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Post by Progenitor A on Jan 12, 2011 9:50:01 GMT 1
A good Horizon programme the other night in which an ex-physicist looked for meaning in the expression '1 degree'. This was in the context of Global Warming where the meaning of '1 degree' increase is not clear.
A beautiful illustration of what it measn was given by an Oxford Mathematician.
Meanwhile here was a demonstarion of the quantum effect in action in a macro world where Nitrogen was cooled to 4K where it became absolutely quiescent - there was no activity going on at all in the liquid because it was at thermal equiibrium. The container in which it was kept - a small glass cylinder closed at one end by a bung (that at greater tempertaures sealed the liquid nitrogen in) at the super-cooled liquid stage became porous and the Nitrogen seeped out. Other odd things happen such as the Nitrogen flowing up th eside of the container and out of the top.
Fascinating
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Post by speakertoanimals on Jan 12, 2011 14:00:18 GMT 1
This is actually superfluid Helium II, NOT nitrogen.
Super-cooled is the wrong phrase to use, since it doesn't just mean extra-cooled, but a specific state, where a liquid such as water is cooled past its freezing point yet still remains liquid -- that is supercooled water, which can spontaneously freeze if disturbed. One good example (on Mythbusters I think), was where you put beer in the freezer. When removed, still liquid, but if you bang the bottle, spontaneously freezes.
The flow of the Helium II up the sides of the container and out through the bung is because the capillary forces on the helium won out over gravity and viscous forces.
You get normal liquid helium between about 4K (boiling point), and the lambda point of about 2K (helium I). Below the lambda point, you get the helium II state, a superfluid state, which does weird stuff like the creeping effect mentioned.
I think the confusion with nitrogen might have arisen because liquid nitrogen is used in cryogenics..................
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Post by principled on Jan 17, 2011 11:20:31 GMT 1
HI STA Can you give us a layman's explanation of why this happens? Thanks P  |
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Post by speakertoanimals on Jan 17, 2011 17:14:22 GMT 1
In liquid water, the molecules wander about, with no permanent bonds between molecules.
In ice, the molecules form a definite crystal structure, and they no longer wander about, but stay bound to their neighbours.
So, what happens when we melt ice is easy to see -- as we raise the temperature, moelcules vibrate more and more (normal thermal motion), until they vibrate so much that they break out of the crystal structure, and become a liquid.
Going the other way though -- if we have a liquid and cool it, the molecules loose thermal energy, but they don't necesarily form the crystal structure -- they need soemthing to start forming a crystal o, if you like. So if you had a seed crystal in there, then the crystal would grow from that.
But without something to start crystallization, you can get the liquid remaining liquid, even though the crystal is the preferred state.
Such a state is metastable, and if you disturb it, then the crystal forms. The disturbance can be physical (bang the bottle).
Same goes for superheated liquids, in effect. I can heat coffee in the microwave, so that the temperature of the liquid is above the boiling point. If there isn't something to start a steam bubble (like a scratch on the container), then the water can remian in the metastable, superheated state. Disturb it (by taking it out, or adding instant coffee), and the whole cup can violently boil, generating a LOT of steam, which isn't good if your face is over the cup, or your hand holding the handle.
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Post by principled on Jan 17, 2011 18:36:39 GMT 1
Thanks STA
I have to say that I have experience (!) of the superheated + microwave . It was only AFTER the event that I did some research to find the cause...I'd like to say it was empirical research but...
Anyway, getting back to your answer. I can appreciate the need for some form of "seeding" to cause crystallisation or to superheated water to boil, but how does that "seeding" actually start the process? Taking the example of the physical shock and the water freezing, what is it about the shock (and I suppose the shockwave passing through the metastable water) that causes it to convert into its preferred crystalline structure?
P
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Post by speakertoanimals on Jan 17, 2011 19:30:43 GMT 1
Seeding is simple, in that the molecules that are sloshing about come up against the existing crystal, and find that becoming part of that is preferable.
With them all still sloshing about, you can view it as an undecided crowd -- they all know they should move off, but until somefew make a move, the rest won't go, and if no one person is confident enough to do it on their own, then it doesn't happen!
Although complicated by the fact that you can only supercool to a certain extent!
It's hard to deal with, because ordinary physics just says -- ice is preferred state according to thermodynamics, hence explaining why you get this metastable stuff is actually quite hard! It's like a pencil balanced on its point -- falling over is more favourable in energy terms, but unless there is something to pick one direction over another (the crystal starts HERE), then it will remain poised.
I guess the balanced pencil is the best analogy I can come up with, where the smallest nudge converts you from poised pencil state, to the stable fallen-over state.
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