I understand density. What I don’t quite get is why density is the cause of layering in fluids. Suppose I release one helium atom. What force causes it to ignore gravity and float upward?
It’s not ignoring gravity, it’s being displaced by heavier gases around it - a balloon full of helium floating up through air is doing exactly the same thing as a balloon full of air floating up through water.
It doesn’t ignore gravity. Just the other gasses in the atmosphere ignore gravity even less and get pulled down harder, pushing the helium out of the way. And the only “out of the way” direction is up.
The collisions with all the other air molecules. One of the things about gases is that on average, all of the molecules have the same amount of kinetic energy, regardless of their mass. But helium atoms have less mass than nitrogen or oxygen molecules, so to have the same amount of kinetic energy they do, they have to be moving faster (on average). This extra speed allows the helium atoms to get higher off the ground (working against gravity) than the average nitrogen or oxygen molecule can.
I’m glossing over some details here, but that’s the basic jist of it.
Same reason you “ignore gravity” when you float in a pool of water. The water is denser than you are, so it wants to be underneath you, and it’ll push past you until it is.
Suppose you had a balloon filled with a gas that’s denser than air. You let it go, and it sinks, right?
Now imagine that you’ve got a whole atmosphere of a planet filled with that gas that’s denser than air. And you’ve got a balloon filled with air. You let the balloon go. What happens? The dense gas sinks, right? That means it falls under the balloon. Which pushes up the air-filled balloon.
The helium filled balloon rises in air for the exact same reason a CO2 filled balloon will sink in air, and for the exact same reason wood will rise in water while rocks sink.
For fun, take a drive with a helium balloon. When you come to a stop, the balloon will move backwards in a manner exactly opposite to you. When you accelerate, the same thing will happen, you’ll be pushed back in your seat while the balloon will surge forward.
I don’t think any of you guys are answering the question. He understands what happens. He doesn’t understand why. You’re just stating tautologies like “It sinks because it falls down” and “It rises 'cause it gets pushed up.”
Which is…?
The same reason, which is…? The question was “why does density matter”, not “does density matter?”
Why?!
Free helium, not in a balloon, doesn’t really rise. Or it does, but only incidentally: What it’s really doing is just spreading out and mixing with all the other gases. In the long term, gases don’t displace each other, they just mix.
A very elementary analogy:
Imagine a big bowl of dog food. (The Earth, for the purposes of this analogy.) There are hundreds of dogs trying to get to it. The biggest, strongest dogs (the heavier molecules) succeed in pushing their way all the way to the food bowl. The little dogs (the lighter molecules) want to get to the food too, but they’re pushed out of the way by the larger dogs, so they end up at the outer edges of the pack. The medium size dogs can push past the little ones, so they do, but they can’t get past the big ones, so they form a layer somewhere in the middle.
In the real world, instead of molecules being ‘stronger’ than each other and fighting for dog food, it’s gravity that is making everything move toward the center of the Earth. Gravity acts differently with different things, though - the more mass something has, the harder it pulls. (We usually call this ‘weight’, or say that things are ‘heavier’ and ‘lighter’, although technically those terms aren’t quite right.) The heavier molecules don’t push the lighter ones in any particular direction other than ‘out of my way’, so theoretically the light molecules could go anywhere. (I do mean ‘push’ literally here, molecules in gas form literally just knock around bumping into each other.) However, from the small molecule’s point of view, if they stay closer to the dog food bowl they’ll just get pushed by another large molecule, then another one, and sooner or later they’ll get pushed upwards where hey, suddenly they’re no longer getting pushed, so they just stay there. Eventually when enough of the light molecules get pushed upwards and stay there (because they’re not getting bumped by heavy molecules any more), we call it layers.
This is doubtlessly very imprecise, and possibly quite childish, but I hope it’ll help.
To expand on this(yeah, bad pun) an individual helium atom is going to try to follow a parabolic trajectory like a ball would.(IE it really travels just like any other ball and falls just like any other ball) From what I remember of gen chem the difference is it will bounce off of other gas particles(molecules and atoms) and that will change its velocity.(IE direction and speed) I guess somebody else can do the math since there’s a formula to figure out how far it’ll travel before it hits another particle. Of course since helium is light it tends to have a higher speed on average in the first place(these speeds are I think a bell curve for what it’s worth) so it has a better chance off getting to escape speed after a few bounces. (But it falls just like very thing else.)
