Cause of Atmospheric Pressure

[AEBanner]

There seems to be a fairly general acceptance of the idea that atmospheric pressure is caused by the weight of the molecules in the atmosphere.  This idea is supported even by Wikipedia.  I am not convinced that this is true.  I think that atmospheric pressure is caused by "air" molecules colliding with the Earth's surface.  This is not a new idea, of course, but it may not be widely held.  If true, however, surely the "air has weight" explanation should be put to rest, as in Wiki and other websites.

If I am wrong in believing the "molecular collisions" explanation, I should welcome comments that would put me right.

However, in order to support my rejection of the "air has weight" theory, please consider the following thought experiment.  

According to this idea, all the air molecules are subjected to the force of gravity, and so the weight forces of the molecules act vertically downwards towards the centre of the Earth.  So far, so good.

Now, suppose a smooth, plane surface is placed on the Earth's surface, but at an angle to the surface.  This plane will intercept the downward weight forces of the molecules, and so will cause a sideways force as well a a reduced vertically downwards force, by a simple resolution of forces. In the absence of friction, the inclined plane will then move sideways,  and, hey, we've got a way of getting energy from the atmosphere!!   I wish.

This also begs another question about the "air has weight theory, but maybe better to leave this for another time. 

 

[StringJunky]

Air pressure is the no.of molecules colliding with a surface in a unit of time. It's the velocity of the molecules that influences air pressure. Gravity just limits their trajectory, dependent on their mass and velocity

Edited November 7, 2018 by StringJunky

[AEBanner]

Many thanks.  That is in line with my thinking

 

[swansont]

53 minutes ago, AEBanner said:

Now, suppose a smooth, plane surface is placed on the Earth's surface, but at an angle to the surface.  This plane will intercept the downward weight forces of the molecules, and so will cause a sideways force as well a a reduced vertically downwards force, by a simple resolution of forces. In the absence of friction, the inclined plane will then move sideways,  and, hey, we've got a way of getting energy from the atmosphere!!   I wish.

What about the molecules below the surface?

[Strange]

47 minutes ago, AEBanner said:

There seems to be a fairly general acceptance of the idea that atmospheric pressure is caused by the weight of the molecules in the atmosphere.  This idea is supported even by Wikipedia.  I am not convinced that this is true.  I think that atmospheric pressure is caused by "air" molecules colliding with the Earth's surface.  This is not a new idea, of course, but it may not be widely held.  If true, however, surely the "air has weight" explanation should be put to rest, as in Wiki and other websites.

As far as I can see, it is both of these. It can't just be the air molecules colliding with the Earth's surface (or any other surface, come to that). Otherwise, why would the pressure be different at the top of a mountain? Air obviously does have weight (see also: hot-air/helium balloons). And remember that the air molecules collide with one another as well, and the weight of the air is what stops it flying off to space (and means that the pressure is higher at low altitudes).

 

[swansont]

We know that water pressure in a tube is due to the weight of the column. That's easily confirmable. Why should air behave differently?

[beecee]

1 hour ago, swansont said:

We know that water pressure in a tube is due to the weight of the column. That's easily confirmable. Why should air behave differently?

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.

[AEBanner]

14 hours ago, swansont said:

What about the molecules below the surface?

According to the "air has weight" theory, the weight of molecules below the inclined plane do not act upon the plane, and so they are not relevant to this thought experiment.

[Strange]

13 minutes ago, AEBanner said:

According to the "air has weight" theory, the weight of molecules below the inclined plane do not act upon the plane, and so they are not relevant to this thought experiment.

Imagine a fluid made of (frictionless) spheres. The weight pushing down would also push back up on the other side of the plane. And this is what happens with water or air. The fact that the collisions between the molecules in these fluids cause them to have less density is not really relevant. 

[AEBanner]

12 minutes ago, Strange said:

Imagine a fluid made of (frictionless) spheres. The weight pushing down would also push back up on the other side of the plane. And this is what happens with water or air. The fact that the collisions between the molecules in these fluids cause them to have less density is not really relevant. 

Here you are involving the "molecular collision" ideas and not the simple "air has weight" theory, which is what the thought experiment is about. 

[swansont]

26 minutes ago, AEBanner said:

According to the "air has weight" theory, the weight of molecules below the inclined plane do not act upon the plane, and so they are not relevant to this thought experiment.

