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Fool wrote: ↑Tue Aug 15, 2023 9:18 am (...)
Gasses never contract during an expansion, even when cooling past their, stp condensation point. This can be observed by subjecting water to a vacuum chamber. Sure at first air cools below the dew point and a cloud forms, but ...

This appears to be a contradiction. "never"?

I think this may be one of those odd phenomena (to us anyway) like the triple point where water freezes and boils at the same time.

Gas "expands", transferring energy to the piston then immediately "contracts".

Actually, in all likelihood, only some of the gas is heated, expands to drive the piston, loses energy and contracts while simultaneously other molecules are heated, transfer energy and cool and "contract".

So with some of the gas molecules gaining energy from heat input and "expanding" while other molecules are giving up energy to the piston and 'contracting", the actual overall increase in total VOLUME due to expansion of the gas is zero.

Yes, the piston moves and the volume increased as a consequence of the piston receiving kinetic energy but it leaves behind a vacuum.

What I don't believe is possible is for the gas to be heated and for its internal energy to increase "expanding and doing work on the piston" and for the gas to then stay expanded while work goes out, and for it to then be necessary to remove additional "internal energy" from the gas by cooling the gas at the sink further in order to contract the gas so that the piston can return.

If that were the case it would be a violation of conservation of energy.

You put so many Joules of heat in, take out an equivalent amount of "work" in Joules, then, ... Remove another equivalent number of Joules to the sink?

Impossible!

Yet the academics put forward this malarkey without blinking an eye.

You've measured this "contraction"? Have data showing an internal tensile force? Shown a piston being pulled back when the open end is in space or a vacuum? No sir. You will always see the cork pushed out of a gun in space. Expansion forever no contraction. Do not confuse an atmosphere pushing a piston back in with "contraction".

A Stirling that runs with connecting rod removed, will not run in space. It will blow the piston right out.

Can you show contraction on a PV diagram?

Have you measured contraction on an indicator diagram?

Fool wrote: ↑Tue Aug 15, 2023 4:30 pm You've measured this "contraction"? Have data showing an internal tensile force? Shown a piston being pulled back when the open end is in space or a vacuum? No sir. You will always see the cork pushed out of a gun in space. Expansion forever no contraction. Do not confuse an atmosphere pushing a piston back in with "contraction".

A Stirling that runs with connecting rod removed, will not run in space. It will blow the piston right out.

Well,..

In this video, how does a vacuum form inside the paint can?

The paint can is rigid. When the water cools it condenses creating a vacuum inside the can.

If the can was even stronger, more rigid, would the water still condense and leave a vacuum inside the stronger can?

What happens first? Does the water condense first or does the can get crushed by atmospheric pressure first?

Logically, IMO, the water vapor must condense before a vacuum forms. In my mind, that seems like a very clear demonstration of "internal tension force", because logically the water vapor needs to condense before there could be a vacuum and there must be a vacuum before atmospheric pressure can crush the can.

Would you not agree?

It might be interesting to do this crushed can experiment inside a vacuum chamber.

My prediction would be that the water vapor will still condense into a liquid at a low enough temperature and still create a vacuum inside the can (or better a thick glass jar so the condensation or the water vapor inside, if any, can be observed)

Or does the can get crushed and the water vapor condenses simultaneously? That would be interesting to see. But even if that were so, would it not demonstrate that there was some actual tensile force?

Or, why would the can spontaneously collapse so that the water could condense after the can begins to collapse?

Can water condense in a vacuum?

https://youtu.be/lG899rZ58Ts

Not sure that answers the question.

He does show, the corked bottle has some water in it. Is that water that condensed from steam, or is it water left over from incomplete boiling?

Personally, I think it condensed before he removed the cork. There is already a vacuum in the bottle evidenced by how instantly the inverted bottle fills with water. Much faster than the bottle fresh out of the microwave, there the water condenses more gradually.

Here is another experiment you've probably already seen:

https://youtu.be/PDoMBfSUSnY

There are dozens of these "making water boil with ice" videos. Same question.

Does the cold from the ice reduce the "internal energy" of the water vapor with the consequence that the water vapor condenses in spite of the fact that in doing so it leaves a vacuum?

Most commentators making these videos claim that the low temperature from the ice causes the water vapor to condense FIRST, and then the water boils only after the water condenses creating the vacuum.

I haven't done this experiment myself, so I couldn't tell you, but the general consensus among those who have is that the water vapor condenses first and only then does the water boil, although to me, it looks like the water starts boiling almost the instant the ice is applied so it kind of looks like the boiling and condensing take place simultaneously.

Why does the water vapor condense in a vacuum at all though?

If the ice is applied, why doesn't the water vapor just stay floating around inside the jar?

I think it might be because at colder temperatures water molecules are attracted to each other and that "tensile force" as you called it is strong enough that it results in a vacuum.

