Cause of Atmospheric Pressure

[swansont]

15 hours ago, AEBanner said:

Sorry, but what is this non-existent theory, and please could you refer me to the standard physics explanation?

That pressure is solely due to weight (collisions playing no part), and exerted only in a downward direction. That seems to be your assertion.

15 hours ago, AEBanner said:

I agree that atmospheric pressure is caused by molecular collisions with the Earth's surface.  But I find it difficult to understand how the weight of molecules throughout the height of the air column can affect the surface.  What is the mechanism?  If an air molecule "sits" on the surface, OK its weight is loaded on the surface and so may contribute to the pressure, but most of the other air molecules are still up there in the air, not contacting the surface.  Their weights do not add up together in the air!  How can they?  I cannot see the mechanism.  

Air molecules collide with each other. The rate depends on the number and speed. But there's a bias in the downward direction, from gravity. A molecule traveling some distance along the column (of height h) would gain a KE of mgh. It doesn't matter if it travels directly, or if this is mediated through collisions, because the collisions are elastic. Because the system is in steady state, all of the molecules contribute to the energy of the ones doing the actual colliding. 

15 hours ago, AEBanner said:

Further, if you refer back to my earlier post dealing with a simple thought experiment, I think you might then be convinced that the only viable explanation for atmospheric pressure is the molecular collision one. 

As I and others have said, molecular collisions is part of the explanation — it's the physical mechanism. But it doesn't explain the magnitude of the pressure. And it's similar to any fluid column, where Bernoulli's equation tells us the the pressure at the bottom due to the column of fluid is pgh (p = rho, the density). The molecules in the fluid are moving, and colliding, just as in air. Merely with a higher density, and smaller spacing.

[AEBanner]

16 minutes ago, swansont said:

That pressure is solely due to weight (collisions playing no part), and exerted only in a downward direction. That seems to be your assertion.

You have totally misunderstood my posts on this matter.  My opinion is that atmospheric pressure is due to the collisions of the air molecules with the Earth's surface.  I do not believe that it is caused by the weight of the molecules, and if you read my posts again perhaps then you too will be convinced. 

My first objection to the "air has weight" explanation is that I cannot see a mechanism whereby the weight of air in the column is applied to the surface, since almost all the molecules are not in contact with it.  Each molecule certainly has its own independent weight but it's up there in the air, not touching the surface.  

Another objection is that, as I'm sure you will agree, air pressure acts in all directions, but with the "air has weight" theory the pressure would only be downwards, in line with gravity.

Or then again, perhaps you're just trying to wind me up.

[mistermack]

I think what's confusing the OP is the buoyancy effect. Air SEEMS to be weightless because it's floating in other air. 

Take a giant balloon full of water, it's obvious that the contents are very heavy. But put the same balloon in a swimming pool, and it floats around, apparently weightless. It's the same for any fluid, floating in a similar fluid. 

It's exactly the same for a gas. If you have a huge balloon full of air, you can't feel the weight, because it's floating in air, like the water balloon floating in water. So the apparent weightlessness of the air in a balloon is an illusion, caused by the buoyancy effect.

If you sealed a rigid container of one cubic metre, and placed it on a weighing scales, in a vacuum room, and gradually pumped the air out of the room, the weight reading of the container would gradually increase, as the air was pumped out, and when you had nearly all of the air removed, the container would actually register more than a kg more. 

If the OP were to read up on buoyancy, it should become clear.  

https://en.wikipedia.org/wiki/Buoyancy  

[Strange]

17 minutes ago, AEBanner said:

You have totally misunderstood my posts on this matter.  My opinion is that atmospheric pressure is due to the collisions of the air molecules with the Earth's surface.  I do not believe that it is caused by the weight of the molecules, and if you read my posts again perhaps then you too will be convinced. 

You seem to have misunderstood what swansont was responding to (I think you did the same with my earlier answer). 

He was answering your question: "Sorry, but what is this non-existent theory"

The "non-existent theory" is that "pressure is due solely to weight". 

So you are making a straw man argument.

19 minutes ago, AEBanner said:

My first objection to the "air has weight" explanation is that I cannot see a mechanism whereby the weight of air in the column is applied to the surface, since almost all the molecules are not in contact with it. 

Why are you ignoring two pages of explanations of this mechanism?