No, that's not correct. You are citing a non-existent theory, aka a straw man.

The physical mechanism of pressure is molecular collisions. The amount of atmospheric pressure is due to the weight of the column of air. Both are part of the standard physics explanation.

As has been pointed out, we know air has weight. Blow up a balloon or pump up a  ball or a tire. Compare the weight before and after. Gases have mass, and therefore they have weight. That's a known.

[Strange]

6 minutes ago, AEBanner said:

Here you are involving the "molecular collision" ideas and not the simple "air has weight" theory, which is what the thought experiment is about. 

Then why would the balls press down if not because of their weight and the weight of those above?

Obviously, if the balls /molecules did not push against / collide with each other then there would be no pressure. 

[DrP]

Is it worth pointing out  PV = NRT.  ?

I always saw pressure as the weight of the molecules pushing down....  I have always associated the speed of the molecules and their collisions with temperature rather than pressure - which from PV=NRT would increase the pressure anyway from the ideal gas relationship.

Like - if I was in a tower full of plastic balls above me - I would feel more pressure the more balls loaded on - the 'speed' of the balls would be zero and wouldn't be relevant but there would still be pressure.   As molecules are not stationary in the air due to them having energy and moving around this probably adds to the pressure from weight alone. Higher temperature means more energy and faster moving particles...  and by PV=NRT more pressure too.    In the case of the plastic balls in the tower - imagine they were all battery powered vibrating balls  -  the experience would be more uncomfortable the more balls there are as well as if they were vibrating faster with it.

42 minutes ago, Strange said:

Obviously, if the balls /molecules did not push against / collide with each other then there would be no pressure

In my ball example they have no motion - it is purely the weight that would give the pressure...  add in motion on top of that and it increases.    Sorry if this is superfluous - I thought it might be worth pointing out this in respect to PV = NRT.

[Strange]

19 minutes ago, DrP said:

In my ball example they have no motion - it is purely the weight that would give the pressure...  add in motion on top of that and it increases.    Sorry if this is superfluous - I thought it might be worth pointing out this in respect to PV = NRT.

Exactly. The fact that molecules in a gas bounce off each other has exactly the same effect as the balls in your (or my) example constantly pushing one another. The collisions are a "push"; just a way of transferring the force from one to another.

So it is the weight that determines how much pressure there is, but it is the collisions that allow the pressure to be transferred as a force.

[swansont]

1 hour ago, DrP said:

Is it worth pointing out  PV = NRT.  ?

I always saw pressure as the weight of the molecules pushing down....  I have always associated the speed of the molecules and their collisions with temperature rather than pressure - which from PV=NRT would increase the pressure anyway from the ideal gas relationship.

PV = NRT works when you have decoupled from a system exposed to atmosphere, too. In a rigid container, for example, where volume is constant — pressure and temperature changes will be tied together, or you could increase the pressure by pumping more gas into the container.

Another problem with the naive "air has weight" model described earlier is that it does not explain why there is air pressure on the sides of a container. 

1 hour ago, DrP said:

Like - if I was in a tower full of plastic balls above me - I would feel more pressure the more balls loaded on - the 'speed' of the balls would be zero and wouldn't be relevant but there would still be pressure.   As molecules are not stationary in the air due to them having energy and moving around this probably adds to the pressure from weight alone. Higher temperature means more energy and faster moving particles...  and by PV=NRT more pressure too.    In the case of the plastic balls in the tower - imagine they were all battery powered vibrating balls  -  the experience would be more uncomfortable the more balls there are as well as if they were vibrating faster with it.

In my ball example they have no motion - it is purely the weight that would give the pressure...  add in motion on top of that and it increases.    Sorry if this is superfluous - I thought it might be worth pointing out this in respect to PV = NRT.

Another thing to point out is that even though the pressure is from the weight of the air in the column above you, it's only a small fraction of the molecules that actually collide with you. But they get their energy and momentum from other collisions, all with a downward bias, owing to gravity. So it's not weight plus the collisions. The collisions have their impact because of the weight. The weight tells you what the pressure is, the impacts are the mechanism for exerting it.

