Maxwell's Demon: Exists, Hard at Work at Phase Interfaces

[klimatos]

In his famous thought experiment of 1867, James Clerk Maxwell postulated a creature (later called a “demon” by Lord Kelvin) who could discern the speeds of gaseous molecules and who could pass selected molecules through a partition or deny them passage on the basis of those speeds. The essence of that “demon” was its ability to bar or pass molecules based on their speed.

May I suggest that the demon exists and is hard at work at every phase interface (water-vapor, water-ice, ice-vapor, etc.). The role of “demon” is played by the collective forces of molecular attraction. Water molecules moving toward the water surface from below will become vapor molecules only if their speed equals or exceeds the “escape velocity”. They will remain water molecules if their speed is less that that velocity. This is a selection based on speed.

The removal of faster (hotter) molecules from the water or ice phase reduces the temperature of that phase. The kinetic energy lost in overcoming the cohesive forces goes on the books as potential energy; i.e., the enthalpy of vaporization or the enthalpy of fusion.

Thus we see that Maxwell’s Demon does exist. He goes by many names: force of intermolecular attraction, surface tension, surface energy and so on. But he does exist and is hard at work doing exactly what Maxwell envisaged him doing.

 

[fluidistic]

In his famous thought experiment of 1867, James Clerk Maxwell postulated a creature (later called a “demon” by Lord Kelvin) who could discern the speeds of gaseous molecules and who could pass selected molecules through a partition or deny them passage on the basis of those speeds. The essence of that “demon” was its ability to bar or pass molecules based on their speed.

May I suggest that the demon exists and is hard at work at every phase interface (water-vapor, water-ice, ice-vapor, etc.). The role of “demon” is played by the collective forces of molecular attraction. Water molecules moving toward the water surface from below will become vapor molecules only if their speed equals or exceeds the “escape velocity”. They will remain water molecules if their speed is less that that velocity. This is a selection based on speed.

The removal of faster (hotter) molecules from the water or ice phase reduces the temperature of that phase. The kinetic energy lost in overcoming the cohesive forces goes on the books as potential energy; i.e., the enthalpy of vaporization or the enthalpy of fusion.

Thus we see that Maxwell’s Demon does exist. He goes by many names: force of intermolecular attraction, surface tension, surface energy and so on. But he does exist and is hard at work doing exactly what Maxwell envisaged him doing.

My (very limited) understanding of Maxwell's demon is that it contradicts the 2nd law of thermodynamics.
More precisely you start with a container made of 2 parts with a uniform temperature and you end with a cold and a hot part without having done any work, contradicting the 2nd law.
In your example you can start with air and liquid at same temperature and end up with only air remaining but at a uniform temperature.By the way in your example it's true that the liquid becomes colder when it evaporates but the air also try to keep it "hot" so as to minimize the temperature difference between air and liquid. I think that's why the liquid finishes evaporating.

On the other hand if the air is saturated in liquid, the liquid won't evaporate anymore. Some fast molecules might leave it but then some fast molecules of air will enter into it.

To me the demon is not just to be able to choose the faster molecules, it's what I wrote in starting this post.

 

[klimatos]

"My (very limited) understanding of Maxwell's demon is that it contradicts the 2nd law of thermodynamics."

It does.

"More precisely you start with a container made of 2 parts with a uniform temperature and you end with a cold and a hot part without having done any work, contradicting the 2nd law."

Correct. That is how Maxwell described it.

"In your example you can start with air and liquid at same temperature and end up with only air remaining but at a uniform temperature."

I did not postulate isothermal conditions. They are not essential. The selection occurs regardless of the temperature of the water and the temperature of the humid air.

"By the way in your example it's true that the liquid becomes colder when it evaporates but the air also try to keep it "hot" so as to minimize the temperature difference between air and liquid."

The air does not "try to keep the water hot" If the air is warmer than the water, then heat will flow from the air to the water. If the water is warmer than the air, then heat will flow from the water to the air. If the two are isothermal, then there will be no heat flow.

"I think that's why the liquid finishes evaporating."

