h a l f b a k e r yExpensive, difficult, slightly dangerous, not particularly effective... I'm on a roll.
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Hi there,
I had this idea,
use of very thin sheet of elastic material so that molecules
and atoms bounce of it, so that heat transfer is minimised
(
the atoms and molecules loose less of their kinetic energy)
possible ? existing ?
[link]
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It's a great idea apart from the physics bit. |
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i see :-) thanks for the answer |
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I can't see this working in the real world; but it's not immediately ruled out by theory. |
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There are two possibilities for efficient molecular reflection: |
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1) A trampoline-like skin, of negligible mass compared to the mass of the molecule to be reflected (like bouncing a brick off a trampoline). |
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2) An extremely massive, extremely high modulus substance (like bouncing a super ball off a brick wall) |
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I can't think of any real-world candidates for 1) (it would probably need to be a field, rather than matter), but neutronium might qualify for 2). |
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It would need an impervious layer of molecules...a
layer of graphene? And behind that, a vacuum, I think.
However, the vacuum will not support the graphene
against the pressure of the molecules on the other
side.
Which brings me to think about the interesting
similarity between gas temperature and pressure in
terms of things jiggling around. Only average velocity
is different, no? |
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//Only average velocity is different, no?// Yes. To be precise, temperature and pressure are directly related to the RMS average of velocity. |
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//However, the vacuum will not support the graphene against the pressure of the molecules on the other side.// And even if it could, the layer of graphene is made of many carbon atoms, and is therefore massive compared to any gas molecules. Momentum, and therefore energy, and therefore heat, will be transferred during collisions. |
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Another way of putting it is that it's difficult to make the impedance mismatch very large, because everything - including the gas and the surface - is made of similar stuff, i.e. atoms and molecules. |
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I suppose the hypothetical layer of Graphene would
quickly reach a steady state where no more energy
could be absorbed (it reached the same temperature!). |
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Yes, but that's what everything does. If it then did not transfer much heat to the structure or space on the other side, that would simply make it a low thermal mass, good insulator - of the ordinary kind - rather than a heat mirror. |
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//There are two possibilities for efficient molecular reflection// |
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What about electric charge - positive reflecting postive and negative reflecting negative? |
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//A trampoline-like skin, of negligible mass// ... //it would probably need to be a field, rather than matter// |
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I'd already cunningly included the third possibility in the first. |
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I thought it was too good to last... |
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There is no such thing as 'heat reflective' paint, or glass, or paint for glass. I'm going to bun this idea, simply because the author says 'heat' and means 'heat' (not light, of any wavelength). |
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Right. The idea is to reflect hot atoms.
A strong laser field can perfectly reflect atoms if tuned slightly away from an absorption line. But this works for practical laser powers (up to 1 watt) only for atoms that are not too hot (say one microkelvin above abs zero). Solution: increase laser power to 10000 watts. |
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// //Only average velocity is different, no?// Yes. To
be precise, temperature and pressure are directly
related to the RMS average of velocity.// |
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Sorry to hijack this a bit...If a piston very slowly
compresses some gas, assuming no heat loss the gas
will considerably increase its temperature (increase
the average velocity). Where does the increase in
velocity come from? Logic says it can only come from
the velocity of the piston, no matter how slow the
piston. |
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// It's a great idea apart from the physics bit //
[marked-for-tagline] |
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// the velocity of the piston // |
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the kinetic energy of the piston/cylinder system … |
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//Logic says it can only come from the velocity of
the piston, no matter how slow the piston.// |
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If I remember correctly, you are right. If the
piston moves quickly, it gives those atoms that
hit it a large additional speed. If it moves slowly,
it gives the atoms that hit it a smaller boost, but
more atoms will hit it during its (slower) motion.
Either way, the total energy delivered to the
atoms of the gas is the same. |
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Actually that prompts a wildly tangential
question, which I may pose in a different post. |
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okay, having done a bit of research for some other post, I can say that atomic(literally) Newtonian KE isn't the only form of energy storage in a gas. |
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