I've been thinking about air pressure and vacuums recently and a (for me at least) interesting question came to mind:
If there is an area with lower pressure than the surrounding air, the air distributes itself equally until the pressure is even everywhere.
For simplicity's sake let's say it's not only a low-pressure area but a complete vacuum so we can ignore any confusing forces that might still happen somehow otherwise.
Now to the question: As the air from the outside fills the vacuum, meaning it moves, some force must be acting on this air. I can not find the counter-force to this though. So my question is: What is the counter force of pressure force acting on the gas to fill a vacuum (or any other area with lower pressure in a gas)?
I have though about the following 3 solutions, but both have some major issues with them:
- If the vacuum "pulls" the air in, there must be a counter-force acting on the vacuum . This isn't possible though, as force cannot act on a "nothing". Also, vacuum doesn't pull in the surrounding but it is pushed in by the surrounding pressure.
- If the surrounding air "pushes" onto itself with its own pressure (which it does), a counter-force would have to act on it (the surrounding air itself). This cannot be true either, because that would mean that either the vacuum would not fill up (because the air should stop moving) or, at a certain distance the air should not be affected at all (which it isn't, that is true but only because the surrounding area is so large the effect is negligible).
I hop this description is clear enough, if not, feel free to ask for an explanation.
EDIT: Alright, I think I have understood it now. So the vacuum fills up because gas molecules move "randomly" around all the time and bump into each other. If there is a vacuum somewhere, they can freely move into there because there is nothing to bump against, so a lot of them end up in there, causing the pressure to equalize, because when it is, the molecules continue bumping around but do not "favour" a certain direction because there is less resistance there anymore.
Wind exists, because of course when many molecules have moved a certain direction, there are less of them where they used to be, which again is a lower-pressure area so it also fills up. This happens untill everything is mostly calm again.
Also, does this mean that pressure is the sum of the forces acting on the walls by the molecules randomly hitting the walls?
Is this correct?
Best Answer
The answer to your dilema is that there is no need for any counter force.
Statistically it's likely that many molecules would come into the region where the vacuum was - however no force pushes them there.
In the kinetic theory of gases, intermolecular forces and collisions are ignored, but there will be, by chance, many molecules travelling towards the vacuum region. They continue in a straight line with no force acting, as in Newtons 1st Law.
Since there were no molecules in the vacuum region to leave, it just happens by chance that a short time later, more molecules will be in the region than before and the vacuum region has been 'filled'.