Cause of Atmospheric Pressure

[AEBanner]

16 minutes ago, Strange said:

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.

The collisions between molecules do not transfer mass from one to another, only momentum.

And YES !!!, some collide with the surface and are reflected, and the change in momentum provides an impulse.  When integrated over the surface for all the impacting molecules, this creates the atmospheric pressure.  Its easy.

 

[Strange]

6 minutes ago, AEBanner said:

The collisions between molecules do not transfer mass from one to another, only momentum.

Who said they transfer mass? You do know weight and mass are different things, don't you?

Quote

And YES !!!, some collide with the surface and are reflected, and the change in momentum provides an impulse.  When integrated over the surface for all the impacting molecules, this creates the atmospheric pressure.  Its easy.

Good, you seem to be getting there. Now, why does pressure depend on altitude? And why does it depend on the density of the gas? Do you think it could be the weight of the gas above?

Remember, weight is a force. And pressure is force per unit area. And force as you correctly say is transferred by collisions (both between molecules and to the surface).

[studiot]

22 minutes ago, AEBanner said:

This was first raised by swansont and I was asking him for help in finding appropriate references.

 

Why waste swansont's time when you have refused to acknowledge the standard Physics expalantion I wrote out for you, complete with calculations?

I further state categorically that the atmosphere's pressure on the surface of the Earth can be directly linked to the weight of the air molecules and would be zero if they had no weight.

What a pity you don't want to know.

 

https://www.google.co.uk/search?source=hp&ei=xwDmW77IEsuWgAaS75PAAg&q=kinks+tired+of+waiting+youtube&oq=kinks+tired+of+waiting&gs_l=psy-ab.1.1.0l4j0i22i30k1l6.1568.6112.0.8102.22.21.0.1.1.0.224.2536.0j16j2.18.0....0...1c.1.64.psy-ab..3.19.2554...0i131k1.0.ztboyc68TiQ

 

[AEBanner]

42 minutes ago, Strange said:

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.

[Please note that this response is meant to replace a recent one where I made a mistake with the attribution.]

 

The collisions between molecules do not transfer mass from one to another, only momentum.

And YES !!!, some collide with the surface and are reflected, and the change in momentum provides an impulse.  When integrated over the surface for all the impacting molecules, this creates the atmospheric pressure.  Its easy.

[Strange]

2 minutes ago, AEBanner said:

[Please note that this response is meant to replace a recent one where I made a mistake with the attribution.]

How about answering people's questions instead of wasting time like this.

[AEBanner]

6 minutes ago, studiot said:

Why waste swansont's time when you have refused to acknowledge the standard Physics expalantion I wrote out for you, complete with calculations?

I further state categorically that the atmosphere's pressure on the surface of the Earth can be directly linked to the weight of the air molecules and would be zero if they had no weight.

What a pity you don't want to know.

Thank you for response and for your efforts on my behalf.  But to be fair, I really do want to know; it is simply a case of finding the required time when so many posts need answering.

[Strange]

1 minute ago, AEBanner said:

But to be fair, I really do want to know; it is simply a case of finding the required time when so many posts need answering.

So is that the problem, you are not bothering to read any explanations, just posting the same replies without even thinking?

[studiot]

21 minutes ago, AEBanner said:

Thank you for response and for your efforts on my behalf.  But to be fair, I really do want to know; it is simply a case of finding the required time when so many posts need answering.

So why did you waste your time and mine with this reply, instead of answering the very simple GCSE  questions for 15/16 year olds I asked?

Was it you complained of a wind up?

Edited November 9, 2018 by studiot

[StringJunky]

@AEBanner This is where the weight of a gas comes from, that I've gathered from the discussion link following it: Picture a box with a molecule inside going up and down, hitting the top and bottom. Every time the molecule hits the surface there is a change of momentum, which exerts a force. The force at the top is less than the force at bottom because the molecule’s velocity is increased by gravity as it travels down, so the force exerted on the bottom is greater than the force exerted on the top. There is thus a net force on the bottom. This difference is the weight of the molecule. https://physics.stackexchange.com/questions/280282/why-do-gases-have-weight

Edited November 9, 2018 by StringJunky

[Strange]

7 minutes ago, StringJunky said:

@AEBanner This is where the weight of a gas comes from, that I've gathered from the discussion link following it: Picture a box with a molecule inside going up and down, hitting the top and bottom. Every time the molecule hits the surface there is a change of momentum, which exerts a force. The force at the top is less than the force at bottom because the molecule’s velocity is increased by gravity as it travels down, so the force exerted on the bottom is greater than the force exerted on the top. There is thus a net force on the bottom. This difference is the weight of the molecule. https://physics.stackexchange.com/questions/280282/why-do-gases-have-weight

Excellent visualisation.

Now all @AEBanner needs to understand is that for most molecules, the upper and lower surface will be other molecules. So the momentum/force from all the molecules above is transferred down (and accumulated at each downward bounce) until it reaches the Earth.

