contract on their own, or that the outward pressure of any gas (by definition) is greater than any inter-molecular forces, or gravitational forces, or dark energy.
Besides, surely the whole point of this discussion is to try and pick holes in established, over-simplified ideas?

That is not what I was referring to. I was referring to this criticism of the use of the word "vacuum":

Tom,
There is no such thing as a vacuum - the word is a useful concept, but in reality there is no such thing as a negative gas pressure.

The term "vacuum" is very very commonly used to describe a zone of pressure below atmospheric pressure. A "vacuum cleaner" for example. Nobody needs a lecture every time they happen to use the word vacuum to describe.. a vacuum.

Your assertion that "gasses NEVER contract on their own" is also just wrong.

Your assertion: "the outward pressure of any gas (by definition) is greater than any inter-molecular forces" applies to an "Ideal Gas".

The only problem is, like a "reversible engine" there is no such thing as an "IDEAL GAS".in reality

REAL gases have extremely strong and powerful intermolecular and intramolecular forces of attraction and repulsion, they are simply not apparent under most ordinary circumstances because, as we find them in nature, they are, for the most part in balance, but as soon as we start manipulating the temperature and pressure that balance is changed.

Your "by definition" is the definition of an "Ideal gas".. That definition does not apply to real, actual, physical material gases like the air we breath or the air or gas inside a Stirling engine.

If you can forget about the "Ideal gas Law" for a minute and just use some common sense and straightforward logic, what is the actual sequence of events:


https://youtu.be/pYoKaLR0P8M


Was the "gas" inside the metal drum squeezed down by atmospheric pressure causing the "gas" to condense into a liquid?

Or when cooled, did the "gas" contract and condense into a liquid "on its own". FIRST and only then atmospheric pressure crushed the can?

We know that all gases will condense at a low enough temperature, but they most certainly begin to contract when cooled even slightly.

I've actually become quite skeptical of the "kinetic theory" generally.

I've always wondered why, if gas molecules are REALLY zipping around at nearly the speed of sound or whatever it is supposed to be, why are we not physically torn to shreads by all these "bullet" like gas molecules constantly striking our skin and sensitive tissue, like our eyes?

Why, if a container full of CO2 is taken out of the freezer, this gas can be seen, under thermal imagery, to act like a liquid and can be poured from one container into another?

Infact, ordinary cold air behaves in the same way, (like a liquid), pooling on the floor as it spills down when we open our refrigerator doors. In a chest freezer the cold air will stay obediently inside the freezer when the lid is raised, it does not expand or dissipate at the speed of sound in all directions.

If you need written confirmation of these simple observable facts, then read up thoroughly on "Real gas" as opposed to "Ideal gas" and then you will find that gases actually do "contract on their own" ALL THE TIME.

When gases are cooled inside a container they are drawn closer together, and it is this drawing together that reduces the pressure on the walls of the container.

That is how a Stirling engine operates, by manipulating the temperature of the gas, causing it to expand and contract inside a rigid container.

Tom Booth wrote: ↑Fri Aug 18, 2023 4:47 pm
We know that all gases will condense at a low enough temperature, but they most certainly begin to contract when cooled even slightly.

I've actually become quite skeptical of the "kinetic theory" generally.

You've likely heard of solids, liquids, and gasses. Well, after a liquid vaporizes, the gas continues to experience changes. To keep this simple, consider water which vaporizes at 212F STP and then enters a two-phase region with unique properties since it's close (or relatively so) to a liquid state. However, once this gas (steam) gets to its "critical point/s" it behaves very similar to an ideal gas. As I recall, the critical points for steam are 704F and 3206psi after which steam is "supercritical". Most common gasses are supercritical and behave very similar ideal gasses where kinetic energy is the dominate force. Any substantial attractive and repulsive forces are down in the two phase region approaching the liquid state, not in the supercritical region (or under unique, usually man made conditions such as plasma, etc).

Your cafeteria approach to thermo is abysmal, this isn't a subject that suits online learning.

Tom Booth wrote: ↑Fri Aug 18, 2023 4:47 pm
When gases are cooled inside a container they are drawn closer together, and it is this drawing together that reduces the pressure on the walls of the container.

Where do you get this stuff (the unicorns ???). If I take a fixed gas volume at 600k and cool it to 300k, the gas molecules move slower and we can measure this as less pressure, but they haven't "drawn closer together" !!!

Matt, I'm afraid in ridiculing and poking fun at me you are just revealing your own ignorance regarding REAL gas behavior.

You need to stop relying on your high school physics textbook or wherever you get your information from and do some actual research.

I'm not going to waste my time spoon feeding you like a spoiled baby that just keeps spitting out the food.

Whoa! Guys, those are some pretty harsh words. Tom, you've ask for proof that your theory is wrong. We're attempting to do so. It's understandable that stubbornness on both sides leads to frustration.

You've asked me to do a number of searches. Unfortunately for me this is frustratingly getting pretty close to a god of the gaps scenario. I've done some searches, and have found many misleading uses of the term "gas contraction", and "gasses are not ideal", I have not found any real gas attraction, or contraction. Sure, there are gaps. So, you will need to provide direct evidence of negative pressure or a web link showing such. I can't do an exhaustive search to locate something logically deniable.

