so this board has provided a wonderful service, at least for me. most of the pointless ponderings that keep me up at night now have a friend who understands. way too many random questions that i have thought about (knight rider physics, q-tips and why they feel so good, etc) pop up on here before i can type them. the answers to my helium balloon question make me feel wright at home, although it is a weird electronic home that only exists on my cathode ray. but hell, at least you guys can’t ground me.
anyway, i (think i) know why helium balloons rise. but what about free helium? does it also rise? if so, is it for the same reasons?
But on a serious note - I would assume that the gas would diffuse through out the atmosphere. IIRC - regular vulcanism and biological influences maintain the current gas mix in the atmosphere.
It is the relative lightness of the concentrated helium that makes the balloon lighter than the surrounding air.
Yes, of course. Why would it suddenly pick another reason simply because it’s not in a balloon.
“If you put helium in a balloon and let go of the balloon, the balloon rises until it pops. When it pops, the helium that escapes has no reason to stop - it just keeps going and leaks out into space. Therefore, in the atmosphere there is very little helium at any given time. The helium that is there comes from alpha particles emitted by radioactive decay (see the Nuclear Radiation article for details on alpha decay). In places that have a lot of uranium ore, natural gas tends to contain high concentrations of helium (up to 7%). This makes sense, since the decay of uranium emits lots of alpha particles and a natural gas pocket tends to be a sealed container underground. Helium is cryogenically distilled out of natural gas to produce the helium we put in balloons.”
wait, wait. just did a thought experiment. water is denser than gaseous nitrogen, so the ocean sinks below the atmosphere. anchors are denser than water so they sink to the bottom. helium , following the same logic, should rise to the top of the whole affair.
wait wait wait- i took a bathroom break and just got back. the anchor only sinks because all of the atoms of iron are tightly bound to one another, so (a) the force of gravity is pulling on a lot more paticles than a piece of, say, styrofoam of the same volume, and (b) the particles themselves can’t disperse. but a gas can, in fact does.
and the balloon was pushed up because of the net force of milions if not billions of air particles smashing into the balloon. but helium is smaller than said particles. there is no net force.
and tc, no disrespect, but wouldn’t your “it has no reason to stop so it keeps going up” argument lead to all gases except the one with largest weight/abundance ratio escaping from earth forever? gravity still tugs on the helium, just not as strong.
as to the weight/abundance ratio, is tc’s idea correct, and this is why there’s so much goddamned nitrogen around? oh god my head hurts…
Helium is in fact constantly leaking into space from the top of the earth’s atmosphere. However, it’s also being produced by various radioactive decay pathways, so the concentration is fairly stable.
Helium does end up escaping, but the mechanism’s a little different than that of a rising balloon. As jb_farley correctly reasoned, buoyancy has little effect on free gasses, since they tend to diffuse into each other-- That’s why the atmosphere remains a mixture of nitrogen and oxygen, instead of all the nitrogen layering out at the top. Note that there’s still some buoyant effects: If you produce pure CO2, for example, in a test tube, you can “pour” it out, and it’ll “puddle” on the ground for a while before diffusing. In the end, though, diffusion still wins.
What actually happens with helium, is that at any given temperature, gas molecules will have a certain spread of speeds. At the same temperature, all gasses will have the same kinetic energy per particle, so hydrogen and helium, for instance, will have higher speeds on average than nitrogen and oxygen: Specifically, a helium atom has one seventh the mass of a nitrogen molecule, so it’ll have an average speed approximately 2.65 times higher than the nitrogen molecule. It so happens, that with the temperatures typical on Earth, this puts a significant number of the helium atoms at above escape speed, so the helium gradually all flies off into the wild black yonder.
