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Thread: <Gas behavior>

  1. #91
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    Quote Originally Posted by profloater View Post
    Quick thought. If you replace the helium balloon with a lead solid model of it, the buoyancy force is identical and is caused by a pressure gradient in the surrounding fluid. The helium (or the density )is not relevant to the buoyancy force.
    And if you remove the source of the pressure gradient (gravity), the buoyancy force disappears.

    Grant Hutchison

  2. #92
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    Right, but you see, the key point here is we must understand differently the cause of the gas pressure itself (which is internal random motions, a property of the gas itself that requires no participation from any walls) from how we understand the various external causes of pressure gradients (which could be due to walls, or gravity, or electric fields, or the history of the preparation of the system-- even there, nothing special about walls). Not doing that is precisely the error being made by all those many sources that say gas pressure is "produced by" walls! Once you understand this, you won't be able to look up "gas pressure" without a handy source of antacid nearby.
    Last edited by Ken G; 2018-Dec-29 at 03:16 PM.

  3. #93
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    Quote Originally Posted by Ken G View Post
    It almost sounds like you don't realize that an "ideal gas" that obeys the "ideal gas law" is not the same thing as a "gas." A "gas" is a more general entity, I told you above what it requires. What's more, all simple gases (not ideal gases, I mean any gas with isotropic nonrelativistic velocities) obey p = 2/3 KE/V, but they do not all obey the "ideal gas law." If you don't know these things, you really shouldn't dig yourself any deeper. Come on, you don't really know anything about gases, now do you? All you can do is insert contextually isolated verbiage from sources you don't really understand, and it's not lending any insight into this discussion, since you're not open to learning anything, and it appears all the others already have.

    Here are the key takeaways that most seem to be getting:
    1) gas pressure is 2/3 KE/V, that's just exactly what it is (for simple nonrelativistic gases, which include but are not limited to ideal gases), and
    2) forces on walls are action/reaction pairs to what gets called "the normal force," show up in all kinds of situations, are constraint forces from how walls work, and in no way distinguish gas pressure from any other forces, including standing on the floor or leaning against the wall. There is no reason whatsoever to single out gas pressure and pretend that that force, compared to all the other ways of pushing and pulling on objects, somehow comes from a wall! That so many sources say that is just a broad misconception.
    In particular:
    Come on, you don't really know anything about gases, now do you? All you can do is insert contextually isolated verbiage from sources you don't really understand, and it's not lending any insight into this discussion, since you're not open to learning anything, and it appears all the others already have.
    This is completely inappropriate. Do not make public judgements about other members' abilities nor their interest in learning.

    If this rudeness continues, there will be points for the next infraction.
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  4. #94
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    All right, those words did sound personal. I don't actually have anything personal in mind, there is, in this thread, the information needed to understand gases and what causes both gas pressure (random particle motions)and what causes forces on gas from gas pressure (pressure gradients), as well as the difference between all that and the mundane elements of the normal force which do not "produce" gas pressure any more than the normal force of my bottom on my chair right now "produces" the force of gravity. I present all this for those who can get it, and I doubt I can say it any more clearly, so I can't imagine why I would need to add anything more.
    Last edited by Ken G; 2018-Dec-29 at 03:58 PM.

  5. #95
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    Quote Originally Posted by Ken G View Post
    Right, but you see, the key point here is we must understand differently the cause of the gas pressure itself (which is internal random motions, a property of the gas itself that requires no participation from any walls) from how we understand the various external causes of pressure gradients (which could be due to walls, or gravity, or electric fields, or the history of the preparation of the system-- even there, nothing special about walls).
    You don't need to persuade me.
    I'm bemused anyone finds this objectionable.

    Grant Hutchison

  6. #96
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    Isn't it remarkable how many sources talk about gas pressure as if it was an interaction with a wall instead of a property of a gas? You rarely see such situation-dependent thinking in such a completely general physics concept. I'm normally pretty impressed by how carefully thought out physics concepts are in the widespread sources on the topic, so this one is pretty disappointing. Maybe it's the chemists!

  7. #97
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    I guess, given the importance of Bernoulli and osmotic pressure in my line of work, I've long been used to thinking of pressure as implying energy density.