Gross vaporization continues as long as a water/vapor interface exists. Net vaporization continues until the number of incoming vapor molecules equals or exceeds the number of outgoing molecules.

"On the other hand if the air is saturated in liquid, the liquid won't evaporate anymore. "

The air is never saturated with liquid, it may or may not be saturated with water vapor. Even when saturated, gross vaporization continues even after net vaporization stops. See my argument in the paragraph above.

"Some fast molecules might leave it but then some fast molecules of air will enter into it."

Continually, in both cases, regardless of water temperature, vapor temperature, or vapor pressure.

"To me the demon is not just to be able to choose the faster molecules, it's what I wrote in starting this post.

I disagree. In Maxwell's letter to Tait, it is clear that Maxwell was concerned only with molecular selection. The "finite being" was just a way to bring this about. My hypothesis is that this selection occurs at every phase interface.

 

[russ_watters]

The implication of "molecular selection" in Maxwell's demon is a violation of the second law of thermodynamics and that's the point of the thought experiment. What's happening at a water/air interface does not violate the second law of thermodynamics. It is not an example of Maxwell's demon in action: The molecules that go to gas do not get "selected", they escape on their own due to their energy.

 

[D H]

I disagree. In Maxwell's letter to Tait, it is clear that Maxwell was concerned only with molecular selection. The "finite being" was just a way to bring this about. My hypothesis is that this selection occurs at every phase interface.

It is clear in Maxwell's Theory of Heat (page 338-339, http://ia700104.us.archive.org/6/items/theoryofheat00maxwrich/theoryofheat00maxwrich.pdf ) that Maxwell is concerned with using his demon as a means of violating the second law of thermodynamics.

Evaporation and boiling do not violate the second law of thermodynamics. These processes act to increase rather than reduce entropy.

 

[klimatos]

The implication of "molecular selection" in Maxwell's demon is a violation of the second law of thermodynamics and that's the point of the thought experiment. What's happening at a water/air interface does not violate the second law of thermodynamics. It is not an example of Maxwell's demon in action: The molecules that go to gas do not get "selected", they escape on their own due to their energy.

I made no mention of the Second Law in my post. I am well aware that vaporization increases the entropy of the system. My focus was on molecular selection phenomena at the interface. I maintain that when some molecules escape and others do not the use of the term "selection" is justified.

"Selection" does not require conscious thought and selection by itself does not violate the Second Law. The Ranque-Hilsch vortex tube separates hot and cold air molecules without conscious thought and does not violate the Second Law.

The cohesive forces at the various interfaces perform exactly the same functions ascribed to Maxwell's "demon". That was and is the point of my posting.

 

[klimatos]

It is clear in Maxwell's Theory of Heat (page 338-339, http://ia700104.us.archive.org/6/items/theoryofheat00maxwrich/theoryofheat00maxwrich.pdf ) that Maxwell is concerned with using his demon as a means of violating the second law of thermodynamics.

Evaporation and boiling do not violate the second law of thermodynamics. These processes act to increase rather than reduce entropy.

Violating the Second Law was Maxwell's concern. It is not mine. I am concerned with the mechanism (molecular selection) Maxwell chose to use, not the end result. I can see no reason why molecular selection (the defining characteristic of the "demon") should violate the Second Law. It certainly does not do so with vaporization.

 

[FroChro]

Violating the Second Law was Maxwell's concern. It is not mine. I am concerned with the mechanism (molecular selection) Maxwell chose to use, not the end result. I can see no reason why molecular selection (the defining characteristic of the "demon") should violate the Second Law. It certainly does not do so with vaporization.

Well, my knowledge of thermodynamics is pretty limited, but I would like to point out a slight difference in those two processes. Evaporation is selection of molecules based on "testing" all molecules - all molecules "are trying to fly away", but only some do pass the test. Maxwell demon can choose which will be tested, in fact he gives slow molecules impassable test (flying through impassable barrier) and gives to fast molecules test they can't fail (flying through open space). This ability to choose how to affect specific molecule may indeed decrease entropy, while affecting all molecules by same effect does not.