20 minutes ago, AEBanner said:

Another objection is that, as I'm sure you will agree, air pressure acts in all directions, but with the "air has weight" theory the pressure would only be downwards, in line with gravity.

Why are you ignoring two pages of explanations of this mechanism?

 

[studiot]

17 hours ago, studiot said:

 

The force exerted on the ground by a pile of bricks of standing area 1 square metre is equal to the weight of the bricks.
The pressure is the weight divided by that area.

Are you saying that this does not apply to a column of air standing on the gorund?

 

I'm still waiting for an answer to my question.

[Strange]

10 minutes ago, studiot said:

 

I'm still waiting for an answer to my question.

Good questions.

10 minutes ago, studiot said:

Are you saying that this does not apply to a column of air standing on the gorund?

And, if not, why not?

[swansont]

57 minutes ago, AEBanner said:

You have totally misunderstood my posts on this matter.  My opinion is that atmospheric pressure is due to the collisions of the air molecules with the Earth's surface.  I do not believe that it is caused by the weight of the molecules, and if you read my posts again perhaps then you too will be convinced. 

I don't know how to be clearer. The standard physics explanation is that the pressure is equal to the weight of the molecules and the mechanism is molecular collisions. Both of these are part of the explanation. It is not one or the other.

If g were reduced, the pressure would go down (all else being equal)

 

57 minutes ago, AEBanner said:

My first objection to the "air has weight" explanation is that I cannot see a mechanism whereby the weight of air in the column is applied to the surface, since almost all the molecules are not in contact with it.  Each molecule certainly has its own independent weight but it's up there in the air, not touching the surface.  

What is missing from the explanation I gave?

The same can be said of a column of water. Only a thin layer is in contact with whatever layer you pick, and yet all of the molecules above it contribute. 

57 minutes ago, AEBanner said:

Another objection is that, as I'm sure you will agree, air pressure acts in all directions, but with the "air has weight" theory the pressure would only be downwards, in line with gravity.

Which is one reason that it's a misinterpretation. Nobody versed in physics thinks that that explanation has merit. It is not what people are referring to when they talk about the weight of the column telling you what the pressure is.

57 minutes ago, AEBanner said:

Or then again, perhaps you're just trying to wind me up.

One could say the same thing about you and your preposterous interpretation of "air has weight"

[Phi for All]

3 hours ago, AEBanner said:

Or then again, perhaps you're just trying to wind me up.

!

Moderator Note

You need to understand that the professional scientists who've responded to your posts are trying to help you learn. Science discussion here is all about clarity, and understanding. There is no conspiracy by all these people to wind you up.

You're ignoring the knowledge being offered in favor of ideas that are being refuted with each reply. That's preaching, or soapboxing, and it's against our rules. If you can't offer solid evidence to support your ideas, then the folks who can support what they claim win the tip of the balance, yes? The best supported explanation is what mainstream science works with every day. 

So please, either try to support what you're claiming, or listen harder to the mainstream explanations offered. Discussions requires a fair amount of listening, and it's clear from the replies that you're being listened to, but not returning the favor. 

 

[AEBanner]

2 hours ago, Phi for All said:

Or then again, perhaps you're just trying to wind me up.

I wish to apologize to the moderator,  and to swansont , and to everyone else for this remark.

I fully accept that the posts are intended to help me understand the explanation for atmospheric pressure.  But I am still not convinced by some of the explanations offered.    

I shall try to deal with these points in answer to the various individual posts.

 

[studiot]

Enough hand waving. We need some mathematics.

There is a connection between the kinetic theory and the fluid mechanics.

I have very quickly scribbled out a simple mathematical derivation of this connection in the attachments.

 

[beecee]

32 minutes ago, AEBanner said:

I wish to apologize to the moderator,  and to swansont , and to everyone else for this remark.

I fully accept that the posts are intended to help me understand the explanation for atmospheric pressure.  But I am still not convinced by some of the explanations offered.    

I shall try to deal with these points in answer to the various individual posts.

 

Why not ask yourself why some gases appear heavier then others? CO2, Cl are two examples of which I personally have been involved with hundreds of times, and as per my previous post in this thread....Isn't this positive proof that since some gases are heavier then others, that one can presume the atmosphere also has weight?

