The discussion about Maxwell’s demon and the related test has already lasted for about twenty-five days. Many people contributed their interesting opinions, most of which are valuable or helpful. We thank Physics Forum and all the participants very much. Nevertheless, it looks as if the discussion is a little divergent. In order to focus the attention in a better way on the most important problems, we list here again some main points of Maxwell’s demon and our experiment as follows, including some important quotations as reference. It amounts total five points.
1. The second law of thermodynamics.
2. Maxwell’s original idea, the first model.
3. Ehrenburg et al, the second model.
4. Replace electrons to molecules.
5. The problem of energy.
1. The second law of thermodynamics
In 1850 and 1851, Clausius and Kelvin established the second law of thermodynamics. They presented two famous equivalent statements of this law.
Clausius: It is impossible to device a process whose only result is to extract heat from a reservoir and to eject it to a reservoir at a higher temperature.
Kelvin: It is impossible to device a process whose only result is to convert heat , extracted from a single reservoir, entirely into work.
According to this law, all practical processes in nature are irreversible, proceeding from useful energy to waste one, like a one-way traffic. And the universe is thus destined to go forward to its eventual state, the “heat death.”
Such a law sounds strange. Nevertheless, numerous of observations and experiments confirm its validity. Few people dare to think that the “universe traffic” should be a round way one.
2. Maxwell’s original idea, the first model of demon
In 1871, the prominent British physicist James Clerk Maxwell firstly challenged this law. The following is his original words.
One of the best established facts in thermodynamics is that it is impossible in a system enclosed in an envelope which permits neither change of volume nor passage of heat, and in which both the temperature and the pressure are everywhere the same, to produce any inequality of temperature or of pressure without the expenditure of work. This is the second law of thermodynamics, and it is undoubtedly true as long as we can deal with bodies only in mass, and have no power of perceiving or handling the separate molecules of which they are made up. But if we conceive a being whose faculties are so sharpened that he can follow every molecule in its course, such a being, whose attributes are still as essentially finite as our own, would be able to do what is at present impossible to us. For we have seen that the 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 ones 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.
Maxwell is a great prophet in the history of physics. He had predicted displacement current, electromagnetic waves, distribution law of the speed of molecules in a gas, the crisis of the classical physics (hydrogen heat capacities at different temperatures), and so on, and all these predictions were verified one by one, ten years, thirty years, and even fifty years after his death. We think that there is still an important prophecy of his not yet verified so far, the prophecy about the demon. It is very likely that this one is his greatest prophecy.
In Maxwell’s conceiving, the demon allows only the swifter molecules to pass from A to B, and only the slower ones to pass from B to A, and the system will change from a state in which both the temperature and the pressure are everywhere the same, to a new state in which the temperature of B is higher than that of A.
This may be referred to as the first model of the demon, (see Fig.1. *)
3. The second model of the demon
The following words are quoted from W Ehrenburg’s general review about the history of Maxwell’s demon, showing a second model.
As it happens, there is an easier way to design a perpetual motion machine that employs the services of such a sorting demon. Beginning with equal pressure and temperature on each side of the division, the demon could, by opening and closing the shutter at the right times, allow molecules to pass only from portion A to portion B. This would eventually result in a difference in pressure between the two sides, a situation even more adaptable than a difference in temperature.
W. Ehrenberg, Maxwell’s Demon,
Scientific American, pp 103-110 (1967)
By the first model, a difference in temperature is finally obtained. This is a process opposite to heat transfer.
By the second model, a difference in pressure is finally obtained. This is a process opposite to gas free expansion.
In traditional thermodynamic, both heat transfer and gas free expansion are very typical and essential irreversible processes.
We approve of Ehrenberg’s opinion that the second model is much easier to realize than the first model, (see Fig.2 *).
4. Replace electrons to molecules, a version of Ehrenberg’s model
Very similar to the gas molecules, thermal electrons ejected by a cathode in a vacuum tube obey Maxwell’s speed distribution law, too. Nevertheless, electrons are much easier to harness than neutral molecules. In our design and experiment, we replace electrons to neutral molecules.
In a vacuum tube, two Ag-O-Cs cathodes A and B are settled on an insulated base, as shown in the following diagram:
IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIAAAAAAAAIIBBBBBBBBIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII
IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII
where IIII is some insulation material, such as mica or quartz.
According to A H Simmer et al, Ag-O-Cs cathodes, due to their lowest work function, eject thermal electrons considerably at room temperature.
Common experience of using Ag-O-Cs cathodes shows that their thermal electron emission overpasses all other materials. The emission density of different samples may differ from each other for several grades, the typical value ranges from 10-11 to 10-14 A/cm2.
--Photo-emissive Materials, Chap.10, Sec.7.1, A. H. Sommer, 1968
The technique of photoelectric tubes and photoelectric multipliers demands an Ag-O-Cs cathode a lower thermal electron emission. If the thermal electron emission is high, the tube or the multiplier is of bad quality. In our experiment, quite contrary, higher thermal electron emission is expected.
When a stable uniform magnetic field is applied to the tube in the direction parallel to the interval between cathodes A and B (perpendicular to this paper and entering it), part of the electrons ejected by A can fly overpass the interval and fall into B, and no any electron ejected by B can fly overpass the interval and fall into A.
Such a working method coincides with Ehrenberg’s second model, (see Fig.3 and Fig.4 *).
The one-way transfer of electrons results in immediately a charge aggregation, with B positively charged and A negatively charged, and a corresponding electric potential difference is established between B and A. The device can thus supply electric power to an exterior load.
5. The problem of energy
One may ask, where comes from this electric power?
It comes from the heat extracted from the surrounding air.
As soon as A is positively charged and B negatively charged, a static electric field appears
above the interval between A and B. The direction of the electric field is such that it impedes the succeeding electrons to fly from A to B. Nevertheless, the electrons have thermal energy, or speed. The electrons overcome the impediment and fly from A to B. In so doing, they slow down, and their “temperature” falls down (slightly), resulting in a drop down (very slight) of the temperature of the whole tube. The tube then extracts heat from the surrounding air to compensate its temperature drop. The whole process converts heat extracted from the air to electric power to supply an exterior load, without producing any other effects.
It is clear and simple that the energy here is conserved. What’s new and important in this experiment is Maxwell’s idea of converting waste energy into useful energy. We think we have realized his dream.
* Note: Fig.1 through Fig.4 are in our arxiv paper Realization of Maxwell’s Hypothesis,
http://www.arxiv.org/physics/0311104
