Introduction. Heat engineering subject. Basic concepts and definitions. Thermodynamic system. Status options. Temperature. Pressure. Specific volume. State equation. Van der Waals equation .
Ratio between units:
1 bar = 10 5 Pa
1 kg / cm 2 (atmosphere) \u003d 9.8067 10 4 Pa
1mmHg st (millimeter of mercury) = 133 Pa
1 mm w.c. Art. (millimeter of water column) = 9.8067 Pa
Density - the ratio of the mass of a substance to the volume it occupies.
Specific volume - the reciprocal of the density, i.e. the ratio of the volume occupied by a substance to its mass.
Definition: If at least one of the parameters of any body entering the system changes in a thermodynamic system, then thermodynamic process .
Basic thermodynamic parameters of the state P, V, T homogeneous body depend on each other and are mutually related by the equation of state:
F(P, V, T)
For an ideal gas, the equation of state is written as:
P- pressure
v- specific volume
T- temperature
R- gas constant (each gas has its own value)
If the equation of state is known, then to determine the state of the simplest systems, it is enough to know two independent variables from 3
P \u003d f1 (v, t); v = f2 (P, T); T = f3(v, P).
Thermodynamic processes are often depicted on state graphs, where state parameters are plotted along the axes. Points on the plane of such a graph correspond to a certain state of the system, lines on the graph correspond to thermodynamic processes that transfer the system from one state to another.
Consider a thermodynamic system consisting of one body of some gas in a vessel with a piston, and the vessel and piston in this case are the external environment.
Let, for example, the gas in the vessel is heated, two cases are possible:
1) If the piston is fixed and the volume does not change, then there will be an increase in pressure in the vessel. Such a process is called isochoric(v = const) going at constant volume;
Rice. 1.1. Isochoric processes in P-T coordinates: v1 >v2 >v3
2) If the piston is free, then the heated gas will expand, at constant pressure, this process is called isobaric (P= const), going at a constant pressure.
Rice. 1.2 Isobaric processes in v - T coordinates: P1>P2>P3
If, by moving the piston, you change the volume of gas in the vessel, then the temperature of the gas will also change, however, by cooling the vessel during compression of the gas and heating during expansion, you can achieve that the temperature will be constant with changes in volume and pressure, such a process is called isothermal (T= const).
Rice. 1.3 Isothermal processes in P-v coordinates: T 1 >T 2 >T 3
The process in which there is no heat exchange between the system and the environment is called adiabatic, while the amount of heat in the system remains constant ( Q= const). In real life, adiabatic processes do not exist, since it is not possible to completely isolate the system from the environment. However, processes often occur in which the heat exchange with the environment is very small, for example, the rapid compression of gas in a vessel by a piston, when heat does not have time to be removed due to heating of the piston and vessel.
Rice. 1.4 Approximate graph of the adiabatic process in P-v coordinates.
Definition: Circular Process (Cycle) - is a set of processes that return the system to its original state. The number of separate processes can be any number in a cycle.
The concept of a circular process is key for us in thermodynamics, since the operation of a nuclear power plant is based on a steam-water cycle, in other words, we can consider the evaporation of water in the core (AZ), the rotation of the turbine rotor by steam, the condensation of steam and the flow of water into the core as a kind of closed thermodynamic process or cycle.
Definition: Working body - a certain amount of a substance that, participating in a thermodynamic cycle, performs useful work. The working fluid in the RBMK reactor plant is water, which, after evaporation in the core in the form of steam, does work in the turbine, rotating the rotor.
Definition: The transfer of energy in a thermodynamic process from one body to another, associated with a change in the volume of the working fluid, with its movement in external space or with a change in its position is called process work .
Thermodynamic system
Technical thermodynamics (t / d) considers the laws of the mutual transformation of heat into work. It establishes the relationship between thermal, mechanical and chemical processes that occur in thermal and refrigeration machines, studies the processes occurring in gases and vapors, as well as the properties of these bodies under various physical conditions.
Thermodynamics is based on two basic laws (beginnings) of thermodynamics:
I law of thermodynamics- the law of transformation and conservation of energy;
II law of thermodynamics- establishes the conditions for the flow and direction of macroscopic processes in systems consisting of a large number of particles.
Technical t/d, applying the basic laws to the processes of converting heat into mechanical work and vice versa, makes it possible to develop theories of heat engines, to study the processes occurring in them, etc.
The object of the study is thermodynamic system, which can be a group of bodies, a body or a part of a body. What is outside the system is called environment. A T/D system is a set of macroscopic bodies exchanging energy with each other and with the environment. For example: t / d system - gas located in a cylinder with a piston, and the environment - a cylinder, piston, air, walls of the room.
isolated system - t / d system that does not interact with the environment.