    Grant Hutchison

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    Me too, so I haven't looked at the common sources much lately. That's why I was so shocked what googling "gas pressure" will get you!

  9. #99
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    Quote Originally Posted by Ken G View Post
    Isn't it remarkable how many sources talk about gas pressure as if it was an interaction with a wall instead of a property of a gas?
    It's not that surprising to me being more in the engineering and the heavy equipment world. When definitions simply state it as a force per unit area it's hard not to have a wall come up (pun intended, of course). It's as if the cart is so big that the horse is hard to see, which makes it all the more interesting.

    Your points have broadened my understanding of it and how it applies to stars is especially interesting. I'm sure there are engineering efforts that require this more accurate definition of gas pressure as well, perhaps fusion reactor containment designs or other energy field work.
    We know time flies, we just can't see its wings.

  10. #100
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    Just for my benefit, I would like to clarify some thoughts I have on this issue at I would say a fairly basic level. So any comments would be appreciated to aid this clarification.

    (1) From Wikipedia:
    Pressure means how much something is pushing on something else. It is expressed as force per unit area: P=F/A

    (2) If I'm banging a nail into a piece of wood, I will be pushing a hammer which in turn pushes a nail which in turn pushes a piece of wood. But to obtain the required end result (work), the piece of wood has to be fixed in position and thus creating an opposing force.

    (3) If the piece of wood was not fixed in position, then my arm,hammer,nail and piece of wood will be pushing against air pressure. If I was doing all of this in a vacuum then the whole assembly of my arm, hammer,nail and piece of wood would not be encountering an opposing force.

    In (2), going by the definition (1) pressure seems to be intuitive as well as scientific.

    In (3) it would seem that in terms of the definition (1) pressure is not present intuitively or scientifically when everything takes place in vacuum.

    But I would assume that in the case of (3) any presence of anything (no matter how weak the opposing force) the other side of my bit of wood would give an opposing force, so then, in terms of the definition, pressure is present scientifically if not intuitively (i.e. atmospheric pressure on the other side of my bit of wood is hardly going to facilitate the nail being driven home).

    So the obvious question I have is, if a gas is expanding into a vacuum where there is no opposing force, is there actually a gas pressure?

    I appreciate that this is sort of going over old ground in that we can ask is there weight without some bathroom scales. There isn't weight as such, but the force of gravity is still there. Place the scales on a descending platform and there will be some weight, place the scales on a free falling platform and there will be no weight. But the force of gravity is there unchanged in all of these scenarios.

    But...place some gas in a container with the container being in a very large vacuum, the definition of pressure in (1) works admirably well. Make a little pinhole and the pressure drops. Remove the walls and the pressure drops to nothing because there is no opposing force, all the gas has as a boundary is a vacuum. But at all times within these scenarios the gas has isotropic momentum.

    In the gravity analogy, we confine weight to the weighing scales and acknowledge that the force of gravity is always present regardless of the status of the weighing scales in terms of its presence or descending motion.

    In dealing with gas, the consensus is that pressure is not confined to the equivalent of the weighing scales, rather it is there all the time, even if the boundary of a gas is a vacuum despite definition (1) suggesting that pressure is something pushing against something else.

    So perhaps, what I should be asking for is a definition of gas pressure that doesn't involve "something pushing on something else".

    In none of the above am I making the suggestion that pressure is "made" by the opposing force of a barrier, just as I would never suggest that a weighing scales makes weight. It just seems to me that the term pressure is just a word to describe the effect of something pushing on something else, I don't see that the term has any intrinsic properties, that it is "something" out there waiting to come into existence like a flame or explosion. It just seems to be a descriptive term when something pushes against something else. You know it is happening when you produce work, its not that the work produces something magical with intrinsic properties of "pressure", rather the intrinsic property is work, be it an explosion from a ruptured gas container to a harmless measurement. It is work that is the intrinsic entity, the term "pressure" just seems to be our way of noting the work in a convenient and easy, manner.

  11. #101
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    Don't forget that in Ken G's model with no collisions between the gas particles, the gas cloud is gravitationally bound, if I am not mistaken. That is what we have with a star. I envision the particles in his model as buzzing around like the stars in a globular cluster. If we could magically turn off the gravity, the cloud would expand, with a corresponding drop in the gas pressure.

