Maxwell's Demon: Chance of Violating the 2nd Law?

[wendten]

http://en.wikipedia.org/wiki/Maxwell%27s_demon"

Maxwell imagines one container divided into two parts, A and B. Both parts are filled with the same gas at equal temperatures and placed next to each other. Observing the molecules on both sides, an imaginary demon guards a trapdoor between the two parts. When a faster-than-average molecule from A flies towards the trapdoor, the demon opens it, and the molecule will fly from A to B. The average speed of the molecules in B will have increased while in A they will have slowed down on average. Since average molecular speed corresponds to temperature, the temperature decreases in A and increases in B, contrary to the second law of thermodynamics
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Szilárd pointed out that a real-life Maxwell's demon would need to have some means of measuring molecular speed, and that the act of acquiring information would require an expenditure of energy. The second law states that the total entropy of an isolated system must increase..

when i read this i was wondering, what if the deamon accuires no information at all and just shoots blind by guessing wherever a approaching particle is A or B..

in most cases the demon would end up with a no separation, but if its lucky enough it might separate them entirely and violate the second law

... just a thought ;-)

 

[Nick89]

The molecules might also divide spontaneously, if "you're lucky enough". The chance of that happening (and the chance of your demon 'accidentally' separating the molecules) are simply so small that they can be regarded as zero.

 

[wendten]

which means that the change of the law being violated is regarded as zero, but not that it is zero?

and if the amount of particles in the containers is just small enough there actually is a good change that a sepperation might be the case

 

[Mapes]

The Second Law tells us that total entropy tends to increase to a maximum, and that significant deviations become less and less likely as a system's size increases.

In your gas chamber example, for example, no physical law says that exactly 50% of the atoms must be on each side. But 50% becomes a better and better approximation as the number of atoms increases. (I believe the deviation goes something like N^{-1/2} for N particles.)

 

[granpa]

http://en.wikipedia.org/wiki/Maxwell%27s_demon"



when i read this i was wondering, what if the deamon accuires no information at all and just shoots blind by guessing wherever a approaching particle is A or B..

in most cases the demon would end up with a no separation, but if its lucky enough it might separate them entirely and violate the second law

... just a thought ;-)

an atom diode would work just as well. it can't separate slow moving from fast moving atoms but it can separate atoms moving one direction from atoms moving the opposite direction

 

[Vanadium 50]

The font springs eternal.

One cannot violate the laws of thermodynamics with an atom diode. It takes work to use an atom diode to move atoms from the left half of a box to the right half, just as it takes work to accomplish the same thing with a piston.

 

[granpa]

the idea is that the motion of the molecules due to heat would move them through the barrier in one direction and once through would be trapped on the other side

 

[Vanadium 50]

You can't create a perpetual motion machine that way.