Air will always try to fill a vacuum.

[vetrox]

I'm in the middle of an argument about engines and have to dumb down something for someone.

So like the title says. My basic understanding is air will always push its way to fill a vacuum to make equal pressure. Or in the case of a car engine. Possitive pressure (say 2xatmo pressure) will try and equalise itself with the atmospheric pressure and take the shortest route to do so.

Does anyone know the technical term for this effect?

Also as this is the case. what keeps all our air from escaping into the vacuum of space and be left with an airless husk of a world? I wanna say the earths magnetic field. Has anyone got a link which could explain this in more detail for me?

Thanks guys.

[sal_vager]

Gravity keeps our air in place, though hydrogen is light enough to escape (that's why we don't have any not locked up like with water)

The term for gasses equalizing pressure is expansion, I think

[How2FoldSoup]

I believe the technical term you're looking for is equilibrium?

I don't know much about atmospheric science though so I can't help you there. If I had to guess it has something gravity and composition of our atmo. I say that because most of Mars' atmosphere has actually leaked away in the last couple billion years yet, ours as has not. As far as I know no one quite knows the answer to the Mars' riddle yet. Once again this is more of a guess than an exact answer so take it with a grain of salt.

[Drunken Hobo]

Also as this is the case. what keeps all our air from escaping into the vacuum of space and be left with an airless husk of a world? I wanna say the earths magnetic field. Has anyone got a link which could explain this in more detail for me?

Quite simply; it's gravity. As you'll know from KSP, to escape Earth's gravitational field, you need to be going escape velocity. The various molecules that make up air simply don't go fast enough to break free of Earth's gravity - like a rocket with not enough delta-V to leave Kerbin's SOI.

Interestingly though, hydrogen molecules (H₂) & helium atoms are so light that occasionally they do go above Earth's escape velocity, and that's why there is very little hydrogen & helium within Earth's atmosphere - it has mostly been lost to deep space. This doesn't help with the helium shortage - the gas from a party balloon ends up in orbit around the Sun.

Earth's magnetic field does help in preventing the solar wind (incredibly fast-moving particles) from interacting with the atmosphere, which would otherwise strip away the heavier molecules in our atmosphere, like oxygen & nitrogen.

[-Velocity-]

I'm in the middle of an argument about engines and have to dumb down something for someone.

So like the title says. My basic understanding is air will always push its way to fill a vacuum to make equal pressure. Or in the case of a car engine. Possitive pressure (say 2xatmo pressure) will try and equalise itself with the atmospheric pressure and take the shortest route to do so.

Does anyone know the technical term for this effect?

Air is just a bunch of molecules zipping about at high speed (hundreds of meters per second at room temperature). Pressure is created by the impact of these molecules into other things. Gases also work a bit differently once you get below a certain pressure at which mean free path length becomes longer than the size of the vessel- viscosity and collisions between molecules (or atoms, if noble) of gas become infrequent and you can no longer really have a flow to suck out the air. You need special kinds of pumps.

But no, I don't really know, at least off hand, a technical term for a gas' propensity to fill a container equally. It's pretty basic, does that really need any terms other than the ones you've already used? It's not diffusion, because diffusion operates regardless of pressure differentials.

Also as this is the case. what keeps all our air from escaping into the vacuum of space and be left with an airless husk of a world? I wanna say the earths magnetic field. Has anyone got a link which could explain this in more detail for me?

Thanks guys.

It's Earth's gravity. In order to escape the Earth, you must reach escape velocity. Particles of gas at room temperature move at an average of like, maybe 500 meters/sec. The escape velocity of the Earth is ~11 km/s. So gas particles cannot escape easily, they have to get very "lucky" to build up enough kinetic energy to escape- unless that particle is hydrogen or helium. The velocity of a gas particle at a certain temperature is inversely related to its mass, so because hydrogen and helium are so light, it's a lot easier for them to obtain the velocity needed to escape Earth.

The magnetic field may play a role though, in protecting the upper atmosphere from the solar wind. Solar wind particles are moving very fast, and would slowly strip away an unprotected atmosphere. That said, a magnetic field is *NOT* required to keep an atmosphere over a long geological time- just look at Venus.

Edited April 17, 2014 by |Velocity|

[vetrox]

Of course...Gravity The mind really does go blank sometimes.

