Maxwell's demon - a new concept

In summary, the idea of a Maxwell's demon is that if you have a container of gas molecules, and the molecules are randomly flying around, then you will often find molecules at different ends of the container. If the molecules have different speeds and energies, then the molecules at the different ends of the container will heat up and the molecules at the middle will cool down.
  • #1
Stanley514
411
2
Could somebody disprove the following concept of a Maxwell's demon:
Let's say we have a very small container, the size and a form of a nanotube, similar to ellipse, and only two gas molecules randomly flying inside. According to probability we will quite often find molecules in the different ends of a such container. If these molecules will have different speed and energy (is it likely to happen?) then we will get the hotter and the colder molecules in the different ends of the container, what is one of the conditions for a Maxwell's demon to start his work. If hotter molecule will stay at one end of the container for a while it will bounce to its walls and heat it. And contra, the cold molecule at the same time will bounce at the other end of nanotube (container) and cool it. In this way a temperature difference between the different ends of nanotube will be created. Then we could imagine a two thermoelectric wires of an atomic thickness which are connected to both ends of the nanotube and to a small electric generator. If we need to scale this system up, we could take a two largest molecules which could only exist in nature and conduct the experiment in a deep space that gravitation and weight of molecules wouldn't interrupt us. Does nature put some fundamental restriction on size of molecules (which could have a thermal motion)?

And also question: is probability of a thermal fluctuations is always the same on all closed systems? Or there could be some systems with increased probability?
 
Last edited:
Physics news on Phys.org
  • #2
Energy and temperature aren't the same thing. The terms hotter and colder refer to macroscopic quantities of particles with macroscopic number of degrees of freedom. It doesn't really make much sense to talk about a hotter molecule and a colder molecule, unless these are really big molecule.

The probability of a fluctuation depends on the size of the fluctuation, so it is given as a power spectrum. Some theoretical values can be obtained from the fluctuation dissipation theorem. http://en.wikipedia.org/wiki/Fluctuation-dissipation_theorem

What do you hope to accomplish with this thermoelectric current? There will be thermal noise in the voltage coming out of the generator. If the temperature of the molecules is greater than the ambient temperature outside the generator, some energy will leak from the molecules into whatever the generator is attached to. But if whatever the generator is attached to is higher temperature than the molecules, energy will leak the other way.
 
  • #3
Khashishi said:
Energy and temperature aren't the same thing. The terms hotter and colder refer to macroscopic quantities of particles with macroscopic number of degrees of freedom. It doesn't really make much sense to talk about a hotter molecule and a colder molecule, unless these are really big molecule.
The probability of a fluctuation depends on the size of the fluctuation, so it is given as a power spectrum. Some theoretical values can be obtained from the fluctuation dissipation theorem. http://en.wikipedia.org/wiki/Fluctuation-dissipation_theorem
What do you hope to accomplish with this thermoelectric current? There will be thermal noise in the voltage coming out of the generator. If the temperature of the molecules is greater than the ambient temperature outside the generator, some energy will leak from the molecules into whatever the generator is attached to. But if whatever the generator is attached to is higher temperature than the molecules, energy will leak the other way.
I do not think that the size of molecules has some principal meaning if we want to talk about them in a terms of their temperature (their speed and energy). But I even appreciate to regard a case with really large molecules. Usually, it is assumed that somehow divison of molecules by their speed is sufficient for a Maxwell's demon to do a usefull work. Up to now most of objections against Maxwell's demons were related to a question on how much energy the demon will use for his own work and that he will spend more energy for his work that will be obtained from temperature differences he will create. Many scientists claim they already demonstrated possibility to convert information to energy with help of a demon however demon spend more energy than is gained as result of his work. I'm not sure that a thermal motion of the container is a principal obstacle, otherwise the concept of demon wouldn't have a sense at all. But what is exactly the difference if instead of sorting out many small molecules, demon will sort a two huge only, with the same mass as if it would be a many small molecules? There is no even need to sort a two molecules since they will constantly fly at the different ends of a container according to a pure probability. If a two molecules will have sufficient mass, speed and energy (and difference in energy between each other) the temperature difference they will create suppose to be sufficient to overcome and exceed the thermal motion in container wall or elsewhere in the system.
[/PLAIN] [Broken]

 
Last edited by a moderator:
  • #4
A large molecule has many degrees of freedom. So there are many places for the thermal energy to go.
 
  • #5
You've assumed the "hot" molecule always moves to the same end of your nanotube, and that's right back to Maxwell's Demon.
 
