THERMODYNAMICS

THERMODYNAMICS, a branch of the science of heat, in the broad sense - the science of energy and therefore relates to all physical, chemical, and biological phenomena. It is built on two propositions, called principles, obtained experimentally, free from any hypotheses or theories, and therefore the conclusions of T. cannot be essentially changed by any further development of science. The first principle of thermodynamics is the law of conservation of energy as applied to thermal phenomena. If the energy of any body (or system of bodies), isolated from the external world, is denoted by E, then according to the law of conservation of energy E = const.

(1) regardless of whether the energy E will remain unchanged in form or will be converted into other forms of energy. If the body is not isolated and its energy, changing, will be partially released outward, then in the external world the manifestations of energy will appear, which will be a measure of the change in energy E, which we will denote by ΔE. At the same time, the manifestation of energy can be expressed by mechanical work A, energy of electromagnetic nature P and finally heat Q. Then according to the law of conservation of energy, assuming that all forms of energy are measured by the same units, we obtain: ΔE = A + P + Q

(2) If we assume that the body is at rest, then the change in energy reduces to the change in the so-called internal energy, which is a function of the thermal state, which is determined by the chemical properties, temperature and specific volume of the body. Denoting for one state of the body the internal energy by U₁, for another - U₂ and assuming that changes of electromagnetic nature are excluded, for the change in internal energy ΔU = U₂ - U₁ we obtain the expression: ΔU = A + Q

(3) One can imagine that a body, changing its state successively, will return again to its original state, will, as they say, perform a cyclic process. In this case ΔU = 0 and A = -Q,

(4) i.e., in a cyclic process, when ΔU = 0, work can be obtained from the body at the expense of heat that will be introduced into the body [we say introduced, because in equation (4) Q is obtained with a minus sign]. In other words, from a thermal (steam) machine, mechanical work can be obtained only by expending an equal amount of thermal energy, thus a perpetual motion machine, perpetuum mobile, operating without the supply of heat (or other form of energy) is impossible. Thus the first principle of T. can be formulated. In its full generality it was proved by the works of Mayer, Helmholtz and Joule. The applications of this principle of T. to solving various problems of physics, chemistry, biology are enormous. We will only point out that the calculation of the heat balance of any machine, apparatus and any living organism can be produced only with the help of this principle of T. The second principle of T. notes a characteristic feature of thermal processes, unlike mechanical or electrodynamical processes, namely the tendency to proceed only in one direction, leading to a state of thermal equilibrium. Let us give an example explaining the question. Let us have a steamship, which has a mechanism allowing to transfer heat from a colder body to a hotter one without the expenditure of corresponding work. If we could build such a device, then we could without any material costs make this steamship move across the sea. Indeed, thermal energy could be supplied to the hot furnaces of the steamship by means of the above mechanism from the cold water of the ocean, and such a process would not at all contradict the first principle of T., since the amount of heat taken from the cold water could be exactly equal to the amount of heat delivered to the boiler furnaces by the above mechanism. Meanwhile, practically such a mechanism would be equivalent for humanity to the invention of a perpetual motion machine, as it would allow obtaining energy without material expenditure. Such a process, however, is impossible and therefore the above-mentioned engine, called the perpetual motion machine of the second kind, is impossible. The second principle of T. states that a perpetual motion machine of the second kind is impossible. In other words, it is impossible in a periodically operating machine to transfer heat from a colder body to a hotter one without the expenditure of corresponding work. The second principle, governing the direction of thermal processes, can be derived from other concepts, first indicated by Thompson. In any physical process (mechanical, acoustic, electrical, optical, etc.), as observations show, the amount of energy present in a closed system will always remain the same according to the law of conservation of energy. However, the qualitative distribution of energy will constantly change, and since in any process we always observe the appearance of heat, the total amount of thermal energy in the system will increase at the expense of other forms of energy. Energy will thus change its qualitative composition, will, from the human point of view, depreciate. According to Thompson, the second principle can thus be considered as the principle of depreciation or dissipation of energy. The study of molecular processes occurring when bodies are heated allows for a deeper formulation of the second principle. The state of a system, characterized by the arrangement and velocities of its moving molecules and atoms, allows one to calculate the probability of a given state. For example, the state of a gas system, in which all molecules have translational motion in a limited area, is less probable than the state in which there is observed the so-called molecular chaos, characterized by Maxwell's law of velocity distribution. We can find for molecular processes a certain quantity characterizing the probability of the state, and this quantity is connected with the amounts of heat introduced into our system at corresponding temperatures. We can express the second principle of T. as a principle stating that a system passes in the course of changes from a less probable state to a more probable state. This viewpoint has been developed by the works of Boltzmann, Planck and a whole series of other theorists and finds application in the laws of radiation of bodies. From the second principle of T. in connection with the first law we can obtain general formulas, which allow wide application both in the field of physics and in the field of chemistry. Modern physical chemistry is based entirely on thermodynamics and it gives enormous theoretical and practical applications. In recent decades (since 1906) to the two principles of T. mentioned above, a third is added, which can be formulated as follows: 'by no finite process can a substance be cooled to absolute zero temperature' (Nernst). However, in this formulation this principle is not yet completely obvious. Nevertheless, the experimental data leading to it, such as the tendency of all properties of substances to zero as the temperature decreases to absolute zero, lead to a number of very important conclusions, for example allow to solve the question of the direction of a chemical process on the basis of thermochemical data (heat effect, heat capacity), which was impossible to do using the first and second principles of thermodynamics. flum : G. Helmholtz, On the Conservation of Force, L., 1929; Lazarev P., Modern Advances in Biological Physics, L., 1927; Mitergof O., Thermodynamics of Living Processes, M.-L., 1928; Merzalov P., Brief Course of Thermodynamics, M.-L., 1927; Michelson V., Physics, vol. I - Mechanics and Heat, M.-L., 1933; Khvolson O., Course of Physics, vol. II, Berlin, 1923; Planck M., Introduction to the Theory of Heat, Lpz., 1930.