Real contraction can be observed in the following experiment.

Connect a large strong long steel rod to the inside top of a long heavy duty cylinder. Connect the other end to the inside top of a heavy duty piston. Let the piston hang. Heat the rod and the piston will drop as the rod expands. Cool the rod and it will contract pulling the piston up.

Outside forces and pressure changes will have little effect on the piston height or cylinder volume, only the length of the steel rod. Inside pressure changes will also have little effect. The strength of the rod overwhelms the gas pressure changes rendering them negligible. The rod applies a real tensile force, not just an outside pushing in force as experienced in an implosion.

Would hot steam gas vapor inside a drum, cooling and condensing in outer space, produce any crushing force? No the pressure would always be positive. Zero inward contraction force. Positive outward vapor pressure, even if all the liquid freezes.

I'm just trying to explain it better.

If a gas is expanded to the point of cooling below liquidity, it won't, because as the pressure drops the temperature of that point drops to zero K. It is that the vapor pressure also drops always a gas vapor, molecules getting further apart.

A cloud in a cloud bottle doesn't liquify, and, stays as a cloud/vapor myst. Even when condensed onto colder air molecules. The myst is seen slowly escaping from the bottle.

How can a gas liquefy if the molecules are so far apart that the attractive forces can't see each other. And if any did, the rest would expand to be even further apart.

That is also observed by the point that cooling a gas by heat rejection to the point of liquid, doesn't have a runaway point where adiabatic temperature drop suddenly liquifies all the gas.

Adiabatic expansion won't liquify all the gas. It only provides a temperature drop. As pressure drops, from condensation, the remaining gas sees a lower pressure requiring an even lower temperature. A condition of diminishing returns, as it closes in on zero pressure, never ever getting there. Both Pressure and Low Temperature are needed to liquify a gas.

This has all been measured and is shown by the phase diagram for nitrogen.

Fool wrote: ↑Sat Aug 19, 2023 9:01 am Whoa! Guys, those are some pretty harsh words. Tom, you've ask for proof that your theory is wrong. We're attempting to do so. ...

I'm all for constructive criticism, presenting different viewpoints etc. I've done my best to be polite and patient in the face of repeated put downs and insults. I post links and videos and use illustrations to try to get a point across nice and friendly like and it seems it gets nowhere, just more put downs and insults.

If someone thinks an "ideal gas" is the be all and end all of gas behavior it's pretty obvious they haven't gotten past high school physics.

Fool wrote: ↑Sat Aug 19, 2023 9:01 am ...
. I've done some searches, and have found many misleading uses of the term "gas contraction", and "gasses are not ideal", I have not found any real gas attraction, or contraction. Sure, there are gaps. So, you will need to provide direct evidence of negative pressure or a web link showing such. I can't do an exhaustive search to locate something logically deniable.

Real contraction can be observed in the following experiment.

Connect a large strong long steel rod to the inside top of a long heavy duty cylinder. Connect the other end to the inside top of a heavy duty piston. Let the piston hang. Heat the rod and the piston will drop as the rod expands. Cool the rod and it will contract pulling the piston up.

Outside forces and pressure changes will have little effect on the piston height or cylinder volume, only the length of the steel rod. Inside pressure changes will also have little effect. The strength of the rod overwhelms the gas pressure changes rendering them negligible. The rod applies a real tensile force, not just an outside pushing in force as experienced in an implosion.

Would hot steam gas vapor inside a drum, cooling and condensing in outer space, produce any crushing force? No the pressure would always be positive. Zero inward contraction force. Positive outward vapor pressure, even if all the liquid freezes.

I'm just trying to explain it better.

If a gas is expanded to the point of cooling below liquidity, it won't, because as the pressure drops the temperature of that point drops to zero K. It is that the vapor pressure also drops always a gas vapor, molecules getting further apart.

A cloud in a cloud bottle doesn't liquify, and, stays as a cloud/vapor myst. Even when condensed onto colder air molecules. The myst is seen slowly escaping from the bottle.

How can a gas liquefy if the molecules are so far apart that the attractive forces can't see each other. And if any did, the rest would expand to be even further apart.

That is also observed by the point that cooling a gas by heat rejection to the point of liquid, doesn't have a runaway point where adiabatic temperature drop suddenly liquifies all the gas.

Adiabatic expansion won't liquify all the gas. It only provides a temperature drop. As pressure drops, from condensation, the remaining gas sees a lower pressure requiring an even lower temperature. A condition of diminishing returns, as it closes in on zero pressure, never ever getting there. Both Pressure and Low Temperature are needed to liquify a gas.

This has all been measured and is shown by the phase diagram for nitrogen.

ST_diagram_of_N2_01.jpg

Most of this is just you spouting your own opinions, which are mostly wrong, without even a single actual reference.

You say: whatever you read is "misleading", with no reference, no evidence, just your opinion.