Adiabatic (heat-insulated) system - the system has an adiabatic shell, which excludes heat exchange (heat exchange) with the environment.
homogeneous system - a system that has the same composition and physical properties in all its parts.
homogeneous system - a homogeneous system in composition and physical structure, inside which there are no interfaces (ice, water, gases).
heterogeneous system
- a system consisting of several homogeneous parts (phases) with different physical properties, separated from one another by visible interfaces (ice and water, water and steam).
In heat engines (engines), mechanical work is performed with the help of working fluids - gas, steam.
The properties of each system are characterized by a number of quantities, which are commonly called thermodynamic parameters. Let us consider some of them, using the molecular-kinetic concepts known from the course of physics about an ideal gas as a collection of molecules that have vanishingly small sizes, are in random thermal motion and interact with each other only during collisions.
The pressure is due to the interaction of the molecules of the working fluid with the surface and is numerically equal to the force acting on the unit surface area of the body along the normal to the latter. In accordance with the molecular kinetic theory, the gas pressure is determined by the relation
Where n is the number of molecules per unit volume;
T is the mass of the molecule; since 2 is the root-mean-square velocity of the translational motion of molecules.
In the International System of Units (SI), pressure is expressed in pascals (1 Pa = 1 N/m2). Since this unit is small, it is more convenient to use 1 kPa = 1000 Pa and 1 MPa = 10 6 Pa.
Pressure is measured using pressure gauges, barometers and vacuum gauges.
Liquid and spring pressure gauges measure gauge pressure, which is the difference between total or absolute pressure. R measured medium and atmospheric pressure
p atm, i.e.
Devices for measuring pressures below atmospheric are called vacuum gauges; their readings give the value of vacuum (or vacuum):
i.e., the excess of atmospheric pressure over absolute pressure.
Note that the state parameter is absolute pressure. This is what enters into the thermodynamic equations.
temperaturecalled a physical quantity characterizing the degree of heating of the body. The concept of temperature follows from the following statement: if two systems are in thermal contact, then if their temperatures are not equal, they will exchange heat with each other, but if their temperatures are equal, then there will be no heat exchange.
From the point of view of molecular kinetic concepts, temperature is a measure of the intensity of the thermal motion of molecules. Its numerical value is related to the value of the average kinetic energy of the molecules of the substance:
Where k is the Boltzmann constant equal to 1.380662.10? 23 J/K. The temperature T defined in this way is called absolute.
In the SI system, the unit of temperature is the kelvin (K); in practice, the degree Celsius (°C) is widely used. The ratio between the absolute T and centigrade I temperatures has the form
In industrial and laboratory conditions, temperature is measured using liquid thermometers, pyrometers, thermocouples and other instruments.
Specific volume v— is the volume per unit mass of a substance. If a homogeneous body of mass M occupies volume v, then by definition
v= V/M.
In the SI system, the unit of specific volume is 1 m 3 /kg. There is an obvious relationship between the specific volume of a substance and its density:
To compare the quantities characterizing systems in the same states, the concept of “normal physical conditions” is introduced:
p= 760 mmHg = 101.325 kPa; T= 273,15 K.
In different branches of technology and different countries they introduce their own, somewhat different from the above "normal conditions", for example, "technical" ( p= 735.6 mmHg = 98 kPa, t= 15?C) or normal conditions for estimating the performance of compressors ( p= 101.325 kPa, t\u003d 20? C), etc.
If all thermodynamic parameters are constant in time and the same at all points of the system, then this state of the system is called balanced spring.
If there are differences in temperature, pressure and other parameters between different points in the system, then it is non-equilibrium. In such a system, under the influence of gradients of parameters, flows of heat, substances, and others arise, tending to return it to a state of equilibrium. Experience shows that an isolated system always comes to a state of equilibrium over time and can never get out of it spontaneously. In classical thermodynamics, only equilibrium systems are considered.
State equation. For an equilibrium thermodynamic system, there is a functional relationship between the state parameters, which is called equation of state. Experience shows that the specific volume, temperature and pressure of the simplest systems, which are gases, vapors or liquids, are related thermal equation view state:
The equation of state can be given another form:
These equations show that of the three main parameters that determine the state of the system, any two are independent.
To solve problems by thermodynamic methods, it is absolutely necessary to know the equation of state. However, it cannot be obtained within the framework of thermodynamics and must be found either experimentally or by methods of statistical physics. The specific form of the equation of state depends on the individual properties of the substance.
