# Question Atmospheric pressure

1. Jul 31, 2008

### JHarris

In Physics classes at college I was taught that atmospheric pressure is due to the weight of the air above a body. I understand this and all related equations and concepts.

But just the other day I was reading up on gas laws in a chemistry book. In that book it stated that atmospheric pressure was due to the "kinetic theory of gases". That is atmospheric pressure is caused by the collisions of air molecules with objects, exerting a force (and subsequentially a pressure) on a body. (In the same way that gas exerts pressure on the walls of a propane container). Clearly this contradicts what I was taught at college.

The question is "What acctually is the cause of atmospheric pressure ... weight of air or kinetic theory?"

Also if kinetic theory is not the cause of atmospheric pressure how does it apply to gas in a container and why does it apply only to contained gas and not gas in the atmosphere?

Jonny.

2. Jul 31, 2008

### uart

No, it doesn't contradict what you were taught, it's just a different way of representing the same thing. One explaination is in terms of bulk fluid properites and the other is in terms of a molecular model but they are both correct.

In terms of the molecular model there is a very slight downward component of the random molecular motion due to gravity, which will result in a higher density at lower elevations. But this increased density means more molecular collisions per unit area and hence more pressure. Ultimately however the origin of this increased pressure is still good ol' mass and gravity.

Last edited: Jul 31, 2008
3. Jul 31, 2008

### JHarris

Thank you uart thats cleared things up a lot.

4. Aug 1, 2008

### JHarris

Acctually when I look back at this now I'm a bit confused.

I understand that in the "molecular" explanation that gravity is the driving force, making air closer to the ground denser making air pressure greater etc.

The problem is that I don't see how these are two ways of representing the same thing. But how can dynamic molecular collisions represent the same force as a static weight acting on a planar surface?

From what I understand It must be one explanation or the other? Or am I missing something?

5. Aug 1, 2008

### uart

The molecular collisions are are mechanism by which the weight is transferred.

The connection between a macroscopic model of a system and it's corresponding microscopic model is not always something that's straight forward. However just because you can't see the connection is not a good reason for concluding that only one or the other could possibly be true.

For example one of the first things I ever learnt about electricity was Ohms Law $I = V/R$. Later when learning semiconductor theory I was tuaght that $J = q (n \mu_e + p \mu_h) E$, where J is the current per unit area and E is the voltage per unit length (and n is the volumetric free electron density. p is the volumetric hole density, q is the atomic unit charge and $\mu_e$ and $\mu_h$ are respectively the electron and hole mobility.

So those two equation basically relate the same two quantities (V and I) yet they are very different. Does this meant that only one or the other can be true? Certainly not!

6. Aug 1, 2008

### rcgldr

Both are the cause of atmospheric pressure, but ultimately it's the weight (assuming no vertical component of acceleration of the atmosphere). Pressure is force per unit area, and the only source for forces on the atmosphere is the force of gravity pulling the air "downwards" and the surface of the earth pushing the air "upwards". These combined forces compress the air until it's pressure corresponds with it's weight. The force of pressure itself is due to atoms or molecules of a gas or liquid colliding with a container, which in turn causes the atoms or molecules to collide with each other (without a container, gas expands and pressure goes to zero). Force equals mass times acceleration applies here, the collisions result in accelerations of atoms or molecules and the average of all these collisions produces an average force per unit area.

Note that gas in a container is affected by gravity. Within a container, the pressure decreases with "altitude" inside the container, and this vertical pressure differential will create a net downwards force on the container, that is exactly equal to the weight of the gas inside the container. For example, put 80 cubic feet of air in a scuba tank and the tanks weight increases by about 6 pounds, the weight of 80 cubic feet of air (at sea level). The vertical pressure differential is how the air exerts 6 pounds of downwards force on the scuba tank.

7. Aug 3, 2008

### JHarris

Ah I get it now. Thanks once again.

8. Aug 3, 2008

### rcgldr

I forgot to explain that there are two ways to change pressure. One is to change the speed of the atoms or molecules of a gas (absolute temperature), the other is to change the number of atoms or molecules per unit volume (density). To maintain density, either a full container is required, or a partial container combined with a captive force is required. In the case of the atmoshpere, you have a partial container, the earth, and a captive force, gravity.