Pressure differentials.
For something like a pool of water, the further down you go, the more pressure there is. (The pressure is equal to the weight of all the water directly above it) If you built a hollow cube, filled it with water, and submerged it in the pool, the pressure on the top surface of the cube is going to be lower than the pressure at the bottom surface of the cube (since the bottom surface is further down where there is more pressure). This pressure differential causes an upward force on the cube. However the water cube stays exactly where it is because the weight of the water inside the cube is also pushing down exactly the same amount as the pressure differential is pushing upwards.
But say you took the same hollow cube and filled it with air, then submerged it in the pool. The pressure differential between the top surface and the bottom surface of the cube is going to be the same as the previous example, which again pushes up on the cube. But this time, there is nothing inside the cube weighing it down to counteract that force. So the air cube rises.
The same thing happens in the atmosphere with a balloon of helium. There’s a (tiny) pressure differential between the air at the bottom of the balloon and the air at the top of the balloon. Since the helium isn’t heavy enough to counteract the pressure differential, the balloon rises.
OK, because the heavier objects experience a greater force from gravity than light objects - and if they’re free to flow or move past each other (as is true of liquid water vs a block of expanded polystyrene, or air vs a balloon full of helium), then the things that are pulled hardest will be moved closest to the pulling force, pushing the things that are not being pulled so hard out of the way.
Is this true? On the scale of weeks gases from volcanic eruptions can spread along the ground, killing humans and animals by displacing the normal atmospheric gases. In the long term there is almost no helium in the air because it all rises to the top of the atmosphere and escapes into space.
So it seems that over both long and short time scales gases do not just mix. Lighter gases rise, heavier gases sink. Sure, they will mix to some degree with enough agitation, they are gases after all, but that;s seems very different form what you are claiming.
To clarify what you are saying. Imagine that I fill a huge latex balloon with several million litres of Helium ant ground level. I then pop the balloon. Are you claiming that the resultant “cloud” of helium won’t rise? That if I were to “mark” that helium then at some point in time I am just as likely to find a the atom 500 metres away at ground level as I am 500 metres upwards? If so then what point would that be? 15 minutes after release? 3 days?
This is what I wanted to ask - and why would a bunch of individual helium atoms act collectively differently just because they’re inside a balloon. Or to put it another way, why would helium balloons float in Nitrogen if helium atoms don’t?
The “cloud” of helium will start off fairly small, a few meters across or so, but it’ll expand into a very large “cloud”. The center of that cloud will be high above ground level, but that’s because the cloud is expanding. It would expand in all directions, but the ground is in the way in one direction, so it can hardly expand at all that way. So, yes, the average movement ends up being upwards, just because it can’t go down.
Yep, that’s what I came in to say. If the individual, lighter gas molecules rose, we’d have an almost perfectly stratified atmosphere with pretty much 100% O2 at our level-- O2 being heavier than N2.
You have to have an enclosed volume of the gas to make it lighter than the surrounding air.
As my physics teacher put it, the helium balloon doesn’t rise; it’s just that all the air it’s lighter than falls down around it.
Oh you’d see other effects besides that if it were true. For example you’d expect to see wine, beer, or other alcoholic liquids stratify into a layer of water on top, then a layer of ethanol(which is heavier) and then a layer of even heavier stuff. (Because the liquids in both beer and wine are mixtures just like air is a mixture of gases.)
Isn’t the meta-question, “why are Helium atoms ‘fat?’” Why does a given volume of Helium, at standard temperature and pressure, have so few atoms, so that it masses so very much less than, say, the same volume of Lithium atoms (even allowing for Lithium atoms being heavier in atomic number.) i.e., why does Helium resist compression so much more than Nitrogen or any of the solids?
I think it has to do with the outer electron shell being completely full, as in all of the noble gases…
Edited to Add: I don’t know what the hell I’m talking about, but I’m trying to ask intelligent questions…