Definition 1
A thermodynamic system is a set and constancy of macroscopic physical bodies that always interact with each other and with other elements, exchanging energy with them.
By a system in thermodynamics, they usually understand macroscopic physical forms that consist of a huge number of particles that do not involve the use of macroscopic indicators to describe each individual element. There are no definite restrictions in the nature of material bodies, which are the constituent components of such concepts. They can be represented as atoms, molecules, electrons, ions and photons.
There are three main types of thermodynamic systems:
- isolated - exchange with matter or energy with the environment is not performed;
- closed - the body is not interconnected with the environment;
- open - there is both energy and mass exchange with external space.
The energy of any thermodynamic system can be divided into the energy that depends on the position and movement of the system, as well as the energy that is determined by the movement and interaction of the microparticles that form the concept. The second part is called in physics the internal energy of the system.
Features of thermodynamic systems
Figure 1. Types of thermodynamic systems. Author24 - online exchange of student papers
Remark 1
Any object observed without the use of microscopes and telescopes can be cited as distinctive characteristics of systems in thermodynamics.
To provide a full description of such a concept, it is necessary to select macroscopic details, through which it is possible to accurately determine the pressure, volume, temperature, magnetic induction, electric polarization, chemical composition, mass of moving components.
For any thermodynamic systems there are conditional or real limits separating them from the environment. Instead, they often consider the concept of a thermostat, which is characterized by such a high heat capacity that in the case of heat exchange with the analyzed concept, the temperature parameter remains unchanged.
Depending on the general nature of the interaction of a thermodynamic system with the environment, it is customary to distinguish:
- isolated species that do not exchange either matter or energy with the environment;
- adiabatically isolated - systems that do not exchange matter with the external environment, but enter into an energy exchange;
- closed systems - those that do not have an exchange with matter, only a slight change in the value of internal energy is allowed;
- open systems - those that are characterized by a full transfer of energy, matter;
- partially open - they have semi-permeable partitions, therefore they do not fully participate in material exchange.
Depending on the formulation, the meanings of the thermodynamic concept can be divided into simple and complex variants.
Internal energy of systems in thermodynamics
Figure 2. Internal energy of a thermodynamic system. Author24 - online exchange of student papers
Remark 2
The main thermodynamic indicators, which directly depend on the mass of the system, include internal energy.
It includes the kinetic energy due to the movement of elementary particles of matter, as well as the potential energy that appears during the interaction of molecules with each other. This parameter is always unambiguous. That is, the meaning and realization of internal energy is constant whenever the concept is in the desired state, regardless of the method by which this position was reached.
In systems whose chemical composition remains unchanged in the process of energy transformations, when determining the internal energy, it is important to take into account only the energy of the thermal motion of material particles.
A good example of such a system in thermodynamics is an ideal gas. Free energy is a certain work that a physical body could do in an isothermal reversible process, or free energy is the maximum possible functional that a concept can do, having a significant supply of internal energy. The internal energy of the system is equal to the sum of the bound and free tension.
Definition 2
Bound energy is that part of the internal energy that is not able to independently turn into work - it is a devalued element of internal energy.
At the same temperature, this parameter increases with entropy. Thus, the entropy of a thermodynamic system is a measure of the security of its initial energy. In thermodynamics, there is another definition - energy loss in a stable isolated system
A reversible process is a thermodynamic process that can quickly go both in the opposite direction and in the forward direction, passing through the same intermediate positions, and the concept eventually returns to its original state without expending internal energy, and there are no macroscopic changes in the surrounding space.
Reversible processes give maximum performance. It is impossible to get the best result from the system in practice. This gives reversible phenomena a theoretical significance that proceeds infinitely slowly, and one can only approach it for short distances.
Definition 3
Irreversible in science is a process that cannot be carried out in the opposite direction through all the same intermediate states.
All real phenomena are in any case irreversible. Examples of such effects are thermal diffusion, diffusion, viscous flow, and heat conduction. The transition of the kinetic and internal energy of macroscopic motion through constant friction into heat, that is, into the system itself, is an irreversible process.
System State Variables
The state of any thermodynamic system can be determined by the current combination of its characteristics or properties. All new variables that are fully determined only at a certain point in time and do not depend on how exactly the concept came to this position are called thermodynamic state parameters or basic functions of space.
A system in thermodynamics is considered stationary if the variables remain stable and do not change over time. One version of the stationary state is thermodynamic equilibrium. Any, even the most insignificant change in the concept is already a physical process, so it can have from one to several variable state indicators. The sequence in which the states of the system systematically transition into each other is called the process path.