Edited November 9, 2018 by beecee

[mistermack]

42 minutes ago, beecee said:

Why not ask yourself why some gases appear heavier then others? CO2, Cl are two examples of which I personally have been involved with hundreds of times, and as per my previous post in this thread....Isn't this positive proof that since some gases are heavier then others, that one can presume the atmosphere also has weight?

The same applies to the lighter gases too. A helium balloon can only float upwards because it's LIGHTER than air. If air weighed nothing, that simply couldn't happen. You can buy the proof on a street corner. 

[beecee]

6 minutes ago, mistermack said:

The same applies to the lighter gases too. A helium balloon can only float upwards because it's LIGHTER than air. If air weighed nothing, that simply couldn't happen. You can buy the proof on a street corner. 

yes, I did post this earlier...."Air weight as well is confirmable. When I wore a younger man's clothes and was part of the work force, I always needed to get a "closed vessel clearance" when entering a pit or LTV well...reason being was some gases are heavier then others...CO2 being the most obvious, and on many occasions refrigerated containers, containing "dry ice" [CO2] would be loaded/transferred by the LTV's [load transfer vehicles]...The ice would often drop into the pits/wells underneath the scissor transfer mechanism, so creating a very real hazard".

This was when I worked for QANTAS and obviously as the dry ice sublimed, the lighter air was displaced by the CO2, creating the hazard mentioned. On one occasion a foolish workmate, who had done the maintenance on these LTV's so often, became complacent and failed to test before entering. It was then changed to a rescue exercise when he collapsed at the bottom of the pit. 

[StringJunky]

2 minutes ago, beecee said:

yes, I did post this earlier...."Air weight as well is confirmable. When I wore a younger man's clothes and was part of the work force, I always needed to get a "closed vessel clearance" when entering a pit or LTV well...reason being was some gases are heavier then others...CO2 being the most obvious, and on many occasions refrigerated containers, containing "dry ice" [CO2] would be loaded/transferred by the LTV's [load transfer vehicles]...The ice would often drop into the pits/wells underneath the scissor transfer mechanism, so creating a very real hazard".

This was when I worked for QANTAS and obviously as the dry ice sublimed, the lighter air was displaced by the CO2, creating the hazard mentioned. On one occasion a foolish workmate, who had done the maintenance on these LTV's so often, became complacent and failed to test before entering. It was then changed to a rescue exercise when he collapsed at the bottom of the pit. 

If there was no weather, the gases would stratify according to their respective masses.

Edited November 9, 2018 by StringJunky

[AEBanner]

6 hours ago, swansont said:

I don't know how to be clearer. The standard physics explanation is that the pressure is equal to the weight of the molecules and the mechanism is molecular collisions. Both of these are part of the explanation. It is not one or the other.

I have tried to find published work on the internet for the standard physics explanation for atmospheric pressure.  There is very little, basically Wiki and National Geographic websites.  The standard explanation seems to be that the cause is the weight of the atmosphere. But no attempt is made to explain how this weight is applied to the Earth's surface.  And that is my problem.

So I should really be grateful if you could please provide me with chapter and verse references for the standard physics  explanation.

As far as I can understand this idea, the weights of the individual air molecules combine somehow, and so the total weight then acts upon the surface.

I simply cannot see this, because the molecules are totally independent, they are "up in the air", and not in contact with the surface.

As an example, consider a man standing on bathroom scales on the Earth's surface.  His weight is applied through his feet to the scales, and the scales show his weight.  Now suppose he jumps straight up into the air.  Clearly, he still has the same weight, but it is not connected to the scales, so they show no weight.  So contact with the surface is required if pressure is to be exerted on the surface.

 

 

 

[beecee]

13 minutes ago, AEBanner said:

But no attempt is made to explain how this weight is applied to the Earth's surface.  And that is my problem.

Weight is the force of gravity  acting on objects/molecules etc. The Earth is pulling on all the atmosphere while at the same time, the molecules of the gases in the atmosphere are creating pressure while jiggling. Both play a part, and both are relevant. 

Edited November 9, 2018 by beecee

[AEBanner]

9 hours ago, swansont said:

That pressure is solely due to weight (collisions playing no part), and exerted only in a downward direction. That seems to be your assertion.

No, that is not my assertion.  I believe that the pressure is caused by molecular collisions.

If the explanation for atmospheric pressure was that the weight of the molecules presses down on the Earth's surface due to gravity, then the resulting pressure could only act vertically downwards, and so there would be no pressure sideways or any other direction, which is clearly not the case.  So it would seem that atmospheric pressure is due to molecular collisions, which is my belief.