 

[kcdodd]

Also important to the story is the demons ability to act on a system in thermal equilibrium. If it cannot then it is not really relevant to the context it was created, and so I wouldn't classify it a maxwell demon.

 

[russ_watters]

I made no mention of the Second Law in my post.

Violating the Second Law was Maxwell's concern. It is not mine.

Yes, I understand that. But do you understand that by describing something different from "Maxwell's concern", you're not really talking about Maxwell's demon? Your usage of the word "selection" may well be viable according to the dictionary, but it isn't how Maxwell used the term.

"Selection" does not require conscious thought and selection by itself does not violate the Second Law.

By your definition of "selection" that's true - by Maxwell's, it isn't. Again, you're not talking about the same thing. Now it is a free internet and you're entitled to make up your own thought experiment, call it "klimatos's demon" and discuss it here (with certain limits). So:

"klimatos's demon" states that molecules at a gas/liquid interface are "selected" by the laws of nature to escape from the surface of the liquid if they have a high enough energy or be captured if they don't.

So what's your point? What does "Klimatos's demon" tell us that's useful/interesting about how the universe works?

The cohesive forces at the various interfaces perform exactly the same functions ascribed to Maxwell's "demon". That was and is the point of my posting.

No they don't. Maxwell's demon's selection isn't based on the molecules on their own following the laws of nature; in your version it is.

In any case, if that's all there is to your point, then I just plain don't see this as interesting enough to be worth discussing. Maxwell's demon is an important thought experiment that makes a worthwhile commentary about the second law of thermodynamics. What your version says is pretty trivially obvious. By removing the second law implications, you remove what makes Maxwell's demon interesting! So so what? Is there more to your point than that? Or are you trying to say that Maxwell's demon is a triviality? Are you trying to make an end-run around the second law?:

I can see no reason why molecular selection (the defining characteristic of the "demon") should violate the Second Law.

In your version, it doesn't, in his version it does. Or are you trying to claim that because in your version it doesn't, it shouldn't in his version either?

By the way, the "flaw" or resolution of Maxwell's thought experiment is that the demon itself generates entropy (such devices actually exist). Your version doesn't really have a demon, or rather, you're calling the laws of nature a demon (without a nod to Sagan, I might add...).

 

[timthereaper]

Maxwell's thought experiment is this:

"If we conceive of a being whose faculties are so sharpened that he can follow every molecule in its course, such a being, whose attributes are as essentially finite as our own, would be able to do what is impossible to us. For we have seen that molecules in a vessel full of air at uniform temperature are moving with velocities by no means uniform, though the mean velocity of any great number of them, arbitrarily selected, is almost exactly uniform. Now let us suppose that such a vessel is divided into two portions, A and B, by a division in which there is a small hole, and that a being, who can see the individual molecules, opens and closes this hole, so as to allow only the swifter molecules to pass from A to B, and only the slower molecules to pass from B to A. He will thus, without expenditure of work, raise the temperature of B and lower that of A, in contradiction to the second law of thermodynamics."

The definition of his "demon" is intimately related to this experiment. Unless you're talking about a being that can divide molecules with higher velocity from those with lower velocity in a closed system at uniform temperature that doesn't not permit volume change or heat exchange, it is not Maxwell's demon.

 

[Curl]

I only read the OP, and I'll tell you you're wrong. The demon is supposed to keep the fast molecules on one side. The intermolecular forces don't care, since any particle that left can just as easily return to the liquid.

I've thought about this before for a long time, and I can come up with many of these "pseudo-demons" like this one, but none of them satisfy the requirements posed by Maxwell. It was later shown that such a creature or "mechanism" will need information, which isn't free (see http://en.wikipedia.org/wiki/Entropy_%28information_theory%29" ). Any "luck" which the demon might have is negated by the central limit theorem, and does not violate the 2nd law.

game over

 

[Dougvsmith]

Maxwell's Demon was discussed in the March 2011 issue of Scientific American, pp 55-59. According to the article, in 1929, Leo Szilard resolved the paradox by proposing that the demon collects information each time the trap door is opened and that this information carries entropy that balances the entropy decrease. Apparently, this has been verified by experiment since that time.