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MOST IMPORTANT SCIENTIFIC DISCOVERIES
The second law of thermodynamics. History and essence of scientific discovery
Directory / The most important scientific discoveries The Englishman Humphrey Davy (1788-1829) became a professor at the age of 23, earned many scientific and public awards, and besides, he added the treatment "sir" to his name, was elected president of the Royal Society of London. During his long life in science, he conducted many successful experiments. At the beginning of the nineteenth century, Davy succeeded in melting ice by friction at a temperature below zero. Later, the experience was repeated by the Russian scientist Petrov. Benjamin Thompson (1753-1814), who emigrated from America after the victorious conclusion of the War of Independence and received the title of Count Rumford in Bavaria, published in 1798 the results of experiments on drilling cannon barrels. In one of his experiments, at 960 revolutions of the drill, the temperature of the drilled cylinder rose by 37 degrees Celsius. Davy came to the conclusion that the theory of caloric was incompatible both with Rumfoord's experiments and with his own, and put forward the kinetic theory of heat, according to which heat represents the oscillatory motion of the particles of the body, and for gases and liquids, he also allowed the rotational motion of particles. Jung also joined the vibrational theory of heat. And yet the theory of caloric continued to dominate. The two most fundamental works on the theory of heat relating to the period under consideration, works that have rightfully entered the golden fund of scientific literature, are based on the concept of caloric. The first of these works, Fourier's Analytical Theory of Heat, was published in 1822 in Paris and is the result of his many years of research in the field of mathematical physics. Another essay belonged to the son of the famous French mathematician Lazar Carnot, Sadi Carnot. Nicolò Léonard Sadi Carnot (1796–1832) studied at the Polytechnic School. Since 1814 he has been working as a military engineer, and since 1819 he has been a lieutenant at the general staff. As the son of a republican minister in exile, Carnot could not be promoted and retired in 1828. He died of cholera. The essay Meditation on the Motive Force of Fire, published in 1824, was Carnot's only completed work. Carnot writes: “Heat is nothing but a driving force, or rather, a movement that has changed its form; it is the movement of the particles of bodies; wherever the destruction of the driving force occurs, heat arises in an amount exactly proportional to the amount of the disappeared driving force. Conversely : always with the disappearance of heat there is a driving force. Thus, it is possible to express a general position: the driving force exists in nature in an unchanged amount; it is, strictly speaking, never created, never destroyed; in fact, it changes form, that is, it causes now one kind of movement, then another, but never disappears. According to some ideas that I have about the theory of heat, the creation of a unit of force requires the expenditure of 2,7 units of heat. Regarding these lines, the famous French scientist Henri Poincare exclaimed admiringly in 1892: "Is it possible to express the law of conservation of energy more clearly and more accurately?" As an engineer, Carnot was engaged in the calculation and construction of water engines. But since by that time steam engines were increasingly being used throughout France, the young engineer became interested in creating the theory of heat engines. Back then, science was dominated by the views that heat is a substance. But Sadi Carnot decided to answer one of the most difficult questions in physics; Under what conditions is it possible to convert heat into work? Well acquainted with the calculation of water engines, Carnot likened heat to water. He knew perfectly well that in order for the water mill to work, one condition is necessary - the water must fall from a high level to a low one. Carnot suggested that in order for heat to do work, it must also move from a high level to a low one, and the height difference for water corresponds to the temperature difference for heat. In 1824, Sadi Carnot expressed the idea, thanks to which he went down in history: for the production of work in a heat engine, a temperature difference is necessary, two sources of heat with different temperatures are needed. This statement in Carnot's theory is the main one and is called Carnot's principle. Based on the principle he derived, Carnot came up with the cycle of an ideal heat engine, which no real engine can surpass. The ideal machine, according to Carnot, was a simple cylinder with a piston. The bottom wall of the cylinder has ideal thermal conductivity, it can be placed on a hot surface, for example, on the surface of a heater filled with a mixture of molten and solid lead, or on the surface of a refrigerator, for example, with a mixture of water and ice. Both sources of heat are infinitely large. The second law of thermodynamics states that a perpetual motion machine of the second kind is impossible. This statement is a paraphrase of Carnot's principle, and hence the efficiency of a machine operating on a Carnot cycle cannot depend on the substance used in the cycle. Carnot described the cycle of operation of an ideal heat engine, showed how to calculate its maximum efficiency. To do this, it is only necessary to know the highest and lowest temperature of the water vapor (or any other coolant, as Carnot noted) used in this machine. The difference between these temperatures, divided by the high temperature value, equals the efficiency of the machine. Temperatures must be expressed in degrees of the absolute