Reading The Martian and thinking about things like depressurisation – why does air from a higher concentration feel the need to rush through a small leak with enough force to rip or blow things apart instead of staying put?
What calls it from the vacuum for it to be so obsessed in doing so?
In: Planetary Science
Gas is made up of many smal gas particles that move around at *very* high speeds and bump off of each other. They move around in all different directions.
If there is no or very little gas in an area (vacuum or very low pressure) then gas particles from a high pressure area that are travelling in the direction of that lower pressure region won’t have anything to bounce off of to redirect them back to the high pressure region, so they’ll just keep going in that direction and will wind up filling that space.
A very high pressure difference (like between mars’ atmosphere and the inside of the hab) means lots and lots of gas (air) molecules are travvellibg at very very high speeds towards a lower region (outside the hab) with very little in the way anything to stop them.
Theres probably a bunch of simulations on youtube showing this. Maybe look up something like “gas particles brownian motion simulation” or something.
Air particles are moving around very fast, bouncing off each other at the speed of sound. Though affected by gravity (if present), this energy and bouncing will cause them to spread themselves out fairly evenly. With gravity the pressure at the bottom is higher than the pressure at the top, but it’s a small difference most of the time. Eventually, air pressure reaches its consistent point and it feels like nothing’s moving at all.
At least, until your hole forms in your spaceship or whatever. Now the air that happens to find its way into the hole goes out… and let’s be honest, is never coming back again. So now there’s fewer molecules in that area bouncing around, so the bouncing around of other molecules wins the war of numbers and pushes themselves towards that hole… But that just means more air heads for the hole and slowly (or quickly) escapes, never to return again.
Over time, pressure keeps dropping and dropping until it’s so low it might as well be zero. 1% air pressure to a human might as well be 0%.
A vacuum is not a force. But in general, “balance” is what forces want and a vacuum will be filled by the force, if it can.
Molecules bounce off each other. Put a cluster of vibrating marbles in a box and imagine what happens.
Think about water. If you have a water line that springs a leak you will suddenly have water spraying into the room. Why? Because the water is pressurized. It’s being pushed on by other water molecules and the walls of the pipe and is pushed out the leak.
In a spaceship it’s the same thing just with air. The air in the spaceship is pressurized. It’s not that the vacuum outside is pulling the air molecules out, it’s that other air molecules and the walls of the spaceship are pushing the air out into space. If the damage to the vessel is bad enough the leak can grow bigger until the vessel blows apart, that’s just crack propagation.
Gas is a statistical model. The theoretical model of a gas is just a bunch of particles moving in straight lines until they bump into something, like the walls of a container or other gas particles, at which point they bounce off that thing and keep going in a different direction. Phenomena like temperature and pressure are the *averages* of the speed etc. of billions and billions of these particles but individual gas particles are still just flying about in straight lines until they bounce off something.
In a pressure leak situation you can imagine that there are a billion particles bouncing off any given 1 cm^(2) patch of wall each second on the high-pressure side (these numbers are wildly not to scale) and there are only a thousand bouncing off the low-pressure side. If we remove that 1 cm^(2) patch those 1 billion particles from the high pressure side that would have hit it just don’t, and keep going, into the low pressure side. Some may bounce off the thousand particles coming the other way from the low pressure side but even if every one of the thousand low-pressure side particles intercepts a high-pressure side particle, there is still nine-hundred and ninety-nine million, nine-hundred and ninety-nine thousand particles going from the high-pressure side to the low-pressure side and in comparison none going the other way.
So gas is made up of these very tiny things called molecules. Scientists have found out that gas molecules love to dance, but don’t like bumping into each other because they are shy and like their personal space.
Now when there are a lot of gas molecules in an area, they like to spread out so that each and every molecule has about the same space to dance on the dance floor. If some of the gas molecules leave the dance floor there is now more personal space for the rest of the molecules to have for themselves.
So what happens is that as more molecules leave, each molecule that is still on the dance floor has more personal space. That is why gasses, like air, fill the space they are in.
If there are more gas’s molecules outside the dance floor than in, the gas molecules from outside will wiggle onto the dance floor since they will have more personal space to dance.
>why does air from a higher concentration feel the need to rush through a small leak with enough force to rip or blow things apart instead of staying put
2 main points here.
1. In gas form, the molecules are moving quite freely, flying about in all directions bouncing off the walls of the container. Relatively similar pressures inside the container and outside? The forces imparted on the container by all these collisions are relatively in balance. Dial up the pressure? Now there are either more collisions occurring on the higher pressure side, or collisions imparting more energy (or both). This translates to the macroscopic scale as a force on the walls of the container. More pressure difference, more force imbalance.
2. The container has some properties based on its shape, materials, quality of materials, etc. The combination of these properties determines the container’s ability to withstand forces. If the pressure difference is cranked-up high enough the forces overcome the containers ability to hold its shape, and it starts to deform. Typically this deformation weakens the container further, so a catastrophic failure occurs (boom). Same goes for introduction of a small hole (leak).
Regarding why the air doesn’t just stay put… we call that a solid. Technically we could cool air far enough for it to solidify (and just stay put), but it doesn’t do us any good for breathing at that point.
If you want to know why molecules move more freely in a gas vs liquid vs solid, it becomes a discussion of the balance of kinetic energy vs intermolecular forces, but that seems to be getting out of the scope of your question. Phases of matter or phase change would be searches of you want to dive into the rabbit hole.
Say you’re playing billiards. At the start of the game, the balls are racked. When someone takes the break shot, do the balls stay in the racked configuration or do they spread out across the table?
It’s the same with air molecules, except there is no felt on the table to slow them down.
I think you should ask the opposite question: why would a block of air stay together? There’s no bonds holding them together so Gases fill whatever space they’re contained in. You wouldn’t have a scuba tank where all the air sinks to the bottom right? Well now you have a scuba tank in space, and pop, the scuba tank disappears.
Now you have a bunch of gas atoms bouncing off of each other but the walls that were keeping them in place are now gone, if you’re in the middle, there’s no escape, you keep bouncing around, but if you’re on the outside, you bounce against the atoms on the inner side of you and start going out. But instead of hitting the wall that’s been keeping you in to turn you around, you don’t turn around, you just keep going, and going, and going until you hit something else. The guy who bounced you out hits his inner neighbor and then starts going out. He won’t hit you because you’re not turning around, and so on and so forth.
From an engineering standpoint:
Why gas fills containers:
Gases fill containers. Meaning, any one molecule (of potentially trillions) has an equal chance of being in any position in the system. The system is a space with a boundary. Like a box, a balloon, your lungs
This is due to that gravity and intermolecular interactions are super tiny in “normal” gases (i am talking about air. They have so much momentum that they just spread out everywhere
Since they are so light, and airy (theres a pun back), gravity has very little effect on them (look up gravitation equations for the math)
So what is a vacuum:
A vacuum is simply a space with little to no gas molecules.
Pressure is force divided by area. The same force you get from pushing on something.
If you crash into your bedroom door, you put force on it. Gas molecules do the same. So pressure measures how much force a bunch of gas molecules are hitting the container with
Vacuum= no pressure = no force = no momentum = no molecules
Maybe i did a eli15 but i hope this helps!
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