[studiot]

19 hours ago, AEBanner said:

There seems to be a fairly general acceptance of the idea that atmospheric pressure is caused by the weight of the molecules in the atmosphere.  This idea is supported even by Wikipedia.  I am not convinced that this is true.  I think that atmospheric pressure is caused by "air" molecules colliding with the Earth's surface.  This is not a new idea, of course, but it may not be widely held.  If true, however, surely the "air has weight" explanation should be put to rest, as in Wiki and other websites.

 

Yes one simple model involves the use of the weight of a fluid column.

So What?

One simple model of the Earth, universally used in navigation and cartography, invoves the use of a sphere.

Neither models are perfect and for the atmosphere that model is particularly defective and difficult to work with.
The weight of a fluid column model is much more suited to incompressible fluids like water and shorter columns.

More sophisticated models acknowlege that the particles of the fluid have mass and momentum.
And, as you (nearly) say, the static pressure is the result of molecular collisions with the fluid boundaries.
But even these are not perfect.
 

 

[AEBanner]

5 hours ago, swansont said:

No, that's not correct. You are citing a non-existent theory, aka a straw man.

The physical mechanism of pressure is molecular collisions. The amount of atmospheric pressure is due to the weight of the column of air. Both are part of the standard physics explanation.

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

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.  

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. 

 

[Strange]

1 minute ago, AEBanner said:

But I find it difficult to understand how the weight of molecules throughout the height of the air column can affect the surface.  

Then why does the pressure change with altitude?

Quote

What is the mechanism? 

The collision of molecules 

4 minutes ago, AEBanner said:

Sorry, but what is this non-existent theory

That pressure is caused by weight (alone).

[AEBanner]

2 hours ago, studiot said:

Yes one simple model involves the use of the weight of a fluid column.

So What?

I have tried to show that this "air has weight" theory for atmospheric pressure is wrong and should be deleted from wherever it appears.

[StringJunky]

11 minutes ago, AEBanner said:

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

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.  

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. 

 

The higher molecules increase the incidence of the lower molecules colliding back with the earth's surface gain, thus increasing the pressure on it.  I think the trajectory of air molecules will always end up downwards due to gravity, in the absence of collisions.

 

Edited November 8, 2018 by StringJunky

[AEBanner]

5 minutes ago, Strange said:

Then why does the pressure change with altitude?

Simply because the density of the air decreases with altitude, and so there are fewer molecules, and therefore fewer collisions.

2 minutes ago, StringJunky said:

The higher molecules increase the incidence of the lower molecules colliding back with the earth's surface gain, thus increasing the pressure on it. 

You seem to be talking about the molecular collision theory here, which has nothing to do with the "air has weight" dilemma I have to explain how the molecules up in the air can "communicate" their weights to the Earth's surface.

16 minutes ago, Strange said:

That pressure is caused by weight (alone).

 

12 minutes ago, StringJunky said:

The higher molecules increase the incidence of the lower molecules colliding back with the earth's surface gain, thus increasing the pressure on it.

I think I've already covered this in my other responses.

[StringJunky]

17 minutes ago, AEBanner said:

You seem to be talking about the molecular collision theory here, which has nothing to do with the "air has weight" dilemma I have to explain how the molecules up in the air can "communicate" their weights to the Earth's surface.

Transfer of momentum is the 'information'.

[studiot]

21 minutes ago, AEBanner said:

I have tried to show that this "air has weight" theory for atmospheric pressure is wrong and should be deleted from wherever it appears.

 

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?

[StringJunky]

1 minute 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?

All the molecules in the brick are in a rigid formation. He can't see how an unconnected column of air molecules give a combined pressure (weight) on the Earth.

[Strange]

33 minutes ago, AEBanner said:

Simply because the density of the air decreases with altitude

Why?

8 minutes ago, StringJunky said:

All the molecules in the brick are in a rigid formation. He can't see how an unconnected column of air molecules give a combined pressure (weight) on the Earth.

As I and others have said, it is because the air molecules collide with one another (and with the Earth and everything else around).

AEBanner hasn't explained why this isn't explanation isn't acceptable (even though it also seems to be the explanation he is providing - go figure).

 

38 minutes ago, AEBanner said:

You seem to be talking about the molecular collision theory here, which has nothing to do with the "air has weight" dilemma I have to explain how the molecules up in the air can "communicate" their weights to the Earth's surface.

The collisions transfer the weight of the air.