What is the counter-force to suction force?

Question

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:

1. 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.
2. 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.

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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?

Answer 1

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'.

Answer 2

As the air from the outside fills the vacuum, meaning it moves, some force must be acting on this air.

It might be useful here to contrast the "push back" of compressed air with the "push back" of a compressed metal, for instance. For the latter (and for all familiar condensed matter), bulk stiffness is what we call "enthalpic"; in other words, compressing the material increases the internal energy. The molecules resist being pushed together for electrostatic reasons; i.e., involving one of the fundamental forces. If we differentiate the internal energy (in joules) with respect to the compression distance (in meters), we obtain a force (in newtons).

In contrast, in the ideal gas, stiffness is what we call "entropic." Compressing the gas (at constant temperature) does not increase the internal energy, and thus, differentiating the internal energy by the compression distance gives... nothing. The resistance to compression—and tendency to expand without limit—arises solely from our tendency to more often notice arrangements that occur more often. (This is one statement of the Second Law of Thermodynamics.) None of the fundamental forces are involved; there are simply many more arrangements possible when a gas spreads to occupy its container.

This distinction may be connected with the challenge you've found of identifying some "force" that pushes a gas into a vacuum. No such enthalpic force exists.

Answer 3

'Pressure' of a gas is generated because of the collisions between gas molecules-gas molecules or gas molecules-wall of the container. Unless they collide, gas molucules move in straight lines with uniform velocity, which requires no external force (The impulsive forces act at the collisions are internal forces to the system).

If you put some gas molecules into a bottle, they spread throughout the bottle because there are no barriers inside the bottle. But they cannot go through the wall of the bottle. If you open the lid of the bottle, some of them come out to equalize the pressure inside and outside, because they like to minimize the number of collisions between them [ peace-loving :) ]. That is simply called they move due to pressure difference. But the real reason is you removed the barrier which prevented them from moving freely.

Imagine this way, you are running along a straight line and you see a door closed, so you turn back and again run. Next time you come and the door is opened so you freely move through the opened door. Did any force act on you? No.

In your example, the air is moving to the vacuum simply abolishing the pressure difference (This is explained in more details above) with uniform velocity. Therefore no force acts on the air. Thus no counter-force!

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In response to comments:

Suppose there are two adjacent rooms. One room is filled with air molecules and the other one is a vacuum. There is a door between the rooms. When the door is closed the air molecules just hit the door and bounce back because they can't penetrate. If you open the door, the air molecules those were coming towards the door will enter the other room. Until door is opened, this process happens. What happens if the pressures of two rooms become equal? The process continues. The air molecules are still moving through the door. But you can't observe it. That is why we say that the air diffuses abolishing the pressure difference. But this is true unless you close the door. If you close the door at the middle of the procedure, the pressures of two rooms will not become equal.

All in all, what I wanted to imply through this example is that no external force acts on the air to push it to fill up the vacuum.

Hope this helps.

Answer 4

If I understand correctly, this question is about Newton's third law. If a gas exerts a pressure on something, something must provide an opposite force to the gas. Keep in mind that counter-forces in Newton's third law don't prevent motion. When I push a ball with my finger, the ball moves forward, my finger wants to move back. I stop my finger moving back with my arm, and my arm now wants to move back. I stop my arm with my legs connected to the ground, so the ground wants to move back. The ground is enormous, so we don't notice it moving back.

Let's imagine a layer of air called l or L. In front is our vacuum, V. Behind it is the rest of our air, A. Now I have a text diagram:

V l A

Microscopically, air molecules from A are colliding with air molecules from L, so whenever a molecule from L gets a kick, air molecules from A get a kick in the opposite and equal direction. This is true of any layer in the gas.

Now let's think about V and A as regions of space where particles can move in and out, like voxels in Minecraft, and l is the boundary between two voxels. The equal/opposite 'force' is more like this: in the L rest frame, equal numbers of particles of L are going towards V and towards A. Hence, equal and opposite momentum is moving both towards V and towards A. This is $F = \frac{dp}{dt}$, a change in momentum over time for a region of space.

The vacuum region V gains momentum from A because it is gaining stuff with velocity. So it experiences a positive force in the wind direction.

The air region A is losing momentum to V as particles with momentum move from A to V. So it experiences the opposite and equal negative force.

Now, after a moment, the vacuum region V is no longer a vacuum. It is now a region with mostly vacuum, and a thin layer of stuff that moved into it.

If you don't want to account for the force this way, and let's imagine there are no collisions either, then you just follow a bunch of particles not experiencing any forces. There are no forces, but because there are more particles on one side than the other, the ones who were already moving toward the vacuum end up continuing, hence filling up the vacuum. Because in a gas, we assume the particles were moving in random directions, so some were bound to point towards the vacuum.

Answer 5

A farmer has 10 acres of land which is fenced off from the surrounding area and divided in the middle into two portions of 5 acres each. There is a gate in the middle that allows passage between the two halves. He puts his sheep in one half of the land with the gate closed. One day he decided to open the gate and left it open. After a day or two, he found out that some of the sheep are in the formerly empty other half of the land. The sheep start filling the empty half just by wondering around. The same applies to how air fills a vacuum. No one pushed the sheep to go to the other side, they just wondered into it because sheep tended to move around. Air molecules are not stationary. They move around and stumble into the unfilled vacuum. Just as there is nothing to stop the sheep from moving around. There is no counterforce.

Continuing with this analogy, the sheep bumping into each other and the fence would be the pressure. The more sheep there are, the more chance that they bump into each other and the more likely that some of them got pushed into the empty half of the land. The higher the pressure the faster the vacuum will be filled.

When I say there is no force to push the sheep, I meant no external force. The sheep will still bump into and pushing against each other.

Answer 6

As the air from the outside fills the vacuum, meaning it moves, some force must be acting on this air.

Force is needed for acceleration. If the air is travelling with a constant velocity, no force is needed. Of course, some force was needed to get the molecules moving, and that force with a collision with another molecule. That molecule will then travel in the opposite direction. So analyzed at a molecular level, what we see as "suction" is not producing a force itself. Rather, it is produced by already existing forces.

Analyzing the system macroscopically, however, there will be a "missing" momentum; the molecules that go through the hole will take some momentum with them, causing the remaining molecules to have a net momentum in the opposite direction. The molecules that go through the hole would have, had the hole not been there, hit the wall, and imparted a force on the wall. There is therefore an imbalance, with more molecules hitting one side than the other, and that will cause a net force.

Answer 7

Creating a vacuum does require an outside force. To create it the pump is constantly pushing air out. To maintain it the walls of the sealed chamber are being pushed on from the outside air.

Creating a high pressure zone is similar, with the pump pushing air in and then walls keeping air in (walls being pushed on outward by the higher pressure gas inside).

Answer 8

(This probably should be couched as a comment, but I do not have enough points to leave one apparently so I will answer with this thought experiment:)

You have probably seen a glazier moving some glass using suction cups. A flat piece of rubber is pushed against the glass, and the middle is pulled up, leaving a partial vacuum between the glass and the rubber.

The glass may then be handled by using the handles attached to the "suction" cups.

Now imagine you are on a space station, and desirous of moving large pieces of glass around which constitute the lens of a telescope you are constructing. Fortunately, you have brought your harbor freight 6-in diameter suction cup glass handlers from the surface of the Earth to assist you in this project.

After you discover that they don't work at all, you will contemplate whether in fact on Earth the suction sucks, or if the atmospheric pressure pushes.

"Counterforce" deliberations are unlikely to enlighten the mind in this situation, (IMHO)

Regards