It was Diffusion i was thinking of but now that you have pointed it out, it does indeed not mean what I was trying to explain. Which is fine it was just sitting in my memory not wanting to come out. Thats what happens when you try to remember something you learnt 10 years ago Knew it began with a D because I had dispersion in my head but thats not even close.

"Air will rush to fill a vacuum" as long as thats true (which i already knew) then my argument stands valid. I just thought there was a word for it. That does explain why I couldnt find one

[Sillychris]

From a thermodynamic point of view, the air's need to reach equillibrium resulst from nature wanting to maximize entropy and minimize free energy.

[K^2]

From a thermodynamic point of view, the air's need to reach equillibrium resulst from nature wanting to maximize entropy and minimize free energy.

That's really the best way to think about it. Gravitational potential energy is going to contribute to free energy, so while clumping together around a planet reduces system's entropy, thereby increasing free energy, the reduction of gravitational potential energy offsets it. But only to a point. Which is why atmosphere isn't at the bottom of the oceans.

The only draw back is that this requires basic understanding of entropy and free energy, which aren't the most intuitive topics.

[vetrox]

The only draw back is that this requires basic understanding of entropy and free energy, which aren't the most intuitive topics.

Precisely. I only need to know enough to make a combustion engine work

[FREEFALL1984]

"Air will rush to fill a vacuum" as long as thats true (which i already knew) then my argument stands valid. I just thought there was a word for it. That does explain why I couldnt find one

A simple way of looking at it is that the entropy of the system forces the gas from a high energy situation to a lower energy situation, imagine two air cylinders connected with a pipe with a valve located on it. You fill one of the tanks with compressed air, in doing so you're essentially packing kinetic energy into the first cylinder. Then you open the valve between the two cylinders and what happens. Entropy causes the gas leaks to between cylinders until the pressure is equal and the same amount of kinetic energy is stored in both cylinders, if you released all the air from both cylinders the gas would diffuse and lose energy until it matched that of the surrounding environment, likewise if gravity suddenly didn't exist the atmosphere would (thanks to entropy) float away and diffuse throughout the universe, but thankfully the gravity is applying enough force to create what could be considered an entropy "barrier", Due to the magnetosphere among other variables, the atmosphere cannot gain enough energy to reach escape velocities and climb higher than this "barrier" If the magnetosphere was to disappear (like mars 4 billion years ago) then the atmosphere would be swept into space, slowly at first but as the atmosphere itself thins the process would increase in speed, until it neared a point where the sun could no longer provide enough energy for the atmosphere to reach escape speeds. by which time the atmosphere would be thin and low and would probably be unable to support surface life and we would all be either dead or living deep underground where gravity is strong enough to hold a dense atmosphere in. Which is why if there ever was going to be life on mars, its had the choice of either evolving to live underground or evolving to live on an increasingly hostile surface.

[Dwayne_Knight]

I'm wondering what the disagreement is?

I find at high speeds gasses in confined systems behave increasingly like fluids with flow characteristics and viscosity playing a role.

[Seret]

On a very simplistic level it's just the relationship between pressure and force. If you have a difference in pressure then you must have a force being applied. If there's no force opposing that (such as a reaction force from a vessel wall) then the system is unstable until the pressure difference goes away.

[Tinyboss]

The molecules of a gas aren't stuck to one another, and they're all flying around at speeds whose average is proportional to the temperature. A vacuum is just the absence of anything for them to bounce against! So they just keep moving, away from where they are now.

[Doozler]

If you're trying to explain it to someone else, how about an analogy? If you have a room full of people and you start off with more people on one side that the other, then if they all push in random directions then they will eventually spread out - there's more push on one side of the room than the other.

Analogies can be a bit dangerous, but an angry crowd is my go-to for gasses.

[-Velocity-]

If you're trying to explain it to someone else, how about an analogy? If you have a room full of people and you start off with more people on one side that the other, then if they all push in random directions then they will eventually spread out - there's more push on one side of the room than the other.

Analogies can be a bit dangerous, but an angry crowd is my go-to for gasses.

An airless room full of tennis balls bouncing around at hundreds of meters per second seems a better and more accurate analogy to me.

[General Zod]

I think its called diffusion.

[Snark]

Hello, and welcome to the forums! 

...However, given that all the prior discussion in this thread was six years ago, I think it's safe to say that everyone involved has since moved on.  Accordingly, locking the thread to prevent confusion.  If anyone has related questions on this topic, please feel free to spin up a new thread.