Cause of atmospheric pressure

  • Thread starter ovais
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  • #1
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Main Question or Discussion Point

Hello All,
It is said that earth has an envelope of gases (air) surrounding it and that due to gravity the weight of this envelope of air exerts a force: on the surface of earth and any thing on it. The weight of air column on any surface divided by the area of the surface is always a constant quantity no matter what is the value of that area, and this very quantity is what we call the atmospheric pressure at that place. This seems quiet simple to understand. Things however complicate the matter (at least for me) when then the kinetic model of gas speaks of molecular collision (with the contact surface) as a reason for gas pressure.Imagine an open empty vessel (with air) at atmospheric pressure as per the kinetic model the molecules of the gas continue collide with the cylinder wall and thereby exerts pressure on the walls and every where inside the vessel. Now I sealed the vessel top keeping the inside pressure the same. The gas inside the vessel pushes the vessel walls outward trying to burst the vessel but as such the outer air also exert equal pressure to vessel walls inwards the vessel is thus neither collapsing nor bursting. But look what happen if this air filled sealed vessel is taken to a zero gravity space with no atmosphere?
The two possible answers will be very different if one is answered thinking gravity(weight) and other molecular collisions as a cause of molecular collision.
If gravity is zero the weight of air should be zero, so is the pressure inside the sealed vessel further remembering that no pressure is acting from out side in empty space the vessel thus due to zero weight of air in it; should have zero pressure in it and zero pressure out and will neither burst nor collapse. But according to kinetic model molecules should continue to collide (I thing independent of gravity) and due to collision with walls should exerts pressure to bursts the vessel.
These two different answers posing a problem in understanding the real cause of atmospheric pressure.
I mean if earth is considered not rotating(chances of escaping air is zero) and it suddenly lose gravity, than as per the weight of air column concept air pressure should be zero, while the fact that molecules in gas are always in random motion is at its place, and it doesn't justifies zero pressure in absence of gravity.
I will be highly thankful if anybody explain what am i missing.
With Regards, New Year Best Wishes!!
 

Answers and Replies

  • #2
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the kinetic model of gas speaks of molecular collision (with the contact surface) as a reason for gas pressure.
What they mean is that this is the mechanism by which the pressure acts on the surface, not that this is the cause of the pressure. The pressure is due to the air being compressed, in this case by gravity, in a bicycle tyre by a pump. The air in your sealed cylinder will remain compressed, unless the walls of the cylinder are free to expand, so the pressure will stay the same. Observe what happens when a weather balloon rises. As the balloon gets higher and the external pressure drops, the balloon inflates.
 
  • #3
D H
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But look what happen if this air filled sealed vessel is taken to a zero gravity space with no atmosphere?
It's still going to be at pressure, of course.

One key difference between your sealed container and a planetary atmosphere is that the atmosphere isn't sealed. The planet's gravity more or less keeps the atmosphere bound to the planet without a need for sealing it in. Take your sealed container out into space and open the seals and the container will soon become empty.

The kinetic theory of gases still works quite nicely for describing a planet's atmosphere. You just need to add in the fact that the atmosphere is in hydrostatic equilibrium, and in the case of the Earth, that the atmosphere is mostly heated from below.
 
  • #4
D-C
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You wrote;
"the kinetic model the molecules of the gas continue collide with the cylinder wall"

Maybe it would help to remember the molecules are also colliding with each-other.
 
  • #5
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I want yo know what is the exact cause of pressure, gravity or molecular collisions?
 
  • #6
D H
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It's not an either-or question. You are creating a false dichotomy.
 

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