Unfortunately, confusion with terms and detailed description still exists, because the same variable in thermodynamics can be both independent and the result of adding several system functions at once. Therefore, terms such as "state parameter", "state function", "state variable" can sometimes be considered as synonyms.
THERMODYNAMIC SYSTEM
THERMODYNAMIC SYSTEM
The set of macroscopic bodies, to-rye can interact with each other and with other bodies (external environment) - exchange energy and matter with them. T. s. consists of such a large number of structural particles (atoms, molecules) that its state can be characterized macroscopically. parameters: density, pressure, concentration of v-in, forming T. s., etc.
THERMODYNAMIC EQUILIBRIUM), if the parameters of the system do not change over time and there is no k.-l in the system. stationary flows (heat, in-va, etc.). For equilibrium T. with. the concept of temperature is introduced as a parameter that has the same value for all macroscopic. parts of the system. The number of independent state parameters is equal to the number of degrees of freedom of the T. s. The rest of the parameters can be expressed in terms of the independent ones using the equation of state. Holy Island of equilibrium T. s. studies equilibrium processes (thermostatics); Holy islands of non-equilibrium systems -.
In thermodynamics, the following are considered: closed thermocouples that do not exchange matter with other systems, but exchange matter and energy with other systems; adiabatic T. s., in which it is absent with other systems; isolated T. s., which do not exchange either energy or matter with other systems. If the system is not isolated, then its state can change; change in the state of T. s. called thermodynamic process. T. s. can be physically homogeneous (homogeneous system) and heterogeneous (heterogeneous system), consisting of several. homogeneous parts with different physical. St. you. As a result of phase and chem. transformations (see PHASE TRANSITION) homogeneous T. s. can become heterogeneous and vice versa.
Physical Encyclopedic Dictionary. - M.: Soviet Encyclopedia. . 1983 .
THERMODYNAMIC SYSTEM
The set of macroscopic bodies, to-rye can interact with each other and with other bodies (external environment) - exchange energy and matter with them. T. s. consists of such a large number of structural particles (atoms, molecules) that its state can be characterized macroscopically. parameters: density, pressure, concentration of substances that form T. s., etc.
T. s. in balance (cf. thermodynamic equilibrium) if the parameters of the system do not change over time and there is no k.-l. stationary flows (heat, matter, etc.). For equilibrium T. with. the concept temperature How state parameter, having the same value for all macroscopic. parts of the system. The number of independent state parameters is equal to the number degrees of freedom T. s., the remaining parameters can be expressed in terms of independent ones using state equations. Properties of equilibrium T. s. studies thermodynamics equilibrium processes (thermostatics), properties of non-equilibrium systems - thermodynamics of nonequilibrium processes.
The following are considered in thermodynamics: closed thermal systems that do not exchange matter with other systems; open systems exchanging matter and energy with other systems; adiabatic T. s., in which there is no heat exchange with other systems; isolated T. homogeneous system) and heterogeneous ( heterogeneous system) consisting of several homogeneous parts with different physical. properties. As a result of phase and chem. transformations (see phase transition) homogeneous T. s. can become heterogeneous and vice versa.
Lit.: Epshtein P. S., Course of thermodynamics, trans. from English, M.-L., 1948; Leontovich M. A., Introduction to thermodynamics, 2nd ed., M.-L., 1951; Samoilovich A, G., Thermodynamics i, 2nd ed., M., 1955.
Physical encyclopedia. In 5 volumes. - M.: Soviet Encyclopedia. Editor-in-Chief A. M. Prokhorov. 1988 .