Whatever else I might reference you will likely just repeat and reaffirm your own opinion, endlessly so what's the point? Believe whatever you want.

But just for example: you say:

If a gas is expanded to the point of cooling below liquidity, it won't

That's is false. You have no basis for saying so. It's your uninformed opinion, and it is wrong.

Gases are liquified in just that way on an industrial scale all day long every day all over the world, but the processes are often proprietary, custom built turbines designed for specific gases etc. and the companies using such methods are not all that inclined to share information.

Here is one video involving steam. The same basic principle is used in liquifying gases, but at higher pressure than steam, so more Work is extracted than with steam so often the gas condenses before leaving the turbine and the gas is cooled before the turbine so after-cooling may not even be necessary.

Regardless, you have in this steam condenser the continual formation of a vacuum right at the turbine exhaust. I think he said 1 to 2 psi (absolute). About 14 psi below atmospheric pressure.


https://youtu.be/9jomP35DGL4

Many gases are liquified by expansion through a turbine in much the same way.

Fool wrote: ↑Sat Aug 19, 2023 11:54 am https://www.chem1.com/acad/webtext/gas/gas_6.html

Is that your example of "many misleading uses of the term "gas contraction""?

Or what exactly.

From the linked article..

Attractive forces: At very close distances, all molecules repel each other as their electron clouds come into contact. At greater distances, however, brief statistical fluctuations in the distribution these electron clouds give rise to a universal attractive force between all molecules. The more electrons in the molecule (and thus the greater the molecular weight), the greater is this attractive force. As long as the energy of thermal motion dominates this attractive force, the substance remains in the gaseous state, but at sufficiently low temperatures the attractions dominate and the substance condenses to a liquid or solid.

I've underlined the significant part.

Suppose you have a very long cylinder sealed at one end.

Insert into the cylinder a plunger with a valve. Push the plunger in with the valve open. Once half way or so into the cylinder close the valve.

(Or the valve could also be on the closed end of the cylinder.)

In theory, there is 15 pounds per square inch atmospheric pressure and the gas molecules inside will expand "forever' so it should never take more than 15 pounds of force to pull the plunger all the way out regardless of its length if the surface area of the plunger is equivalent to 1 inch.

Intuitively I think that the plunger would become extremely hard to pull at some point.

The cylinder started out half full of air so it should never be a complete vacuum inside.

Suppose we hang the cylinder from a rooftop and hang a 20 lb weight on the plunger, will the plunger pull all the way out with a 20 lb weight, regardless of the length?

What if we cool the cylinder with the 20 lb weight hanging on the plunger. Will the plunger be drawn in by the "contracting gas" with ANY cooling, even though atmospheric pressure is more than compensated for by the 20 lb weight?

If the plunger were pushed ALL the way in at the start so there is no air at all in the cylinder, would it be easier or more difficult to pull out the plunger? Would the "attractive forces" of the air molecules trying to hold together be more difficult to draw apart than a total vacuum?

I don't have definite answers to any of these questions, but maybe some such experiments could be used to measure the attractive force, if any, at various temperatures.

VincentG's highlight says:

As long as the energy of thermal motion dominates this attractive force, the substance remains in the gaseous state, but at sufficiently low temperatures the attractions dominate and the substance condenses to a liquid or solid.

Rather vague.

Obviously, to me anyway, a 55 gallon drum full of water vapor does not just all condense into a liquid at once with the snap of a finger at a certain temperature does it?

I mean, the molecules are at first, attracted a little closer, then a little more.

IMO there is a continuum of attraction and repulsion that are always finding a balance.

Cool the gas a little... Attraction increases a little. A little more cooling, a little more attraction etc. etc

I spent some time searching for any text or video of the experiments just described but could not find anything

If it takes more than 15 pounds of force to pull a 1 inch surface area plunger out of a cylinder, (when cooled slightly) can it be assumed that the extra force is due to the molecular attraction of the gas particles in the cylinder?

A rather strange consequence of "NON-IDEAL" gas behavior is that when a gas is compressed, the pressure will eventually start to increase less and less.

As the molecules are forced closer together, they start to become more and more attracted.

I don't know how much, if at all, this sort of thing comes into play in an engine. I think certainly in a diesel engine or a fire piston.

A little LTD Stirling?

Well, if cooling and heating a few degrees has an effect then pretty obviously work input and output does also, as a result of cooling of heating due to changes in "internal energy".

In a gas, changes in internal energy are reflected in the temperature. There is no "latent heat" as in a liquid or water vapor (steam).

I don't really care if it is a result of internal molecular attraction or external atmospheric pressure or some combination of both, the effect is the same. "moleculat attraction" or being pushed together by atmospheric pressure.

https://byjus.com/chemistry/deviation-f ... behaviour/
It looks just the opposite.

The link before the last, is a web page describing the differences between ideal and real gasses. I agree with it. It was posted here for people to read a different wording of my point. It also shows charts depicting a positive pressure for gasses regardless how low the pressure goes. It also shows that expanding a gas leads further away from it becoming a liquid and to an area where it behaves more ideal.