Cause of Atmospheric Pressure

[AEBanner]

17 hours ago, mistermack said:

Because of the constant movement of molecules, the pressure at the bottom operates in all directions, but all lateral pressure is balanced by the equal and opposite pressure of the surrounding air.

Thank you for your contribution.

[Strange]

On 12/11/2018 at 12:17 PM, AEBanner said:

The horizontal pressure problem has gone away.

I realized a few hours ago that there are no such pressures in the atmosphere, of course, provided there is no wind.

Err, what!?

No. Just no.

There is no wind inside the room I am in (or on the International Space Station, if you are going to claim it is the wind outside the building) and yet the air exerts pressure in all directions.

Air is a fluid. The constant collisions between molecules distributes the momentum (and therefore pressure) in all directions.

[DrP]

On ‎08‎/‎11‎/‎2018 at 3:37 PM, swansont said:

In a rigid container, for example, where volume is constant

Yea - I know it is for the ideal case, sorry I might not have been clear. I mentioned it because it shows a relationship between molecular movement (temperature) and pressure.

17 minutes ago, Strange said:

yet the air exerts pressure in all directions.

... This box in space brings me back to PV=NRT.   No gravity, but pressure on the walls of the box from the molecular collisions of the air. Increase the temperature and the molecules move faster and impact the walls harder with more force and thus more pressure. It shows that as temperature increases (and the molecules move and vibrate faster) then so does the pressure.

[studiot]

I have been away for a few day, but i see this topic is still live.

The analysis I gave was a classical one using macroscopic variables and what engineers call a 'control volume' which in the limiting case can be integrated.

If you wish to construct a kinetic model then instead of using a control volume, you need to use separating or cutting planes (splitting the column into two 'free bodies') and the equilibrium constraint condition.

So consider a horizontal cutting plane anywhere across a colum in equilibrium.

Since the column is in equilibrium there is no change of composition with time.
Time is then the variable of integration.

This means that

The time average number of molecules passing upwards must be equal to the time average number of molecules passing downwards through the section.

If this were not so then gas would accumulate on one side or the other of the cut plane.

Since in the simple model all molecules have the same mass, this can be related to the momentum or kinetic energy concerned.

 

The horizontal section is almost the simplest, if we now consider a vertical section we can perform the same analysis.

Finally, as is so often the case in this type of analysis we need to consider slant sections at any angle between horizontal and vertical.

[AEBanner]

5 hours ago, Strange said:

There is no wind inside the room I am in (or on the International Space Station, if you are going to claim it is the wind outside the building) and yet the air exerts pressure in all directions.

Air is a fluid. The constant collisions between molecules distributes the momentum (and therefore pressure) in all directions.

I am a little puzzled by your response above.

Of course, your statement in itself is correct, but I was referring to the pressures acting horizontally from the vertical column of air, as discussed in the treatment given by studiot.  Two or more such adjacent columns would, presumably, have equivalent horizontal pressures, which would cancel out, so making no overall horizontal pressure within the body of the gas. 

I think you may not have understood what I was saying.

On 11/11/2018 at 8:53 PM, swansont said:

I don't see what the problem is. The center of mass doesn't move in the transverse direction. (Momentum, of course, is not conserved in the z direction, as there is a net force)

Very good.  Thank you for contribution.

Thank you  studiot  for your recent contribution.

Edited November 13, 2018 by AEBanner
comment in wrong post.

[Strange]

21 minutes ago, AEBanner said:

I think you may not have understood what I was saying.

You may be right although I don't see where wind comes into it.

[AEBanner]

On 11/13/2018 at 7:57 PM, Strange said:

You may be right although I don't see where wind comes into it.

It doesn't.

[studiot]

On 13/11/2018 at 7:35 PM, AEBanner said:

Of course, your statement in itself is correct, but I was referring to the pressures acting horizontally from the vertical column of air, as discussed in the treatment given by studiot.  Two or more such adjacent columns would, presumably, have equivalent horizontal pressures, which would cancel out, so making no overall horizontal pressure within the body of the gas.

One point I need to bring out more clearly is the difference between the treatment of collisions of gas molecules with a boundary and internal collisions between molecules of gas, in the kinetic theory.

Collisions with a boundary are much easier to treat and lead directly to Boyle's Law.
This is because boundaries are considered not to move during the collision, thus making the calculation of change of momentum particularly simple.

On the other hand, inter molecular collisions can take a variety of histories from full head on to grazing incidence and everything in between.
Both molecules are affected by the collision and the calculation of momentum depends upon this.
Thus some kind of 'average' collision need to be introduced within the body of the gas.

[tinkerer]

On 11/7/2018 at 3:26 PM, 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?

Both are "fluids", which classically behave predictably.