[StringJunky]

5 minutes ago, Strange said:

Excellent visualisation.

Now all @AEBanner needs to understand is that for most molecules, the upper and lower surface will be other molecules. So the momentum/force from all the molecules above is transferred down (and accumulated at each downward bounce) until it reaches the Earth.

I think it's a kind of momentum diffusion when lots of molecules are involved, although strictly it applies to fluids in the explanations I've seen.

Edited November 9, 2018 by StringJunky

[Strange]

2 minutes ago, StringJunky said:

I think it's a kind of momentum diffusion when lots of molecules are involved, although strictly it applies to fluids in the explanations I've seen.

And the other definition of pressure is momentum flux, which makes this even clearer.

[beecee]

21 minutes ago, StringJunky said:

@AEBanner This is where the weight of a gas comes from, that I've gathered from the discussion link following it: Picture a box with a molecule inside going up and down, hitting the top and bottom. Every time the molecule hits the surface there is a change of momentum, which exerts a force. The force at the top is less than the force at bottom because the molecule’s velocity is increased by gravity as it travels down, so the force exerted on the bottom is greater than the force exerted on the top. There is thus a net force on the bottom. This difference is the weight of the molecule. https://physics.stackexchange.com/questions/280282/why-do-gases-have-weight

10 minutes ago, Strange said:

Excellent visualisation.

Now all @AEBanner needs to understand is that for most molecules, the upper and lower surface will be other molecules. So the momentum/force from all the molecules above is transferred down (and accumulated at each downward bounce) until it reaches the Earth.

Eloquently, beautifully and admirably put by both parties! A joy to behold!! 

 

[StringJunky]

5 minutes ago, Strange said:

And the other definition of pressure is momentum flux, which makes this even clearer.

You made me look that up. That makes good sense.

Edited November 9, 2018 by StringJunky

[swansont]

2 hours ago, AEBanner said:

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

Your assertion about the weight of molecules.

2 hours 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. 

You say that's not your assertion, and then go on to say exactly that.

I will repeat: that is NOT what physics claims. You are misunderstanding the model, or are just making this up.

2 hours ago, AEBanner said:

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

It's collisions, and the weight of the atmosphere 

[studiot]

Swansont has mentioned several times that the kinetic theory is the microscopic mechanism and simple newtonian mechanics the bulk theory of fluids.

Bearing this in mind we should not let visions of bonny bouncing molecules run away with us responding to the OP question which was about fluid statics, not fluid dynamics.

In particular care is needed in considerations of momentum and momentum flux. Momentum flux refers to fluids moving in the bulk and shear stresses.

There are no shear stresses in fluid statics, so the OP question was about normal stresses only.

Aligning  the kinetic theory with fluid statics is actually quite tricky, in some ways more tricky than for fluid dynamics because of this.

So we need to take care not to offer confusion to the OP on this matter.

[AEBanner]

16 hours ago, studiot said:

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.

Thank you for an excellent post.

You have solved my misunderstanding for me, and I am truly grateful.  I now see clearly where I went wrong in trying to deal with the mass of the molecules.  Thank you, too, to everybody else for their help, and I am pleased to say I no longer need references to standard work on this matter.

But, I now have a new problem.  I thought that pressure at any point in a gas acts equally in all directions.  The analysis above deals with the downward effects of gravity, so what happens "sideways"?  

 

 

 

14 hours ago, Strange said:

So is that the problem, you are not bothering to read any explanations, just posting the same replies without even thinking?

Please refer to my very recent reply to studiot

[Strange]

9 minutes ago, AEBanner said:

But, I now have a new problem.  I thought that pressure at any point in a gas acts equally in all directions.  The analysis above deals with the downward effects of gravity, so what happens "sideways"?  

There is a simplification in the descriptions so far, that the forces only act vertically. As the collisions are random and the molecules can be treated as (crudely) spherical. This means that when the molecules collide, they will bounce of in random directions. This distributes the momentum (and therefore force) in all directions. This is what makes gases and liquids a fluid.

[studiot]

14 minutes ago, AEBanner said:

Thank you for an excellent post.

You have solved my misunderstanding for me, and I am truly grateful.  I now see clearly where I went wrong in trying to deal with the mass of the molecules.  Thank you, too, to everybody else for their help, and I am pleased to say I no longer need references to standard work on this matter.

But, I now have a new problem.  I thought that pressure at any point in a gas acts equally in all directions.  The analysis above deals with the downward effects of gravity, so what happens "sideways"?  

Yippee, progress. Another +1 for that.

But please let us have some more feedback for this and any further questions.

You say you understood the mathematical development. This is good as I don't care what  level you are at, I just want to post that that level.

Did you also understand the later post about the difference between fluid statics and fluid dynamics and in particular the bit about shear and normal stresses?

If I talk about random walks for molecules would you have heard of these?

Finally I note that a few posts back you correctly noted that the conditions for momentum to change, resulting in a force, is when a molecule is what you called reflected ie bounces back from a boundary.