  12. #102
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    Quote Originally Posted by Hornblower View Post
    Don't forget that in Ken G's model with no collisions between the gas particles, the gas cloud is gravitationally bound, if I am not mistaken. That is what we have with a star. I envision the particles in his model as buzzing around like the stars in a globular cluster. If we could magically turn off the gravity, the cloud would expand, with a corresponding drop in the gas pressure.
    That helps - that allows my definition of pressure being "something pushing against something else". It also allows me to continue to prefer the term "work" to pressure. The gravitational field is providing an opposing force to the isotropic particle velocities, so work is being done there. Magically switch off the gravitational field and the gas particles have no opposing force, so no work gets done. But all of the particle velocities are still intact.

    So the work is there with the gravitational barrier in place, reduce the force of that barrier and less work gets done. Remove it completely and the work is zero. So work is dependent on the barrier, it isn't there when the barrier is removed.

    ....I think.

  13. #103
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    Quote Originally Posted by Len Moran View Post
    (1) From Wikipedia:
    Pressure means how much something is pushing on something else. It is expressed as force per unit area: P=F/A
    Yes, that is the problem. This is a very limited way to think about pressure, because pressure is a much more general (and powerful) concept than that! It is true that pressure has units of F/A, and that's how it acts when there is a sudden pressure discontinuity (as when a wall is present), but the concept is so much more general than that so it's really unfortunate to get misguided by those units. After all, it also has the units of energy density, and if you want forcelike units, notice that a force is a rate of change of momentum, so F/A is a flux density of momentum. Also, pressure gradients have units of force per unit volume, and both of those latter ways of thinking about gas pressure are powerful when you realize that the gas pressure force is a force on the gas. Defining it as a force on the enclosure of the gas simply confuses the pressure force with the action/reaction pair forced called the "normal force," which appears in many different situations involving surfaces and has nothing to do with the nature of the forces that are making the normal force appear.

