Thermal dynamics volume

Fundamentals of thermodynamics

Thermodynamics studies systems consisting of a very large number of particles. The description of such systems by the methods of classical mechanics is not only impossible, but actually makes no sense. Features of the thermodynamic description arise due to the fact that the behavior of large ensembles of particles obeys statistical laws and cannot be reduced to an analysis of the deterministic evolution of dynamic systems . However, historically, thermodynamics has developed without relying on the ideas of statistical theory, and the main provisions of thermodynamics can be formulated on the basis of a limited number of postulates, which are generalizations of experimental facts. The number of these postulates varies among different authors in accordance with how the axiomatics of thermodynamics is constructed., but traditionally it is believed that four laws of thermodynamics can be distinguished.

Basic concepts of thermodynamics

Thermodynamic systems 

In thermodynamics, physical systems are studied that consist of a large number of particles and are in a state of thermodynamic equilibrium or close to it. Such systems are called thermodynamic systems . This concept is generally quite difficult to define strictly, therefore, a descriptive definition is used, in which a thermodynamic system is a macroscopic system that is somehow (for example, using a real or imaginary shell) isolated from the environment and is able to interact with it . A shell that does not allow heat exchange between the system and the environment is called adiabatic , and the system enclosed in such a shell is thermally insulated oradiabatically isolated . The heat-permeable shell is also called diathermic ; if such a shell prevents the exchange of matter, then the corresponding system is called closed . Systems in which the shell does not interfere with the exchange of either matter or energy are called open .

Thermodynamic equilibrium 

Fundamental to classical thermodynamics is the concept of thermodynamic equilibrium , which is also difficult to logically define and is formulated as a generalization of experimental facts. It is argued that any closed thermodynamic system, for which the external conditions remain unchanged, over time passes into an equilibrium state in which all macroscopic processes stop. At the same time, a variety of processes can occur in the system at the microscopic level, for example, chemical reactions that can proceed both in the forward and reverse directions, however, on average, these processes compensate each other, and the macroscopic parameters of the system remain unchanged, fluctuating relative to the equilibrium value . Fluctuations are studied in statistical physics.

Thermodynamic parameters 

Thermodynamics does not consider the features of the structure of bodies at the molecular level. The equilibrium states of thermodynamic systems can be described using a small number of macroscopic parameters such as temperature, pressure, density, component concentrations, etc., which can be measured with macroscopic instruments. A state described in this way is called a macroscopic state , and the laws of thermodynamics make it possible to establish a relationship between macroscopic parameters. If the parameter has the same value, independent of the size of any selected part of the equilibrium system, then it is called non-additive or intensive , if the parameter value is proportional to the size of the part of the system, then it is calledadditive or extensive ^[18] . Pressure and temperature are non-additive parameters, while internal energy and entropy are additive parameters.
Macroscopic parameters can be divided into internal, characterizing the state of the system as such, and external, describing the interaction of the system with the environment and force fields acting on the system, but this division is rather arbitrary. So, if the gas is enclosed in a vessel with movable walls and its volume is determined by the position of the walls, then the volume is an external parameter, and the gas pressure depends on the rates of thermal movement of molecules and is an internal parameter. On the contrary, if an external pressure is given, then it can be considered an external parameter, and the volume of gas, an internal parameter. It is postulated that in the state of thermodynamic equilibrium, each internal parameter can be expressed in terms of external parameters and the temperature of the system. Such a functional relationship is called the generalized equation of state of the system .

Thermodynamic processes

When external parameters change or when energy is transferred to a system, complex processes can occur in it at the macroscopic and molecular levels, as a result of which the system passes into another state. Equilibrium thermodynamics does not deal with the description of these transient processes, but considers the state that is established after the relaxation of nonequilibria. In thermodynamics, idealized processes are widely used in which a system passes from one state of thermodynamic equilibrium to another, which continuously follow each other. Such processes are called quasi-static or quasi-equilibrium processes ^[20]. A special role in the methods of thermodynamics is played by cyclic processes in which the system returns to its original state, doing work in the course of the process and exchanging energy with the environment.

Principles of thermodynamics 

Zero law of thermodynamics 

The zero law of thermodynamics is so named because it was formulated after the first and second laws were among the well-established scientific concepts. It states that an isolated thermodynamic system spontaneously passes into a state of thermodynamic equilibrium over time and remains in it for an arbitrarily long time if the external conditions remain unchanged ^[21] [22] . It is also called the common beginning ^[23] . Thermodynamic equilibrium implies the presence in the system of mechanical, thermal and chemical equilibria, as well as phase equilibrium. Classical thermodynamics postulates only the existence of a state of thermodynamic equilibrium, but says nothing about the time it takes to reach it.
In the literature, the zeroth beginning also often includes statements about the properties of thermal equilibrium. Thermal equilibrium can exist between systems separated by an immovable heat-permeable partition, that is, a partition that allows systems to exchange internal energy, but does not let matter through. The postulate of transitivity of thermal equilibrium ^[24] states that if two bodies separated by such a partition (diathermic) are in thermal equilibrium with each other, then any third body that is in thermal equilibrium with one of these bodies will also be in thermal equilibrium. balance with another body.
In other words, if two closed systems A and B are brought into thermal contact with each other, then after reaching thermodynamic equilibrium by the complete system A + B , systems A and B will be in thermal equilibrium with each other. In this case, each of the systems A and B itself is also in a state of thermodynamic equilibrium. Then if systems B and C are in thermal equilibrium, then systems A and C are also in thermal equilibrium with each other.
In foreign literature, the postulate of the transitivity of thermal equilibrium is often called the zero start ^[25] [26] , and the statement about the achievement of thermodynamic equilibrium can be called the “minus first” start ^[27] . The importance of the postulate of transitivity lies in the fact that it allows one to introduce some function of the state of the system, which has the properties of empirical temperature, that is, to create devices for measuring temperature. The equality of empirical temperatures measured with the help of such an instrument - a thermometer , is a condition for the thermal equilibrium of systems (or parts of the same system).