 

 

[AEBanner]

On 11/8/2018 at 7:52 PM, 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.

 

On 11/8/2018 at 7:52 PM, studiot said:

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

Of course, you are correct, it does apply.

16 hours ago, studiot said:

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?

 

Sorry, I must have missed these questions.  My answers are below.

1) Gravitational force of attraction of the Earth

2)Mass*Gravitational Force

3)When a molecule of air impacts upon the Earth's surface, it is "reflected" off again.  There is a change of momentum in this collision, which applies an impulse to the Earth's surface, that is (force*time). When summed for all the impacting molecules over a specified area, we get the total force on that area, and hence the pressure.

OK, ? 

[StringJunky]

3 hours ago, studiot said:

Swansont has mentioned several times that the kinetic theory is the microscopic mechanism and simple newtonian mechanics the bulk theory of fluids.

Bearing this in mind we should not let visions of bonny bouncing molecules run away with us responding to the OP question which was about fluid statics, not fluid dynamics.

In particular care is needed in considerations of momentum and momentum flux. Momentum flux refers to fluids moving in the bulk and shear stresses.

There are no shear stresses in fluid statics, so the OP question was about normal stresses only.

Aligning  the kinetic theory with fluid statics is actually quite tricky, in some ways more tricky than for fluid dynamics because of this.

So we need to take care not to offer confusion to the OP on this matter.

That was covered in the discussion I linked: "Note also that this difference in molecular speeds at the top and the bottom manifests itself as a difference in pressure between the top and bottom of the container, as seen by the pressure-depth relationship for a static fluid: ΔP=ρgΔh. – Michael Seifert"

[AEBanner]

2 hours ago, studiot said:

Yippee, progress. Another +1 for that.

But please let us have some more feedback for this and any further questions.

You say you understood the mathematical development. This is good as I don't care what  level you are at, I just want to post that that level.

Did you also understand the later post about the difference between fluid statics and fluid dynamics and in particular the bit about shear and normal stresses?

If I talk about random walks for molecules would you have heard of these?

Finally I note that a few posts back you correctly noted that the conditions for momentum to change, resulting in a force, is when a molecule is what you called reflected ie bounces back from a boundary.

Thank you for your friendly and helpful response.  I fully understand the reason for your questions above.  Here is my feedback. 

I am a (very) retired physicist. I received my degree in 1959; and perhaps this partly explains some of my posts.

I have no experience with fluid statics or fluid dynamics. 

I am only slightly aware of the random walks ideas.

[studiot]

35 minutes ago, StringJunky said:

That was covered in the discussion I linked: "Note also that this difference in molecular speeds at the top and the bottom manifests itself as a difference in pressure between the top and bottom of the container, as seen by the pressure-depth relationship for a static fluid: ΔP=ρgΔh. – Michael Seifert"

This is why I advised caution.

That equation is incorrect for the atmosphere.

The link you gave has some good replies and some dubious ones like the one due to Seifert.

 

[StringJunky]

32 minutes ago, studiot said:

This is why I advised caution.

That equation is incorrect for the atmosphere.

The link you gave has some good replies and some dubious ones like the one due to Seifert.

 

He's a physics professor. It was likely a perfectly sufficient answer for the level of the OP.  You can rarely be strictly correct with a neophyte because you might bog them down.

Edited November 10, 2018 by StringJunky

[studiot]

Damn and Blast this site editor.

I just lost a significant post it didn't save, like it used to.

I will try to reproduce it, but I fear the result won't be half as good.

42 minutes ago, AEBanner said:

Thank you for your friendly and helpful response.  I fully understand the reason for your questions above.  Here is my feedback. 

I am a (very) retired physicist. I received my degree in 1959; and perhaps this partly explains some of my posts.

I have no experience with fluid statics or fluid dynamics. 

I am only slightly aware of the random walks ideas.

Thank you. A decade (and a bit) before me. Wow.
Perhaps you used C J Smith " A Degree Physics" ?

I'm sure you know quite a bit about the principles then, even though the connections may be a bit rusty.

Fluid statics and dynamcs have the same difference as other parts of mechanics.

They use different equations.

Fluid statics uses the principles and equations of equilibrium (dynamical equilibrium) for fluids.

Dynamics uses Newton's laws along with the conservations laws of momentum and energy.

 

The trick with the analysis of the air column is to know when to use a plane section and when to use a differential section to get volume.
I will come back to that.

 

Meanwhile I note you have posted in Earth Science not Physics, so is your interest in this really in Earth Science?

Here is an extract from a brilliant book about this from an Earth Science perspective with a much more detailed presentation than my scribbling and provides an explanation (compressibility I took for granted) along with an explanation of why Seifert is just plain wrong.

Atmoshere and Ocean - Our Fluid Environments

J G Harvey

 

 

 

 

I don't care if he is God Almighty that was bad Physics in reply to a question about the atmosphere.

24 minutes ago, StringJunky said:

He's a physics professor. It was likely a perfectly sufficient answer for the level of the OP.