    (2) If I'm banging a nail into a piece of wood, I will be pushing a hammer which in turn pushes a nail which in turn pushes a piece of wood. But to obtain the required end result (work), the piece of wood has to be fixed in position and thus creating an opposing force.
    Not so, and this is very much the point. You can hit a nail with a hammer in free space, and accelerate the nail, without any wood present. Is that somehow a different hammer force than if the wood is present? Yet that's how you see gas pressure forces explained, astonishingly.
    (3) If the piece of wood was not fixed in position, then my arm,hammer,nail and piece of wood will be pushing against air pressure.
    See how you are imagining a hammer must push against something? No, the nail has inertia, so the hammer can push on it, there is no need for any resistance. There the work done will simply show up as kinetic energy in the nail, should we really consider that to be a different kind of force?
    In (2), going by the definition (1) pressure seems to be intuitive as well as scientific.
    And wrong! Note how it led you to a wrong conclusion, you thought you cannot push against a nail unless something "opposes it." That's wrong, and it's also wrong for gases-- gases can be pushed around by their own pressures, no resistance needed other than their own inertia.
    So the obvious question I have is, if a gas is expanding into a vacuum where there is no opposing force, is there actually a gas pressure?
    A useful question-- have you ever heard of gas pressure in the early universe? Is that not exactly the scenario you are describing? How about light pressure after the era of recombination (where photons are noncollisional), doesn't that appear in cosmological models? (It happens to be a small contribution, but it could have been large and it would be the same equations.)
    But the force of gravity is there unchanged in all of these scenarios.
    Precisely, and so is the gas pressure. This is very important, gas pressure is just gas pressure-- it doesn't change at all when you put the gas next to a wall. What the wall does is create a discontinuity in gas pressure, which you can agree is something quite different.
    But...place some gas in a container with the container being in a very large vacuum, the definition of pressure in (1) works admirably well.
    And that's the problem-- that's the only situation where it works, just like bathroom scales are the only situation where weight is easily confused for a normal force. But gas pressure is a much more general concept, just like gravity is.
    Remove the walls and the pressure drops to nothing because there is no opposing force,
    Oh really? Remove a all and the gas pressure instantly drops to nothing? See how you are making errors based on the misconception you are analyzing?
    In dealing with gas, the consensus is that pressure is not confined to the equivalent of the weighing scales, rather it is there all the time, even if the boundary of a gas is a vacuum despite definition (1) suggesting that pressure is something pushing against something else.
    Right, now you see it.
    So perhaps, what I should be asking for is a definition of gas pressure that doesn't involve "something pushing on something else".
    Yes, that's what you should be demanding from those sources that let you down so badly. I can give you a conceptual definition, and then a more precise (but more mathematically complicated) one. The conceptual one is that gas pressure quantifies the tendency of a gas to expand. The mathematical one is that gas pressure is the tensor that determines the rate the gas particles carry momentum across imaginary surfaces you can mentally insert anywhere you like without any effect on the gas. This tensor quantity simplifies to a scalar behavior in the simple case of isotropic motions, so that's why this simplification has been inserted so much above. The scalar gives the rate, per area, that momentum is carried across any imaginary surface, with the subtle point I mentioned above that when leftward momentum is carried from left to right, and rightward momentum is carried from right to left, they both contribute positively to the pressure scalar. The reason is that if you make an imaginary box, momentum carried one way into the box adds positive momentum to the box, whereas momentum carried the other way out of the box subtracts negative momentum from the box, both of which produce the same force (when you think of force as vector momentum deposition rate).
    It just seems to me that the term pressure is just a word to describe the effect of something pushing on something else, I don't see that the term has any intrinsic properties, that it is "something" out there waiting to come into existence like a flame or explosion.
    You are right, that is how the word is being used in those sources, but that's exactly the problem. This is a terrible way to think about what gas pressure is, because it is far more powerful to think of it in a much more general way-- a property of the gas itself.
    It just seems to be a descriptive term when something pushes against something else.
    We already have a term for that-- it's called "the normal force." I agree that "the normal force" is a very generic term and is not widely understood outside of physics, because it is really just a constraint force that bears no particular connection to whatever other forces are causing it to appear, but it is much worse to mistake gas pressure for a normal force on a wall. It's just as bad as mistaking gravity for the normal force on the chairs we are sitting on.
    It is work that is the intrinsic entity, the term "pressure" just seems to be our way of noting the work in a convenient and easy, manner.
    And so you see the problem-- gas pressure can do work without any walls, and to see that, simply blow out a candle.
    Last edited by Ken G; 2018-Dec-29 at 08:18 PM.

  14. #104
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    Quote Originally Posted by grant hutchison View Post
    And if you remove the source of the pressure gradient (gravity), the buoyancy force disappears.

    Grant Hutchison
    Yes and it is surprising that pressure is not understood by some as within a gas, (it was the removal of the need for collisions that helped me earlier) when we experience weather such as wind all our lives, (not the other kind of wind) and surely we know that pressure gradient drives winds? No surfaces required.
    sicut vis videre esto
    When we realize that patterns don't exist in the universe, they are a template that we hold to the universe to make sense of it, it all makes a lot more sense.
    Originally Posted by Ken G

  15. #105
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    Quote Originally Posted by Ken G View Post
    And so you see the problem-- gas pressure can do work without any walls, and to see that, simply blow out a candle.
    Well that helps thanks. To understand gas pressure as being a real and proper property of gas in its own right helps to place the kind of definition "pressure is something pushing against something else" in a better context. That definition starts you off with a mindset that sees gas pressure as being "a"(a gas) pushing against "b"(a wall), from that point on it's difficult to think of "a" as being separated from "b" in terms of pressure. But your (as always) detailed look under the conventional approaches allows us to simply start and finish with "a" as a system with a self contained property of pressure.

    That (for me at any rate) is quite a departure from how I have always thought about gas pressure.

  16. #106
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    Apparently that's because it's a departure from how it's normally taught. To the standard approach, a wind isn't a thing, it's something that blows against your cheek. We're usually so adept at abstract concepts in physics, it's odd to find an example that is so pinned down to concrete experience. The topic of gas pressure must have a very different history than we normally find in physics-- temperature and density have much more formal meanings that sound more like intrinsic properties.

