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Physics help. Blind me with science.
joshofalltrades
Class TraitorSmoke-filled roomRegistered User regular
Class TraitorSmoke-filled roomRegistered User regular
Okay, I was a chemistry major and physics stuff always looks so scary.
Basically, I am thinking about the movement of gas particles which are lighter than air and affected by Brownian motion. Here's the basics as I understand them, somebody please correct me if I'm wrong: Gas particles by definition move randomly through the space they occupy. Even though this is the case, there are still ways to predict the motion of a gas; lighter particles will always diffuse/effuse faster than heavier particles because they have more kinetic energy. Additionally, there are sometimes predictable external factors that exert a force on a gas great enough to overcome simple random diffusion/effusion.
I have several questions, and yes some of them are dumb but if I ask something obvious it's more than likely to make sure I am not being confident about something I take as a given that may not be.
1. If a gas is lighter than air, can it be assumed that its motion can not be called random (even though it sort of still is)? Gases such as helium and methane rise in elevation since the ambient atmosphere is denser, and this makes lighter than air gases buoyant. So, is it a safe assumption to say that in aerobic environments, the probability of a lighter than air gas decreasing in elevation is zero or near zero? And if so, does this affect lateral motion at all?
2. Anaerobic environments are something I'm not as knowledgeable about, but if I understand correctly, the lack of free oxygen does not imply a vacuum in all anaerobic cases, such as beneath the soil, correct? So, would the same principle from (1) apply to (2), unless the anaerobic environment was water-saturated and so hypoxia?
3. Pressure is something else that worries me. About the farthest I can go in my thinking here is that greater concentrations of gas will equal greater pressure, which will cause gas to diffuse/effuse into an area of lower concentration/pressure, eventually reaching equilibrium when all of the available space is used. But if you're talking about something small being the only transport option, say, for instance, a small hole in a container, it seems to me that the pressure would be greatly increased in areas of high concentrations, speeding up molecule velocity and therefore rate of effusion into the area of low concentration. If you have a wide open space, there is effectively no pressure affecting diffusion/effusion speed of a gas (assuming it's just there and not coming out of a small container or something).
A corollary to (3) then is that if you have a giant web of spaces connected by equally small pores for gas transport, with a gas fully concentrated in one space, you still can't fully rule out pressure as a force on the movement of a gas through the pore spaces, right? Because concentration in a space changes pressure both in that space and in the space from which gas is traveling into it. This is the crux of my pressure difficulties.
4. Finally, where there is the presence of a liquid in a space where gas in being transported, is there any simple way to predict motion of gas through that liquid depending on if the liquid is subject to high pressure conditions, is or is not in motion, etc., and particularly the "lighter than air" gases? I need a lot of help in this area because this is pretty much pure physics and not my realm at all.
I realize this is a lot to ask for help for, but I know there are some smart people here on PA. Incidentally, I did just recently graduate, and this is not a homework assignment. It's an area of personal curiosity for me and some places I am applying at may expect me to be able to answer questions like this. If you don't want to answer publicly, feel free to PM me. Otherwise... physics go!
Basically, I am thinking about the movement of gas particles which are lighter than air and affected by Brownian motion. Here's the basics as I understand them, somebody please correct me if I'm wrong: Gas particles by definition move randomly through the space they occupy. Even though this is the case, there are still ways to predict the motion of a gas; lighter particles will always diffuse/effuse faster than heavier particles because they have more kinetic energy. Additionally, there are sometimes predictable external factors that exert a force on a gas great enough to overcome simple random diffusion/effusion.
I have several questions, and yes some of them are dumb but if I ask something obvious it's more than likely to make sure I am not being confident about something I take as a given that may not be.
1. If a gas is lighter than air, can it be assumed that its motion can not be called random (even though it sort of still is)? Gases such as helium and methane rise in elevation since the ambient atmosphere is denser, and this makes lighter than air gases buoyant. So, is it a safe assumption to say that in aerobic environments, the probability of a lighter than air gas decreasing in elevation is zero or near zero? And if so, does this affect lateral motion at all?
2. Anaerobic environments are something I'm not as knowledgeable about, but if I understand correctly, the lack of free oxygen does not imply a vacuum in all anaerobic cases, such as beneath the soil, correct? So, would the same principle from (1) apply to (2), unless the anaerobic environment was water-saturated and so hypoxia?
3. Pressure is something else that worries me. About the farthest I can go in my thinking here is that greater concentrations of gas will equal greater pressure, which will cause gas to diffuse/effuse into an area of lower concentration/pressure, eventually reaching equilibrium when all of the available space is used. But if you're talking about something small being the only transport option, say, for instance, a small hole in a container, it seems to me that the pressure would be greatly increased in areas of high concentrations, speeding up molecule velocity and therefore rate of effusion into the area of low concentration. If you have a wide open space, there is effectively no pressure affecting diffusion/effusion speed of a gas (assuming it's just there and not coming out of a small container or something).
A corollary to (3) then is that if you have a giant web of spaces connected by equally small pores for gas transport, with a gas fully concentrated in one space, you still can't fully rule out pressure as a force on the movement of a gas through the pore spaces, right? Because concentration in a space changes pressure both in that space and in the space from which gas is traveling into it. This is the crux of my pressure difficulties.