  • #6
DaleSpam said:
A large molecule has many degrees of freedom. So there are many places for the thermal energy to go.
And what? Where could it go?
 
  • #7
Bystander said:
You've assumed the "hot" molecule always moves to the same end of your nanotube, and that's right back to Maxwell's Demon.
What did you want to tell by that?
 
  • #8
You do not know which end of the tube is hot.
 
  • #9
Bystander said:
You do not know which end of the tube is hot.
Not necessary. First of all if a molecules really big we could monitor their position and speed and it will not take too much energy in comparison to the energy a generator will produce. Secondly, (though I'm not sure) we could use some type of AC generator for which there is no difference in which direction current will flow. A capacitor for example is non polar and could be charged by flow in any direction. We may use a device to charge a capacitors. Not a usefull work would you say? Also we may have a thermal detector which inform us which side is colder and which is hotter. We could connect wires from the container sides with help of some diodes. So when one end of container is hotter current flows. When it is colder current doesn't flow at all and doesn't bother us.
 
Last edited:
  • #10
You might find it useful to compare your ideas to available information on "Johnson Noise" and "Boltzmann Distribution."
 
  • #11
I have a theoretical assumption:
What if we have some hypothetical kind of matter (which is capable to create a structures)
which doesn't have any degrees of freedom and therefore no thermal motion. Subsequently, it doesn't have temperature or its temperature is always equal to zero. In one word properties of such matter is similar to a field which neither have degrees of freedom and subsequently an infinite density. Many scientists believe that a field (for example a magnetic field) is also kind of matter. If we would have such type of hypothetical matter, would it be useful for a Maxwell's demon creation? If yes, does physics postulate absolute impossibility for a such matter to exist?
 
  • #12
So if I understand correctly you have a thermoelectric wire attached to a generator and you hope this will allow you to generate electrical work at equilibrium. But such a generator will consume work as often as it produces it since the voltage and current are uncorrelated.
 
  • #13
DaleSpam said:
So if I understand correctly you have a thermoelectric wire attached to a generator and you hope this will allow you to generate electrical work at equilibrium. But such a generator will consume work as often as it produces it since the voltage and current are uncorrelated.
Why at equilibrium?? There is two huge molecules flying inside of container. They create huge thermal difference, as one of them is fast and the other is slow.
 
  • #14
You aren't doing anything to keep it in one side or the other, so even a big thermal fluctuation is still just a thermal fluctuation.
 
  • #15
DaleSpam said:
You aren't doing anything to keep it in one side or the other, so even a big thermal fluctuation is still just a thermal fluctuation.
Molecules constantly come in contact with container walls and exchange energy with them. As fast molecule will get to one end of the container it will very likely strike this end and heat it. Of course, you may not be able to predict which one end of the container the molecule will strike. However, how do you think AC current generator does work? For example take a look at a free piston engine:
free-pistong-engine-image-01.jpg


So, as you see energy is released each time at a different end of cylinder. Magnet is moved back and forth. Current flows each time in opposite direction. It does produce AC current. I do not claim it, but I guess a similar scheme could be used with help of thermoelectrics. Once a one end of the container becomes hotter current flows in one direction, once the opposite end gets hotter current reverses flow. As a result you may be able to obtain AC current with irregular voltage and frequency. It will look like a natural audio wave.
Something similar to this:
Audio_wave_by_DinkydauSet.png
 
  • #16
You've been trying to execute an end-run around the statistics in statistical mechanics by working with two very large molecules. The way you're thinking about it, you don't need the concept of temperature at all (and your understanding of temperature as the kinetic energy of the molecules doesn't apply in this case, so it's a good thing you don't need it).

The system you've described so far (nanotube, two big molecules) is just a scaled-down version of a perfectly ordinary length of pipe with two perfectly ordinary balls bouncing around in it.
 
  • #17
Nugatory said:
The system you've described so far (nanotube, two big molecules) is just a scaled-down version of a perfectly ordinary length of pipe with two perfectly ordinary balls bouncing around in it.
I thought about it. I think that more likely there is some fundamental restriction on the size of molecules (or particles) which are capable to have a macroscopic thermal motion. If not, then we could have even two neutron stars (which are basically two giant atoms) flying inside some planetary size tube. However, you didn't do your consistent critics of the idea.
 
  • #18
Stanley514 said:
Current flows each time in opposite direction.
This is the key point. To produce a power the current and the voltage must be in the same direction, otherwise you the generator absorbs power rather than producing it. There is nothing in your scheme which makes the current and the voltage both go in the "power" direction at the same time. So your generator will be randomly absorbing mechanical energy as often as it is randomly producing it.
 