  17. #107
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    Quote Originally Posted by Ken G View Post
    ...
    And more fundamentally, I also had not realized that so many common sources so badly conflate the interesting and physically unique specifics of gas pressure with completely generic elements of the everyday normal force, as though the latter "produced" the former. So in all fairness, the misconception being promoted by Selfsim that you were pointing out did not start with him, it starts in these seemingly authoritative sources and the bizarre approach they take of describing a physical effect in terms of what it does when it encounters a surface. It would be like saying you only have a force of gravity on you when you are standing on the ground, or that a refrigerator magnet is only magnetic when it is against a metal surface, which no one would tolerate for an instant but we don't bat an eye when the sources do exactly that with gas pressure! Without Selfsim's participation, I would not have recognized how widespread is this misidentified role of surfaces in gas pressure, and so I would not have understood where Robitaille is getting his misconceptions from. It was quite a surprise, that for some reason, sources that are normally quite dependable cannot tell a "normal force" action/reaction pair from whatever interesting and unique physical effect is the cause of the generation of that force pair! So there is always some learning that occurs in discourses, and in this case I believe I may have succeeded in learning more from Selfsim's involvement than he did from mine, ironically-- though perhaps there is still hope.
    There is no misconception being promoted by SelfSim.

    What we have here, is a clear-cut case of an Against the Mainstream advocation by yourself.

    In all my time reading threads on this site, I have never seen anything as clear-cut as what you are doing, which is what I have been distinguishing for others, all along. There is a line and your argument clearly steps over it.

    And, with your above statements, it seems even you now concur with the depth and breadth of mainstream consensus sharply contrasting with your idea.

    This is not a comment directed at you personally, Ken and I assure you, I do appreciate and understand deeply your overall idea ... however, the responsibility for taking appropriate actions on how this thread should be treated, lies somewhere beyond both of us.

    The point is, that the onus is on yourself for defending your idea with your own objective experimentation evidence in the face of the vast weight of countering mainstream evidence.

  18. #108
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    Quote Originally Posted by Selfsim View Post
    There is no misconception being promoted by SelfSim.

    What we have here, [B]is a clear-cut case of an Against the Mainstream advocation by yourself.
    I thought you might say that. It certainly would seem to be true, but only for someone who really doesn't understand everything else that is being said in this thread. Aren't you ignoring all those posts by other contributors? Why would you do that?

    And, with your above statements, it seems even you now concur with the depth and breadth of mainstream consensus sharply contrasting with your idea.
    Well, what the evidence has shown me is that common "mainstream" explanations of what gas pressure is are often quite poor, as they have led to a stream of false statements by you and other people on the thread who fell victim to those misconceptions. This was demonstrated above. Sadly, it sometimes happens that widespread seemingly good sources can make a hash of some particular concept, and then you have to either use your own brain, or consult more advanced sources. Graduate level textbooks are often the best for the latter option, but are hard to give people access to, so I normally resort to the former option on this forum. Ultimately, it just comes down to whether or not you want to understand gas pressure. You've made your choice, so have the others.

    The point is, that the onus is on yourself for defending your idea with your own objective experimentation evidence in the face of the vast weight of countering mainstream evidence.
    Actually, the arguments I've provided are the only "defense" I require or intend. As I said, the rest is up to each person who reads the thread, I can't explain it any better.
    Last edited by Ken G; 2018-Dec-29 at 10:08 PM.

  19. #109
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    Quote Originally Posted by Selfsim View Post
    What we have here, is a clear-cut case of an Against the Mainstream advocation by yourself.
    Sorry, but that seems like complete nonsense to me, and no more defensible for being delivered in Bold Italic Underline.
    Ken has done nothing here but explain an understanding of pressure with which I've been familiar for decades. Whereas your own erroneous comments about buoyance forces suggest you have a deep misunderstanding somewhere.

    Grant Hutchison

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    Quote Originally Posted by Ken G View Post
    I thought you might say that. It certainly would seem to be true, but only for someone who really doesn't understand everything else that is being said in this thread.
    Appearances can be deceptive.

    How else would a 'foil' going up against your clearly tightly held idea, come away as appearing any other way?

    Anyway how I appear doesn't really matter so much .. as long as the distinction of what 'mainstream' means has been clarified (a little?)

