As science develops, its laws cover ever wider areas, are refined, approach the laws of nature, and become adequate to them. In a generalized form, the nature of the connection between the laws of nature and the laws of science was clearly expressed by A. Einstein: "Our ideas about physical reality can never be final, and we must always be ready to change these ideas." P. L. Kapitsa, who loved paradoxes, even said this: "It is not the laws themselves that are interesting, but the deviations from them."
But the inventors of perpetuum mobile are wrong, counting on a completely possible change in the laws of science, which do not yet permit the operation of perpetual motion machines. The fact is that the laws of science (in particular, physics) are not canceled, but supplemented and developed.
N. Bohr formulated a general position (1923), reflecting this regularity of the development of science: the principle of correspondence, which says that any more general law includes the old law as a special case; it (old) is obtained from the new one when passing to other values of the quantities defining it.
The approval of the law of conservation of energy - the first law of thermodynamics - made attempts to create a perpetual motion machine of the first kind absolutely hopeless. And although they were still going on, the main direction of thought of the creators of perpetuum mobile changed. New versions of perpetual motion machines are being born in full agreement with the first law of thermodynamics: how much energy enters such a motor, exactly the same amount goes out.
As you know, the law of conservation of energy can be formulated in the following somewhat modified form: for all processes of energy conversion, the sum of all types of energy participating in this process must remain unchanged. Such a formulation, although it does not allow the possibility of creating energy from nothing, however, leaves open another way of realizing a perpetual motion machine, the principle of operation of which would be based on the ideal transformation of one form of energy into another.
It was known that work in engines is done when a hot body gives off heat to a gas or steam and steam does work, for example, moving a piston. However, it turned out that there was no way to make the energy from a colder body go to a hotter one. But to create a perpetual motion machine, it is necessary that at the same time work is done.
As a result of the development of thermodynamics, based on the works of Sadi Carnot, Rudolph Clausius showed that a process is impossible in which heat would pass spontaneously from colder bodies to warmer bodies. In this case, not only a direct transition is impossible - it is also impossible to carry out it with the help of machines or devices without any other changes occurring in nature.
William Thomson (Lord Kelvin) formulated the principle of impossibility of a perpetual motion machine of the second kind (1851), since processes are impossible in nature, the only consequence of which would be mechanical work performed by cooling a heat reservoir.
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Investigating the issue of a new type of perpetuum mobile at the beginning of the XX century. the famous German physicist and chemist Wilhelm Ostwald studied. He called the ideal machine, capable of cyclically and without loss, converting energy from one form to another, he called a perpetual motion machine of the second kind. As can be seen, even after the abandonment of the possibility of creating a perpetual motion machine of the first kind, the problem of perpetual motion still remains open. However, perpetual motion machines of the first and second kind are already significantly different from each other. If the function of the perpetual motion machine of the first kind, declared by scientists to be unrealizable, consisted in the continuous performance of useful work without replenishing energy reserves from external sources, then from the perpetual motion machine of the second kind, only the ability to ideally transform energy was required.
According to the first law of thermodynamics, heat is equivalent to mechanical energy, therefore, without contradicting the first principle, it is quite possible to build a machine that takes heat from a body that has the temperature of the surrounding air, or, for example, takes heat from water from large reservoirs and performs due to this mechanical work. If we convert the now received mechanical energy back into heat, then a closed cycle of energy conversion, based on the principle of a perpetual motion machine of the second kind, arises.
However, such phenomena are never encountered in everyday life. In a warm room, a bottle of milk taken out of the refrigerator heats up, and a glass of hot tea cools down. In addition, a cold liquid, when heated, imperceptibly lowers the air temperature in the room, and a hot one increases it. At the same time, it never happens that a cold body cools by itself or a hot one warms up. For such cooling, special refrigeration units are used, which, however, need a constant supply of energy from external sources. At the same time, spontaneous cooling of a cold or heating of a hot body does not at all contradict the first law of thermodynamics. Therefore, it is obvious that the wording of this law should somehow be clarified and supplemented.
The second law of thermodynamics eliminates the incompleteness of the law of conservation of energy, which did not distinguish between reversible and irreversible processes and thereby left an illusory hope for those who did not want to put up with the impossibility of creating a perpetuum mobile. This physical principle imposes a limitation on the direction of processes that can occur in thermodynamic systems. The second law of thermodynamics prohibits the so-called perpetual motion machines of the second kind, showing that the efficiency cannot be equal to unity, since for a circular process the temperature of the refrigerator cannot be equal to absolute zero (it is impossible to build a closed cycle passing through a point with zero temperature).
There are several equivalent formulations of the second law of thermodynamics:
Clausius' postulate: “A circular process is impossible, the only result of which is the transfer of heat from a less heated body to a more heated one” (this process is called the Clausius process).
Thomson's (Kelvin's) postulate: "A circular process is impossible, the only result of which would be the production of work by cooling the heat reservoir" (this process is called the Thomson process).
Another formulation of the second law of thermodynamics is based on the concept of entropy:
"The entropy of an isolated system cannot decrease" (the law of non-decreasing entropy). In a state with maximum entropy macroscopic irreversible processes (and the process of heat transfer is always irreversible due to the Clausius postulate) are impossible.
When statistical thermodynamics was created, which was based on molecular concepts, it turned out that the second law of thermodynamics has a statistical character: it is valid for the most probable behavior of the system. The existence of fluctuations prevents its accurate implementation, but the probability of any significant violation is extremely small. That is, the transfer of heat from a cold body to a hotter one is possible, but this is an extremely unlikely event. And in nature, the most probable events take place.
Read also "Perpetual motion machine of the first kind" and "Perpetual motion machine of the third kind"