[Strange]

20 minutes ago, AEBanner said:

I have tried to find published work on the internet for the standard physics explanation for atmospheric pressure.  There is very little, basically Wiki and National Geographic websites. 

Maybe because you made it up?

21 minutes ago, AEBanner said:

So I should really be grateful if you could please provide me with chapter and verse references for the standard physics  explanation.

You are the one claiming that the "standard physics explanation" is that pressure is just caused by weight. If you can't provide a reference to it, maybe it doesn't exist.

20 minutes ago, AEBanner said:

I simply cannot see this, because the molecules are totally independent, they are "up in the air", and not in contact with the surface.

You know how you said that pressure is due to the collisions of the molecules?

They also collide with each other and so are not independent.

And these collisions are how they are in contact with the surface.

It's really not that hard.

 

4 minutes ago, AEBanner said:

So it would seem that atmospheric pressure is due to molecular collisions, which is my belief.

How about: weight transferred by molecular collisions?

[AEBanner]

7 minutes ago, beecee said:

Weight is the force of gravity  acting on objects/molecules etc. The Earth is pulling on all the atmosphere while at the same time, the molecules of the gases in the atmosphere are creating pressure while jiggling. Both play a part, and both are relevant. 

So far, so good.  But how is that pressure exerted on the surface except by molecular collisions?

[beecee]

3 minutes ago, AEBanner said:

If the explanation for atmospheric pressure was that the weight of the molecules presses down on the Earth's surface due to gravity, then the resulting pressure could only act vertically downwards, and so there would be no pressure sideways or any other direction, which is clearly not the case.  So it would seem that atmospheric pressure is due to molecular collisions, which is my belief.

The atmospheric pressure to to molecular collisions act in all directions. The weight of the molecules in those collisions act downwards via gravity, towards the Earth's surface and centre.

[Strange]

1 minute ago, AEBanner said:

So far, so good.  But how is that pressure exerted on the surface except by molecular collisions?

It is exerted by collisions.

1 minute ago, beecee said:

The atmospheric pressure to to molecular collisions act in all directions. The weight of the molecules in those collisions act downwards via gravity, towards the Earth's surface and centre.

And the random collisions cause it to act like a fluid so the force can be exerted in all directions.

[beecee]

1 minute ago, AEBanner said:

So far, so good.  But how is that pressure exerted on the surface except by molecular collisions?

The same way as gravity acts on a bird flying, or on a plane.

[studiot]

6 minutes ago, AEBanner said:

So far, so good.  But how is that pressure exerted on the surface except by molecular collisions?

 

I don't see how you expect to learn anything if you refuse to engage in discussion.

Post reported.

 

To add to the questions you refuse to answer.

1) What stops the atmosphere evaporating off into space?

2) What is weight?

3) How does the impact of molecule on molecule affect the surface of the Earth?

 

 

 

[AEBanner]

3 minutes ago, Strange said:

26 minutes ago, AEBanner said:

I have tried to find published work on the internet for the standard physics explanation for atmospheric pressure.  There is very little, basically Wiki and National Geographic websites. 

Maybe because you made it up?

26 minutes ago, AEBanner said:

So I should really be grateful if you could please provide me with chapter and verse references for the standard physics  explanation.

You are the one claiming that the "standard physics explanation" is that pressure is just caused by weight. If you can't provide a reference to it, maybe it doesn't exist.

Oh, please!

I would never claim to have made up the standard physics explanation for atmospheric pressure.  This was first raised by swansont and I was asking him for help in finding appropriate references.

What I said was in the references I found on the internet, the main explanation they gave was that of the weight.

[Strange]

6 minutes ago, AEBanner said:

I would never claim to have made up the standard physics explanation for atmospheric pressure.

Your entire argument has been that the standard explanation is weight, and that you think this is wrong. This started with the first sentence of your first post. 

Well, it is wrong. And it is not the standard physics explanation.

However, it is not "just collisions" either.

6 minutes ago, AEBanner said:

What I said was in the references I found on the internet, the main explanation they gave was that of the weight.

Then find some better sources. As you have ignored all the explanations in this thread, I wouldn't be surprised if you have come across more accurate explanations but decided to ignore them.

Maybe read the explanations in this thread before asking again "but how can this work".