Pressure should decrease in zero gravity?

[Ravi Singh choudhary]

Pressure can exist in a container of gas even if there is no gravity. From kinetic theory of gases it is assumed that pressure exists due to momentum transfer between molecules and the wall. Suppose I have a closed container on Earth at sea level filled with air. Now I carried it to zero gravity facility. I maintained all other thermodynamic parameter like volume and temperature. Now pressure should decrease as gravitational force is not pulling individual gas molecules. Therefore there should be less momentum of particles so the pressure. Please just give me insight to my thought whether I am current or not?

 

[Simon Bridge]

Under gravity, the pressure in the container is not uniform... usually the effect is small but in large containers (ie the atmosphere) the effect is quite obvious.

 

[Dale]

I agree with @Simon Bridge. The average pressure should be the same. The difference will only be the vertical pressure gradient.

 

[Chestermiller]

Why don't you just model it and figure it out yourself? Let n be the number of moles in the container and M be the molecular weight. Let's see your analysis.

 

[Ravi Singh choudhary]

Why don't you just model it and figure it out yourself? Let n be the number of moles in the container and M be the molecular weight. Let's see your analysis.

Total mass is always nM, now when container is influenced by gravity some molecules will drift downward and localised higher density region is created opposite what we had uniform density region. Now more number of collisions is there in the downward region while less number of collisions in upward region. So average pressure remains same but molecules are just redistributed in gravity.

 

[Chestermiller]

Total mass is always nM, now when container is influenced by gravity some molecules will drift downward and localised higher density region is created opposite what we had uniform density region. Now more number of collisions is there in the downward region while less number of collisions in upward region. So average pressure remains same but molecules are just redistributed in gravity.

I meant to analyze the problem quantitatively. Assume you have a vertical cylinder of cross sectional area A and height h.

 

[vanhees71]

This is most simply solved by statistical mechanics. Just write down the Boltzmann distribution!

 

[Chestermiller]

This is most simply solved by statistical mechanics. Just write down the Boltzmann distribution!

This sounds interesting. With gravity switched on, using statistical mechanics, what do you get for the pressure at the top of the container, the pressure at the bottom of the container, and the average pressure (in terms of A, M, n, h, g, R, and T)?