Maxwell's demon

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Re: Maxwell's demon

Postby Doug Coulter » Fri Mar 16, 2012 3:52 pm

Now you've done it, I'll have to go dig up that book which has a long-form detailed derivation of 2-T - which I accept. I DO NOT accept the common oversimplification taught to kids in science class, however.
The "real" statement makes the assumptions explicit (over time, in the absence of non linearities in the system and so on) - and in some systems, those assumptions are not satisfied, hence the rest doesn't apply as it would were all the assumptions true. That's the angle I'm attacking this from. It's not like 2T is dead wrong, it's like Newton, then we discover e=mc2 and stuff like that. Only in this case, its classical vs quantum, working in a region where neither of the math-sets work very well, and the "splice" function isn't very well defined, so there's some hope here. Or so I think. In fact, I once did a very careful study of 2T to discover what the assumptions were so I could perhaps find a way to make them not the case, to enable such flights of fancy. I've stated the important ones above.

"On average". 2T allows for things that are "transitory". It just says what the equilibrium will wind up being.
"Nothing nonlinear" - most always the case in nature (but not around black holes, or Hawking radiation wouldn't exist), but not if you do something to deliberately make nonlinearity happen. This provides the possibility of changing "transitory" to "permanent" if you can make energy flow one way with some sort of "check valve" - be it a photon that radiates out of the system, or a diode - or something entirely else.

Someone imagined and built a tiny watch escapement - a mechanical ratchet-diode. Put it in warm water, the thing turns all the time, just one way (no obvious temperature drop in the system anywhere) - and you can see why, clearly. Some big surprises have happened at the nanoscale, where things get interesting in the tug of war between quanta and classical smoothness. In this case, any random fluctuation of atomic jiggling that happens to put force on the wheel in the correct direction can move it. The rest of the time, the ratchet holds it. So you can "select from random data" and be successful with doing it. I just think the floating magnet thing scales better, since you could in principle do it with silicon fab technique we have already.
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Re: Maxwell's demon

Postby chrismb » Sun Mar 18, 2012 5:10 am

I'll have to get back to you later with my explanation that I think might help justify some of this, because I've been suffering a mega-migraine the last few days and can't concentrate that well. I guess too many Maxwell's demons have come to haunt me for saying this all, and they are now batting the wrong blood-vessels/synapses in my head to prove me wrong!
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Re: Maxwell's demon

Postby Doug Coulter » Tue Apr 10, 2012 6:17 pm

Don't mean to make your head hurt. A lot of people's heads are hurting as we get into that very interesting area between pure classical and pure quantum - the math fails at the "splice" point similar to how it does when going from viscous flow to molecular in vacuum systems. Another example of that:
http://phys.org/news/2012-04-carbon-nan ... emote.html
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Re: Maxwell's demon

Postby chrismb » Sun Apr 15, 2012 8:35 am

I owe you 'my' explanation for all this, still. So much on these days.. sometime soon!!...
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Re: Maxwell's demon

Postby Doug Coulter » Tue Jul 03, 2012 6:23 pm

Ah, once a PhD writes it up, it's true, eh?
http://arstechnica.com/science/2012/07/ ... mments-bar
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Re: Maxwell's demon

Postby Doug Coulter » Tue Nov 13, 2012 4:43 pm

Oops. They are catching on to the idea.
http://phys.org/news/2012-11-powering-lasers.html
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Re: Maxwell's demon

Postby Doug Coulter » Wed Nov 14, 2012 10:53 am

The hits just keep coming. All you need is any selection mechanism or nonlinearity. Of course, I was wrong about that until half the PhD's on the planet changed their minds. I guess you have to give them some credit for that, actually, since the facts to justify it in the lab.

http://physicsworld.com/cws/article/new ... es-a-motor
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Re: Maxwell's demon

Postby chrismb » Wed Nov 14, 2012 5:07 pm

Doug, I think you are mistaken. None of these three items propose that work can be done by such mechanisms. They are all showing (actually, theorising) how work can be extracted as entropy increases.

Nothing against the laws of thermodynamics here ....

I still owe you my explanation of why entropy always increases, on average (and certainly, when you use it to do work), I said I would ... been busy of late ....
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Re: Maxwell's demon

Postby Lars Berntzon » Wed Dec 26, 2012 9:02 am

Interesting idea. There must be some sort of work involved when the particle is moving particle in browns movement. Do you really need diodes? What if you have a very thin but long coil, and have a fixed magnetic field over the whole tank and instead have small ferromagnetic particles moving around. Each part of the coil should produce a random noise as the particles move in and out of the coil area, and each winding of the coil adding to the voltage as individual noise sources as RMS-values? Not sure what happens with the EMF force, it would be evened out over the coils length and thus not cause cooling to the particles, probably only giving voltage but no current.
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Re: Maxwell's demon

Postby Doug Coulter » Wed Dec 26, 2012 11:12 am

What happens with a lot of magnetic particles is that they simply clump and become one big one. You want tiny particles, as this means the probability of high velocity motion due to a bunch of the other atoms hitting them is much higher. You need all three axes, as if you only inhibit motion in one axis, randomness will tend to channel all the energy into the other two.

Little particles are favored as it's a lot easier to get other than a "50% coin toss" with fewer tosses. This is why brownian motion isn't noticed at large scales, and indeed was discovered only after the microscope.

You have to have the diodes, else you are limited to Johnson type noise that doesn't add up very well (this is just the noise across a warm resistor - it goes up in voltage with resistor size, but noise power does not). This is the required selection process that violates the unstated assumption in the 2nd law - the assumption that everything is linear and everything is random. We need them to pick the "tails" off the distribution. Randomness then repopulates them.

All these things together mean just about the only way to build one of these is with semiconductor fab type techniques, and at similar scales. The diodes need not be perfect, but do need to be fast, have a very low or zero forward threshold, and "reasonable" front to back ratio - 10's to one at worst, or you cannot sum the output power from zillions of tiny cells.
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