Likely, air in a Stirling engine, when it loses internal energy likewise contracts for a similar reason. The attractive force of the air molecules dominated at lower energy levels.

I forgot the link to the paint can crushing video:

https://youtu.be/O_1J23uLtPY

But there are dozens of others like it.

So, can the attraction between "cooled" gaseous molecules (that have lost "internal energy") create a vacuum?

It looks that way to me.

Does it make any difference if the lower temperature is caused by cooling or as a result of the gas loosing internal energy because of doing "work"

Personally I don't think it makes a difference the gas is going to contract.

Unlike "ideal" gases, real gases have forces of attraction.

Fool wrote: ↑Tue Aug 15, 2023 5:32 pm Can you show contraction on a PV diagram?

Have you measured contraction on an indicator diagram?

Look at any REAL PV tracing of a Stirling engine.

Logically, if there was no contraction of the gas, the internal pressure of the engine would never fall below atmospheric pressure.

Yet, we do know that the pressure does drop below atmospheric pressure and it actually stays below atmospheric pressure during compression/contraction.

How can the internal pressure of the engine continue to drop after BDC without cooling and contraction of the gas.

The gas is not being "compressed" by atmospheric pressure. If it were being "compressed" by the flywheel or some other means it would be higher than atmospheric pressure.

I assume you've already seen the Andrew Hall video demonstration, but just in case:

The PV indicator diagram in Hall's video clearly indicates the engine pressure goes below atmospheric pressure for roughly 1/2 the cycle. Moreso on the return "contraction" stroke than on the expansion stroke.

https://youtu.be/SHyke4hUNOs

Contraction on a PV diagram, by definition, would appear as a negative absolute pressure. A value lower than a perfect vacuum.

All those links are in the positive pressure segment of a PV diagram, even when below atmospheric pressure. Trying to redefine contraction to any compression that is below atmospheric, doesn't change the fact that, from a working gas's and PV diagram's perspective, it is something outside compressing it. Gasses do not pull.

Coming up with phase change process doesn't work either. Look at a PV diagram for nitrogen. The phase change region is still in the positive pressure quadrant. Besides phase change processes are quite a bit removed from the range of pressures and temperatures our Stirling Engines operate.

Gasses, real, don't attract. They don't attract because the attraction force, to become liquid, is way overcome by the kinetic, internal, energy, forcing them to push outward becoming gasses.

Cloud in a bottle results because the air temperature drops faster than the moisture temperature, causing the moisture to condense on the cooler air. The phenomenon is still in the positive pressure quadrant of the PV diagram. Sorry not "real" contraction either.

To understand a PV diagram we need to ignore what the outside world is doing. A PV diagram is valid on Earth, in space, or at the bottom of the ocean. Of course given that the volume, temperature, and total mass stay the same.

Adding outside conditions is misleading and confusing and is inconsequential. Okay? The gas doesn't care what is moving the piston it only pushes, how hard depends only on temperature and volume and total mass. It always pushes.

IMO movement to the right on a PV diagram (increase in volume) represents heat added into the gas that expanded the gas WITHOUT doing work (unless there is a pressure and/or temperature drop, i.e. the increase in volume results in a "partial vacuum" as that phrase is commonly understood by almost everyone).

Movement to the right DOES NOT in my opinion, represent actual work performed by the gas. Work results in a lower internal energy, cooling and "contraction" which, if not for the momentum of the piston and possibly the continued action of SOME high energy particle constituents of the gas, would result in movement to the left.

I am of course, describing what an ACTUAL PV recording using real instruments on a real engine with real gases under real atmospheric conditions in the real world would represent.

Real gases in our atmosphere have forces of attraction and repulsion that in earths planetary gravitational field are in balance. That is, the attractive forces and the repelling forces are in a state of equilibrium.

Any compression of a gas whatsoever, any heating or cooling whatsoever will disturb that equilibrium. Any "work" input or output will alter that equilibrium.

A heat engine is in a constant state of oscillation between extremes of heating expansion, work output, increases and decreases in internal energy, etc.

To imagine a gas can take in heat, expand and output energy as work while also retaining the same energy for later removal to a "cold reservoir' is illogical and defies common sense.

Fool wrote: ↑Wed Aug 16, 2023 7:52 am ...

Gasses, real, don't attract. They don't attract because the attraction force, to become liquid, is way overcome by the kinetic, internal, energy, forcing them to push outward becoming gasses.

...

Sure, except that they do ALLWAYS attract within a certain range, but the attractive force is balanced by the repulsive forces that as you say "push outward".

To say they "don't attract" at all, ever is nonsensical.

Cooling, heating, work input/output of any kind alters the balance.

