How does atmospheric pressure work?

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Discussion Overview

The discussion revolves around the concept of atmospheric pressure, specifically how it is measured and the underlying principles that govern it. Participants explore the relationship between gas molecules, pressure, and the effects of air above a surface, touching on both theoretical and practical aspects.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant expresses confusion about how atmospheric pressure can be felt from air molecules that are not in direct contact, questioning the relationship between molecular collisions and pressure measurement.
  • Another participant explains that the interactions between gas molecules, including those further away, influence the pressure experienced at a given point, suggesting that the weight of the air above contributes to the pressure felt at the surface.
  • A participant introduces an analogy involving a cylinder with a piston to illustrate how pressure can be understood in terms of weight, noting the challenge of visualizing air pressure without a physical piston.
  • There is a practical inquiry about using a sealed container with a piece of paper to demonstrate atmospheric pressure, with a follow-up confirmation that the pressure inside would equal atmospheric pressure if sealed correctly.
  • Another participant adds that if there were a pressure difference, it would cause the paper to bend, linking this to the construction of a barometer.

Areas of Agreement / Disagreement

Participants generally agree on the basic principles of atmospheric pressure and its measurement, but there are varying levels of understanding and visualization of the concepts involved. The discussion remains exploratory without a definitive resolution to the initial confusion expressed.

Contextual Notes

Some assumptions about the behavior of gas molecules and the nature of pressure are not fully articulated, and the discussion does not resolve the complexities of how pressure is transmitted through gases.

yunocell
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From what I know, the pressure of a gas is due to the collisions of its molecules against a surface. So I don't understand why atmospheric pressure is measured by taking the weight of all the air molecules above a surface including the ones that aren't contacting it. Gases have plenty of space between their molecules so I don't understand how I can feel the weight of a gas molecule one mile above me that isn't in contact with anything else. Can somebody please enlighten me? Thanks in advance.
 
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Welcome to PF;
The gas in the air all bumps into each other so how hard the molecules close to you bump into you depends also on how hard all the other molecules bump into each other. So it is reasonable to expect that the air way above you would have some effect.

The reason this effect comes to the weight of the air above a spot on the ground is that the air pressure is what stops all the air being a thin pancake under your feet. There has to be enough pressure to hold the whole atmosphere up.
 
Ok, I think I understand now. Thanks for the answer!
 
If you has a cylinder of air with a piston on top that was being held up by the pressure in the container, then you'd have to figure the pressure in the container in terms of how heavy the piston was right?
It works like that with water pressure too.

The trouble with picturing air pressure it there is no obvious piston ;)

But you should have a decent mind-pic now to go on with.
 
So if I go outside and I get a container and a piece of paper, and I put the piece of paper on top of the container to seal it, does that mean that the inside of the container is 14.7psi (atmospheric pressure) just like the outside?
 
yunocell said:
So if I go outside and I get a container and a piece of paper, and I put the piece of paper on top of the container to seal it, does that mean that the inside of the container is 14.7psi (atmospheric pressure) just like the outside?

Yes.
 
... if there were a difference, then the paper would bend.
This effect can be used to build a barometer.
 

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