  • #19
DaleSpam said:
This is the key point. To produce a power the current and the voltage must be in the same direction, otherwise you the generator absorbs power rather than producing it. There is nothing in your scheme which makes the current and the voltage both go in the "power" direction at the same time. So your generator will be randomly absorbing mechanical energy as often as it is randomly producing it.
So why do you believe that current and voltage will be not in the same direction? When you heat a one end of thermoelectric, voltage and current contradict each other? Why would they?
 
  • #20
Stanley514 said:
So why do you believe that current and voltage will be not in the same direction?
Because they are both thermally fluctuating. Your setup doesn't change that in any way.
 
  • #21
Stanley514 said:
...temperature (their speed and energy).
Again, temperature and energy are not the same thing. Don't claim to violate thermodynamics if you don't have a clear idea of what it is.
 
  • #22
DaleSpam said:
Because they are both thermally fluctuating. Your setup doesn't change that in any way.
If you will take a regular thermoelectric device and will heat a hot sinc of it with help of usual match your voltage and current will fluctuate as well. Because a match may burn uneven. However it doesn't mean that your voltage and current will come in contradiction to each other. Could you explain it in detail on example of a molecule? Thermoelectric effect if based on tendency of electron gas to expend. Once some metallic piece of matter if heated, electrons will try to expand and move from hotter areas to a cooler areas. Everywhere they could. Now imagine that we have a container with the ends made of metal (such as copper) and middle section made of some heat resistant material. Then, when fast molecule will strike a one end (made of metal) electrons will try to flow to the colder end of container. However as it's too difficult for them to overcome thermally resistant barrier (the middle section) it would be easier for them to move through a thermoelectric device to a cold end and simultaneously perform some useful work. Yes, voltage and current may not be stable. However they do not exist separately from each other. They both originate in electron gas expansion.
 
Last edited:
  • #23
Nugatory said:
You've been trying to execute an end-run around the statistics in statistical mechanics by working with two very large molecules. The way you're thinking about it, you don't need the concept of temperature at all (and your understanding of temperature as the kinetic energy of the molecules doesn't apply in this case, so it's a good thing you don't need it).

The system you've described so far (nanotube, two big molecules) is just a scaled-down version of a perfectly ordinary length of pipe with two perfectly ordinary balls bouncing around in it.
Ordinary balls do not experience Brownian motion. (within themselves only).
 
  • #24
Stanley514 said:
Ordinary balls do not experience Brownian motion. (within themselves only).
Neither do the two molecules in a two-molecule system.

The point here is that you can use the methods of statistical mechanics (in which temperature has a definition, and it's more than just the kinetic energy of the particles) everywhere, or you can use them nowhere (in which case there is no concept of temperature). If you mix and match, as you're trying to do, you will get nonsense results.
 
  • #25
Nugatory said:
Neither do the two molecules in a two-molecule system.
So, how will they behaive then?

Do I understand it correct, that second law of thermodynamics doesn't prohibit to obtain useful energy from thermal fluctuations and once you obtained it, you do not have to loose the same amount of useful energy anywhere in time or space and it could be regarded as an pure gain? This is just takes very long time at any known conditions?
Also, probability of fluctuations is not the same in all type of closed systems? For example, with rise of temperature probability of fluctuations will increase? Also in systems with smaller amount of particles probability will increase?

"All thermal fluctuations become larger and more frequent as the temperature increases, and likewise they decrease as temperature approaches absolute zero."
http://en.wikipedia.org/wiki/Thermal_fluctuations

If the experiment is correct, the beads momentarily gained energy from their environment. However, this perpetual motion machine only worked for about two seconds and is not a likely practical device.
http://www.csicop.org/sb/show/has_the_second_law_been_falsified

 
Last edited:
  • #26
Stanley514 said:
If you will take a regular thermoelectric device and will heat a hot sinc of it with help of usual match your voltage and current will fluctuate as well. Because a match may burn uneven. However it doesn't mean that your voltage and current will come in contradiction to each other.
You seem to think that the thermoelectric effect is magic. It is not. The thermoelectric effect is simply a heat engine, and a rather inefficient heat engine at that.

Your suggested setup will not have a temperature difference between the different sides, so there is no temperature difference and therefore no overall thermoelectric voltage. The voltage and current will fluctuate, as they always do, but in your case they will fluctuate about 0, which is the key difference. When you have a temperature gradient, then they fluctuate around some non-zero value. That doesn't happen in your setup.

The mere fact of fluctuations doesn't give you work.
 