    Quote Originally Posted by Ken G
    .. Well, what the evidence has shown me is that common "mainstream" explanations of what gas pressure is are often quite poor, as they have led to a stream of false statements by you and other people on the thread who fell victim to those misconceptions. This was demonstrated above.
    I admitted and disclosed any personally made oversights and misreads. I think you'll find honesty there. My not responding to those posters who defend your idea is merely out of necessity and efficiency .. and no disrespect is intended.

    Quote Originally Posted by Ken G
    Sadly, it sometimes happens that widespread seemingly good sources can make a hash of some particular concept, and then you have to either use your own brain, or consult more advanced sources. Graduate level textbooks are often the best for the latter option, but are hard to give people access to, so I normally resort to the former option on this forum. Ultimately, it just comes down to whether or not you want to understand gas pressure. You've made your choice, so have the others.
    Unfortunately, in the process your argument, you have not provided sufficient evidence for distinguishing the speaker of 'your idea', from a myriad of other countless other conversations of the same type, which I've had with ideologicially driven acolytes, (similar topics), whose basis of agumentation is identical.
    The 'tools' of which you speak, unfortunately, lack the necessary backing to hold up in the face of those arguments.

    As I said, all you've provided here is yet another idea where: 'I'm right and everyone else is wrong'.

    Quote Originally Posted by Ken G
    Actually, the arguments I've provided are the only "defense" I require or intend. As I said, the rest is up to each person who reads the thread, I can't explain it any better.
    You don't have to explain that .. I get it .. and your idea simply doesn't get to the level where I can make use of it, I'm afraid.

    Cheers

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    Quote Originally Posted by Selfsim View Post

    I admitted and disclosed any personally made oversights and misreads. I think you'll find honesty there.
    I'm certain you are not being dishonest, but honesty isn't the issue, misunderstanding is. I see a clear connection between the misconceptions you have about gas pressure, and your errors involving the ideal gas law, the difference between a gas and a fluid, and the source of buoyancy. These are all concepts that follow quite easily from a better understanding of what gas pressure is, yet you resist that better understanding. So your mistakes don't surprise me at all, this is the whole point of what I'm trying to do here-- provide a better understand to help avoid mistakes and false conclusions. I've seen this work well with other posters, and you should have seen that too-- just by reading carefully. Yet you resist. That's a personal thing, it's none of my business.

    As I said, all you've provided here is yet another idea where: 'I'm right and everyone else is wrong'.
    Not everyone else.

    You don't have to explain that .. I get it .. and your idea simply doesn't get to the level where I can make use of it, I'm afraid.
    Yes, I reached that conclusion some time ago. Unfortunately, this means you likely will also not understand gases and fluids, the ideal gas law, and buoyancy, as well as a host of other pressure-related topics that didn't come up, like how gas pressure appears in general relativity. But you don't have to care, as I said, it's a personal choice.
    Last edited by Ken G; 2018-Dec-29 at 10:55 PM.

  22. #112
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    Quote Originally Posted by Selfsim View Post
    Unfortunately, in the process your argument, you have not provided sufficient evidence for distinguishing the speaker of 'your idea', from a myriad of other countless other conversations of the same type, which I've had with ideologicially driven acolytes, (similar topics), whose basis of agumentation is identical.
    And that looks very much like the Argument of the Squid.

    Grant Hutchison

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    I realize that the issue is a bit deeper, but in a sense this reminds me of the question of whether a falling tree makes a sound if there is nobody there to hear it. You can make the argument, which I find fairly silly, that the vibrations in the air, which definitely occur, are not sound until somebody hears them. It seems that Wikipedia and other sources are almost making the same mistake for pressure. So I have no problem with Kenís argument and I also donít understand why itís controversial.


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    Yes you're right, it's a bit like asking if a sound wave is a pressure pattern in the air, or something that happens in your ears. It isn't the semantics we care about, we have to call these things something, it's that we want to understand what is happening in the air-- not what is happening in our ears.

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    Note the reasoning that the condition of pressure summing up as
    4E/6V
    is that M should be equal for every opposite u

    Yes, if Mu do not cancel out then we have a net wind of the bulk.
    But the momentum is given by Mu - pressure by Muˇ2
    You can easily have momentum cancelling out (no net wind) but pressures unequal in opposing directions if the velocity distribution is such that at one u, particles move in one direction, and at another u, in opposite direction.