If the kinetic energy of both molecules, relative to each other, is high enough, their speed will be above "escape velocity". That means the molecular attraction can be ignored, they are gasses, and they will have positive pressure. Zero contraction is possible. This is why they are called a gas. Zero attraction. Positive pressure. The kinetic energy is far above, not balanced, totally overwhelming the attraction, to the point of insignificance, thus becoming a gas. Gas means zero attraction.

The kinetic energy of the molecule is greater than the attractive force between them, thus they are much farther apart and move freely of each other. In most cases, there are essentially no attractive forces between particles.

https://web.fscj.edu/Milczanowski/psc/l ... 0particles.

In a planetary system, escape velocity is the speed where a satellite, from stored momentum, will keep going away from a planet and never stop. Direction of escape velocity doesn't matter, as long as it doesn't hit the planet or it's atmosphere. If headed closer, it will speed up maintaining escape velocity speed for any altitude.

Molecules don't care if on a collision course. They will just bounce and reverse directions. They will maintain the total system momentum/speed. Their attraction effect will be negligible, ignorable, inconsequential.

By definition:
A change in volume is a force over a distance, work. Work is being done anytime there is movement/volume changes.

There is nothing adiabatic about a Stirling Engine, expansion, compression, regeneration. Adiabatics can't happen in a gas volume that is up against a temperature difference.

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. We would be in trouble if there were, as all of Earth's atmosphere would be sucked out into space.

All of the demonstrations of a can/bottle/whatever imploding are proof that atmospheric pressure exists.
I thought that this was a fairly well-known issue with Stirling design - one cannot allow the internal pressure to rise too much, or atmospheric pressure cannot make the piston return?

Fool wrote: ↑Thu Aug 17, 2023 12:23 am If the kinetic energy of both molecules, relative to each other, is high enough, their speed will be above "escape velocity". That means the molecular attraction can be ignored, they are gasses, and they will have positive pressure. Zero contraction is possible. This is why they are called a gas. Zero attraction. Positive pressure. The kinetic energy is far above, not balanced, totally overwhelming the attraction, to the point of insignificance, thus becoming a gas. Gas means zero attraction.

The kinetic energy of the molecule is greater than the attractive force between them, thus they are much farther apart and move freely of each other. In most cases, there are essentially no attractive forces between particles.

https://web.fscj.edu/Milczanowski/psc/l ... 0particles.

In a planetary system, escape velocity is the speed where a satellite, from stored momentum, will keep going away from a planet and never stop. Direction of escape velocity doesn't matter, as long as it doesn't hit the planet or it's atmosphere. If headed closer, it will speed up maintaining escape velocity speed for any altitude.

Molecules don't care if on a collision course. They will just bounce and reverse directions. They will maintain the total system momentum/speed. Their attraction effect will be negligible, ignorable, inconsequential.

By definition:
A change in volume is a force over a distance, work. Work is being done anytime there is movement/volume changes.

There is nothing adiabatic about a Stirling Engine, expansion, compression, regeneration. Adiabatics can't happen in a gas volume that is up against a temperature difference.

Well, certainly, if you search the internet for the text string "In a gas,no attractive forces between particles" you will find something somewhere online that supports your opinion.

In this case your citation is from some student curriculum in what appears to be a course in bio-chemistry at a State college which IMO carries zero weight in this context.

Try Googling "real gases", "real gas behavior", "real gases vs. Ideal gases", "real gases" + attraction, "NON-IDEAL gas behavior", "real gases" + compression OR expansion, "real gas" + pressure, "deviation from ideal" gas behavior.

Air conditioning and refrigeration, heat pumps etc. Depend on NON-IDEAL gas behavior. If all gases were "ideal gases" or behaved as 'ideal" gases there would be no such thing as COOLING in a refrigerator, no heat pumps or air conditioning etc.

MikeB wrote: ↑Thu Aug 17, 2023 12:38 am 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. We would be in trouble if there were, as all of Earth's atmosphere would be sucked out into space.

All of the demonstrations of a can/bottle/whatever imploding are proof that atmospheric pressure exists.
I thought that this was a fairly well-known issue with Stirling design - one cannot allow the internal pressure to rise too much, or atmospheric pressure cannot make the piston return?

"Vacuum" or "partial vacuum" is accepted terminology that everybody understands, at any rate, it's all we have, I've had quite enough of this nit picking over words and ignoring the point

The point of bringing up those examples of "implosion' is that logically, for a can to implode or be crushed by atmospheric pressure there needs to be a "vacuum" or low pressure inside the can. That low pressure must exist before the can collapses under atmospheric pressure.

For the low pressure to exist, the gas particles must first be attracted to each other, otherwise they would never condense, creating a "vacuum" inside a rigid container.

It's simple logic, cause and effect. You don't have an effect without a cause.

Cooling the gas allowed the attractive force to become dominant lowering the pressure. Obviously this happens even without phase change. When cooled, gases contract and pressure is reduced BECAUSE gas particles DO have forces of attraction, ALWAYS, under all conditions all the time.