4. Finally, where there is the presence of a liquid in a space where gas in being transported, is there any simple way to predict motion of gas through that liquid depending on if the liquid is subject to high pressure conditions, is or is not in motion, etc., and particularly the "lighter than air" gases? I need a lot of help in this area because this is pretty much pure physics and not my realm at all.
I realize this is a lot to ask for help for, but I know there are some smart people here on PA. Incidentally, I did just recently graduate, and this is not a homework assignment. It's an area of personal curiosity for me and some places I am applying at may expect me to be able to answer questions like this. If you don't want to answer publicly, feel free to PM me. Otherwise... physics go!
joshofalltrades on
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as for answers, as best as i know:
1) the main force on a two-gas system is going to be gravity, and heavier gases definitely sink in air, just as lighter gases rise. it's why CO2 will flow along the ground instead of immediately rising. eventually, due to brownian motion, you'll have a gradient of concentration from the top of your container to the bottom, with the biggest quantities of light gas at the top and the biggest quantities of heavy gas at the bottom. lateral motion shouldn't be affected unless there's a way for the gases to 'flow' along the floor or ceiling of your container. think of a dip in the floor or a slanted ceiling - both will tend to concentrate more gas.
2) anaerobic simply means without oxygen. you're right, it doesn't imply a vacuum. you can have an anaerobic environment in your intestines, for instance. lots of methane, near-zero oxygen, but pressure can still build up... :P again, gases are lighter than soil and will over time diffuse through the ground upward.
3) i can't comment directly on 3, but the situation sounds vaguely pest-control related, or might even be fungus-related (mycelium, maybe). your instincts are mostly right, but keep in mind that unless your concentration of gas is at significantly higher pressure than ambient atmosphere, your mechanics should be dictated more by diffusion/effusion principles instead of pressure. why? because you've got some air/gas already IN your system at atmospheric pressure, and that's gotta be displaced first.
4) now that i think on it, this sounds a lot more like petroleum engineering/fracking. assuming it is, and the liquid is under pressure, then the gas itself will be partially dissolved in the liquid. as soon as you find a way to relieve that pressure, both the liquid and gas will exit as fast as possible. think of a soda bottle that's been shaken. you get the gas coming out, but it brings with it a ton of liquid...
i've tried to express these in more general terms without going into PV=nRT basics, but i hope that helps.
also! i've been wrong before, so if i screwed up anywhere, by all means correct me.
Registered just for the Mass Effect threads | Steam: click ^^^ | Origin: curlyhairedboy
Thanks for this, I'm going to read it over again a few times and see if I can make all this make sense in my head. And if you're reading this please throw in your two cents! More to read is better than less.
Registered just for the Mass Effect threads | Steam: click ^^^ | Origin: curlyhairedboy
I'm assuming you haven't seen material like that before.
Specifically, I'm trying to set up a small-scale experiment but I need to do a few things first. I need to successfully model the motion of natural gas as it rises from a deep source and encounters horizons of different porosities and permeabilities, among other criteria. Once I have a handle on the motion and can simulate it reliably in 2 dimensions at least, I can set up a small-scale soil test with several different horizon compositions. I will sample the surface-level soil and perform a test on it as a control, then allow small amounts of natural gas to flow through the soil and repeat the test.
Spoilered explanation of why I'm actually discussing the nature of this work:
the extra info does help. i'm gonna give it some more thought. i'm reinstalling windows right now and it's a lot of thumbtwiddling. i'll get back to you.
Registered just for the Mass Effect threads | Steam: click ^^^ | Origin: curlyhairedboy
So far as I know, all of these answers are correct. The only minor quibble I have is with the example in 4, which isn't quite right. The pressure release on the liquid releases the dissolved gas, and although the liquid's pressure has also been released, the liquid will only escape if it remains viscous enough to trap bubbles of the escaping dissolved gas in such a way that the bubbles escape, or if the hole is in the part of the vessel filled with the liquid.
You're right, of course. It has been a while since gen chem.
@Terrendos -- That's an interesting correction, but I am having trouble parsing it. Can you illustrate it a little more?
The second place to puncture it would be near the top. The gas would escape, and the resulting pressure drop inside the bottle would draw out the dissolved carbon dioxide. However, soda has a viscosity, and as a result it will form bubbles when that gas is escaping upward, that foamy head. The bubbles take up a lot more volume than the liquid, so the soda expands upward. In this example, we'll say that the expansion doesn't reach the puncture, in which case only the gas and dissolved carbon dioxide will escape.
The last place to puncture the bottle is just a bit above the liquid line, so that the foam expansion covers the breach. In this case, the pressure differential will drive out both the liquid (as a foam) and the gas/dissolved carbon dioxide.
The distinction may not be important in your field, but I know it's important in some. The Three Mile Island nuclear accident was caused in part by a stuck valve in the pressurizer (basically a bottle filled with water and water vapor) which allowed the vapor to escape. Because the controllers couldn't explain the resulting pressure drop immediately, they were unable to respond in time to prevent a partial meltdown of the reactor core.
Registered just for the Mass Effect threads | Steam: click ^^^ | Origin: curlyhairedboy
In the meantime, I've been keeping my chemistry chops sharp by working practice problems in my college textbook, and, um
playing SpaceChem