  • #27
DaleSpam said:
You seem to think that the thermoelectric effect is magic. It is not. The thermoelectric effect is simply a heat engine, and a rather inefficient heat engine at that.

Your suggested setup will not have a temperature difference between the different sides, so there is no temperature difference and therefore no overall thermoelectric voltage. The voltage and current will fluctuate, as they always do, but in your case they will fluctuate about 0, which is the key difference. When you have a temperature gradient, then they fluctuate around some non-zero value. That doesn't happen in your setup.

The mere fact of fluctuations doesn't give you work.
Here is a real example of obtaining electricity from a thermal fluctuations:

The team made final preparations for their presentation by tuning the device and the data collecting software MatLab.
They were going to graph a live data stream to demonstrate that they had succeeded in extracting electrical energy from liquid crystals after applying arbitrary thermal fluctuations on their device.
http://news.engineering.iastate.edu...students-harness-energy-from-liquid-crystals/
In this case pyroelectricity is used, however.
 
Last edited by a moderator:
  • #28
If you read the article you will see that they have a hot reservoir and a cold reservoir. There is no indication that this is anything other than a heat engine.
 
  • #29
Stanley514 said:
Do I understand it correct, that second law of thermodynamics doesn't prohibit to obtain useful energy from thermal fluctuations and once you obtained it, you do not have to loose the same amount of useful energy anywhere in time or space and it could be regarded as an pure gain? This is just takes very long time at any known conditions?

You cannot obtain useful, macroscopic energy from thermal fluctuations. Macroscopic means that the scale is above the atomic scale. There's also the requirement that the timescale be appropriate for the size scale. Atomic and subatomic systems change states extremely quickly, on the order of picoseconds, nanoseconds, etc. Macroscopic systems are generally on the order of milliseconds or longer.

At the microscopic scale a heat engine may convert heat (energy) from one part of the system into work, but there is an equally likely chance that the heat engine will convert work into heat (since all heat engines can operate in reverse), which is then deposited back into the system, so there is no net flow of energy on a macroscopic scale.

Stanley514 said:
If you will take a regular thermoelectric device and will heat a hot sinc of it with help of usual match your voltage and current will fluctuate as well.

But no energy is generated from the fluctuations. Energy is generated due to a temperature gradient. The fluctuations only mean that the amount of energy generated fluctuates. It's like a campfire. The amount of energy the fire puts out decreases as the wood burns and fuel is lost. Putting more logs onto the fire increases the energy generation. So we can say that the energy generated by the campfire fluctuates as the amount of fuel fluctuates, but the campfire doesn't generate energy these fuel fluctuations, it generates energy from the combustion process.

So your thermoelectric device will generate energy when there is a temperature gradient, with the amount of energy being generated dependent on how big the gradient is. A fluctuation in the temperature gradient changes the amount of energy generated, but the fluctuation is not the source of the energy, the temperature gradient is.
 
  • Like
Likes mfb

1. What is Maxwell's demon?

Maxwell's demon is a thought experiment proposed by physicist James Clerk Maxwell in 1867. It involves a hypothetical creature that has the ability to separate fast-moving and slow-moving particles in a gas, and thus, violates the second law of thermodynamics.

2. Why is Maxwell's demon significant?

This thought experiment raises questions about the validity of the second law of thermodynamics, which states that the total entropy of a closed system will always increase over time. It also has implications for the understanding of information and its role in the physical world.

3. How does Maxwell's demon work?

The demon is imagined to be able to open and close a small door between two chambers of gas, allowing only fast-moving particles to pass through. This causes one chamber to become hotter (as it contains more fast-moving particles) and the other to become colder (as it contains more slow-moving particles).

4. Is Maxwell's demon a real creature?

No, Maxwell's demon is purely a thought experiment and does not exist in reality. It is used to explore fundamental concepts in physics and has not been observed or created in any experiment.

5. Can Maxwell's demon be used to create perpetual motion?

No, although the thought experiment challenges the second law of thermodynamics, it does not actually violate it. The demon's actions require energy, and the overall system would still experience an increase in entropy. Therefore, perpetual motion cannot be achieved using Maxwell's demon.

Similar threads

Replies
8
Views
2K
  • Electromagnetism
Replies
8
Views
1K
  • Electromagnetism
Replies
9
Views
1K
  • Electromagnetism
Replies
5
Views
2K
Replies
17
Views
1K
Replies
12
Views
4K
Replies
78
Views
11K
Replies
2
Views
2K
Replies
7
Views
2K
Replies
4
Views
1K
Back
Top