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    It's not opposing directions that matter (as I mentioned, momentum transport along opposing directions just adds to the gas pressure, never "cancelling out"), but rather perpendicular ones. That's why gas pressure is most generally a tensor. But as I've said many times, to keep things simple, and to make pressure act like the scalar we all know and love, the assumption of isotropic velocities in the fluid frame is necessary. This is the core assumption of a simple gas-- not collisions. Indeed, frequent particle-particle interactions don't make the behavior more gaslike, it makes it closer to a phase transition to a liquid. However, some small collisionality is everyone's favorite way to enforce isotropic velocities, though it can also come from other things (such as the cosmological principle, in the case of cosmic photon pressure, which is highly noncollisional after the era of recombination-- but also small). Another common way to enforce isotropy is to have particles interacting with randomly varying fields, as often happens in collisionless plasmas. A final application where isotropy is maintained noncollisionally is when you have spherical symmetry in a static distribution maintained by self-gravity, such as a slowly evolving nearly spherical galaxy of stars. Arms in spiral galaxies is also a common application of collisionless gas dynamics, though I don't know if the anisotropic complexities there require a tensor pressure treatment.
    Last edited by Ken G; 2018-Dec-30 at 10:48 AM.

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    Quote Originally Posted by Ken G View Post
    This sounds unnecessarily complicated-- can we not simply say that that concept of volume exists independently of the concept of containers, and be done with it?
    I have just presented the caveats, that your explanations omitted, to highlight the pitfalls associated with regarding 'special' cases (with redefined hidden variables) as 'general' cases, which was directly implied due to those omissions.

    In the 'general' case the concept of volume does not exist 'independently of the concept of containers' so you need to provide a method/mechanism (different to mine) to prove that your statement in the 'special' case is truly 'independent of the concept of containers' in the 'general' case.

    The end result is a draw, no side is completely right or completely wrong, a quantum conundrum if you like.

  28. #118
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    Quote Originally Posted by LaurieAG View Post
    I have just presented the caveats, that your explanations omitted, to highlight the pitfalls associated with regarding 'special' cases (with redefined hidden variables) as 'general' cases, which was directly implied due to those omissions.
    Unfortunately, this sentence does not make much sense to me.
    In the 'general' case the concept of volume does not exist 'independently of the concept of containers' so you need to provide a method/mechanism (different to mine) to prove that your statement in the 'special' case is truly 'independent of the concept of containers' in the 'general' case.
    That one, even less.
    I would quite confidently state that the general concept of volume requires no input from the concept of containers, as a universe devoid of containers (say, our universe after the era of recombination) could still find use for the concept of volume and its crucially related notion of density. Hence the concept of volume does not rely on containers, but rather, volume is an attribute of a container. It is also an attribute of a region of empty space. Indeed, the concept of the volume of a container can be regarded as an attribute the container inherits from the empty space that it encloses.
    The end result is a draw, no side is completely right or completely wrong, a quantum conundrum if you like.
    It's not a competition, but what's clear is that one "side" understands gases, and the other does not, leading to a host of misconstrued claims about fluids, the ideal gas law, buoyancy, kinetic theory, and so on. This is the purpose, and it's up to the individual-- all victories are personal victories of achieving, or not achieving, insight.
    Last edited by Ken G; 2018-Dec-30 at 11:00 AM.

  29. #119
    Join Date
    May 2008
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    15,105
    This thread was supposed to be about gas behavior, and not forum member behavior. If a bunch of adult people cannot discuss such a topic without bickering about each other like little kids, we'll just close it.

    For the any next threads: please learn to ignore baiting, and re-read every one of your posts before submitting, to check if you're talking about attributes of an idea, or of a person. And if you find that it is about a person, or not specifically about the topic, simply don't post. It's really not that hard. You don't lose anything by not posting a bunch of already typed words, and gain a lesson in thinking before posting. How's that for a new years resolution?
    Last edited by slang; 2018-Dec-30 at 03:54 PM. Reason: removed unnecessary snark
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