Kelvin scale. This equation is called the second law of thermodynamics, and all technology obeys it. The calculation according to the Carnot formula showed that the first heat engines could not have an efficiency higher than 7–8 percent, and if we take into account the inevitable heat leakage into the atmosphere, then the resulting value of 2–3 percent should be recognized as a significant achievement ... Quite quickly, along with steam, as Carnot predicted, gas was also used in turbines, which can be heated to a high temperature. If the temperature of the hot gas in the turbine is 800 degrees Kelvin (527 degrees Celsius), and the refrigerator reduces it to 300 degrees Kelvin, then the maximum efficiency of the machine, even in the case of an ideal Carnot cycle, cannot be higher than 62 percent. The inevitable heat losses lead, as always, to a decrease in this figure. The best examples of turbines installed in modern power plants have an efficiency of 35-40 percent. Carnot pointed out a specific feature of heat. Heat creates mechanical work only with a thermal "difference", i.e., the presence of a temperature difference. This temperature difference determines the efficiency of heat engines. Paul Clapeyron in 1834 developed Carnot's ideas and introduced a graphic method that is very valuable in thermodynamic studies. In 1850, the first work of Rudolf Clausius (1822-1888) "On the driving force of heat" was published, in which again, after Carnot and Clapeyron, the question was raised about the conditions for converting heat into work. The principle of conservation of energy, requiring only quantitative equality, does not impose any conditions for the qualitative transformation of energies. In this work, Clausius analyzes Carnot's theory from a new point of view, from the point of view of the mechanical theory of heat. Carnot's work had recently been resurrected from the ashes of oblivion by William Thomson (Lord Kelvin) (1824-1907). "Thomson admits," PS Kudryavtsev writes in his book "History of Physics," that Carnot's view that heat in machines is only redistributed, but not consumed, is wrong." But at the same time he points out that if we abandon Carnot's conclusions regarding the conditions for the conversion of heat into work, then insurmountable difficulties are encountered. Thomson concludes that the theory of heat requires serious restructuring and additional experimental research. In his work, Clausius believes that along with the first law, which says, "that in all cases when heat produces work, an amount of heat is consumed proportional to the work received," Carnot's position should be retained as the second law, that work is produced when heat passes from a warmer body to a colder one. This position, according to Clausius, is consistent with the nature of heat, in which there is always a transition of heat "by itself" from a hot body to a cold one, and not vice versa. As the second beginning, Clausius puts forward the postulate: "Heat cannot "by itself" pass from a colder body to a warmer one." The words "by itself" should not mean that heat cannot be transferred at all from a cold body to a heated one (otherwise refrigeration machines would not be possible). They mean that there can be no such processes, the only result of which would be the mentioned transition, without corresponding other "compensatory" changes. This work was followed almost simultaneously in 1851 by Thomson's three papers. Having examined the question of the transformation of various forms of energy from a quantitative point of view, Thomson points out that with the same quantitative value, not all types of energy are capable of transformation to the same degree. For example, there are conditions under which the conversion of heat into work is impossible. Thomson's postulate says: "It is impossible, by means of an inanimate body, to obtain mechanical action from any mass of matter by cooling its temperature below that of the coldest of the surrounding bodies." Developing this position, Thomson in his work of 1857 comes to the well-known conclusion about the dominant tendency in nature to convert energy into heat and to equalize temperatures, which ultimately leads to a decrease in the efficiency of all bodies to zero, to heat death. In 1854, Clausius in his article "On a modified form of the second law of the mechanical theory of heat" proves Carnot's theorem, based on his postulate, and, generalizing it, gives a mathematical expression of the second law in the form of an inequality for circular processes. In subsequent works, Clausius introduces the state function "entropy" and gives a mathematical formulation of the trend, seen by Thomson, in the form of the position "The entropy of the universe tends to a maximum." So, in physics, along with the "queen of the world" (energy), her "shadow" (entropy) appeared. Clausius himself at the end of his work in 1865 writes: “The second law, in the form in which I gave it, says that all transformations that take place in nature in a certain direction, which I took as positive, can occur by themselves, i.e. without compensation, but in the opposite, i.e., in a negative direction, they can occur only if they are compensated by positive transformations occurring simultaneously with them. Applying this principle to the entire universe leads to the conclusion first pointed out by William Thomson. Indeed, if for all the changes taking place in the Universe, the states of transformation in one particular direction constantly prevail in magnitude over transformations in the opposite direction, then "the general state of the Universe must change more and more in the first direction, and thus it must constantly approaching the limit state. Author: Samin D.K.
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