See what the "THERMODYNAMIC SYSTEM" is in other dictionaries:
A macroscopic body isolated from the environment with the help of partitions or shells (they can also be mental, conditional) and characterized by macroscopic parameters: volume, temperature, pressure, etc. For this ... ... Big Encyclopedic Dictionary
thermodynamic system- thermodynamic system; system A set of bodies that can energetically interact with each other and with other bodies and exchange matter with them ... Polytechnic terminological explanatory dictionary
THERMODYNAMIC SYSTEM- a set of physical bodies that can exchange energy and matter with each other and with other bodies (the external environment). T. s. is any system consisting of a very large number of molecules, atoms, electrons, and other particles that have many ... ... Great Polytechnic Encyclopedia
thermodynamic system- A body (a set of bodies) capable of exchanging energy and (or) matter with other bodies (between themselves). [Collection of recommended terms. Issue 103. Thermodynamics. USSR Academy of Sciences. Committee of Scientific and Technical Terminology. 1984 ... Technical Translator's Handbook
thermodynamic system- - an arbitrarily chosen part of space containing one or more substances and separated from the external environment by a real or conditional shell. General chemistry: textbook / A. V. Zholnin ... Chemical terms
thermodynamic system- a macroscopic body separated from the environment by real or imaginary boundaries, which can be characterized by thermodynamic parameters: volume, temperature, pressure, etc. There are isolated, ... ... Encyclopedic Dictionary of Metallurgy
A macroscopic body isolated from the environment with the help of partitions or shells (they can also be mental, conditional), which can be characterized by macroscopic parameters: volume, temperature, pressure, etc. For ... ... encyclopedic Dictionary
Thermodynamics ... Wikipedia
thermodynamic system- termodinaminė sistema statusas T sritis chemija apibrėžtis Kūnas (kūnų visuma), kurį nuo aplinkos skiria reali ar įsivaizduojama riba. atitikmenys: engl. thermodynamic system rus. thermodynamic system... Chemijos terminų aiskinamasis žodynas
thermodynamic system- termodinaminė sistema statusas T sritis fizika atitikmenys: angl. thermodynamic system vok. thermodynamisches System, n rus. thermodynamic system, f pranc. système thermodynamique, m … Fizikos terminų žodynas
Consider the features of thermodynamic systems. They are usually understood as physical macroscopic forms, consisting of a significant number of particles, which do not imply the use of each individual particle to describe the macroscopic indicators.
There are no restrictions on the nature of the material particles that are the constituent components of such systems. They can be represented as molecules, atoms, ions, electrons, photons.
Peculiarities
Let us analyze the distinctive characteristics of thermodynamic systems. An example is any object that can be observed without the use of telescopes, microscopes. To give a full description of such a system, macroscopic details are selected, thanks to which it is possible to determine the volume, pressure, temperature, electric polarization, magnetic induction, chemical composition, mass of components.
For any thermodynamic systems, there are conditional or real boundaries that separate them from the environment. Instead, the concept of a thermostat is often used, which is characterized by such a high heat capacity that in the case of heat exchange with the analyzed system, the temperature index remains unchanged.
System classification
Consider what the classification of thermodynamic systems is. Depending on the nature of its interaction with the environment, it is customary to distinguish:
- isolated species that do not exchange either matter or energy with the environment;
- adiabatically isolated, not exchanging matter with the external environment, but entering into an exchange of work or energy;
- closed thermodynamic systems have no exchange of matter, only a change in the magnitude of energy is allowed;
- open systems are characterized by complete transfer of energy, matter;
- partially open ones may have semi-permeable partitions, so they do not fully participate in material exchange.
Depending on the description, the parameters of a thermodynamic system can be divided into complex and simple options.
Features of simple systems
Simple systems are called equilibrium states, the physical state of which can be determined by specific volume, temperature, pressure. Examples of thermodynamic systems of this type are isotropic bodies that have equal characteristics in different directions and points. So, liquids, gaseous substances, solids, which are in a state of thermodynamic equilibrium, are not affected by electromagnetic and gravitational forces, surface tension, chemical transformations. The analysis of simple bodies is recognized in thermodynamics as important and relevant from a practical and theoretical point of view.
The internal energy of a thermodynamic system of this kind is connected with the surrounding world. When describing, the number of particles, the mass of the substance of each individual component are used.
Complex systems
Complex systems include thermodynamic systems that do not fall under simple types. For example, they are magnets, dielectrics, solid elastic bodies, superconductors, phase interfaces, thermal radiation, electrochemical systems. As parameters used to describe them, we note the elasticity of a spring or rod, the surface of the phase separation, and thermal radiation.
A physical system is such a set in which there is no chemical interaction between substances within the temperature and pressure indicators chosen for the study. And chemical systems are those options that involve the interaction between its individual components.
The internal energy of a thermodynamic system depends on its isolation from the outside world. For example, as a variant of the adiabatic shell, one can imagine a Dewar vessel. A homogeneous character is manifested in a system in which all components have similar properties. Examples of them are gas, solid, liquid solutions. A typical example of a gaseous homogeneous phase is the Earth's atmosphere.
Features of thermodynamics
This branch of science deals with the study of the basic laws of the processes that are associated with the release, absorption of energy. In chemical thermodynamics, it is supposed to study the mutual transformations of the constituent parts of the system, to establish the laws governing the transition of one type of energy to another under given conditions (pressure, temperature, volume).
The system, which is the object of thermodynamic research, can be represented as any object of nature, which includes a large number of molecules that are separated by an interface with other real objects. Under the state of the system is meant the totality of its properties, which make it possible to determine it from the standpoint of thermodynamics.
Conclusion
In any system, there is a transition of one type of energy to another, thermodynamic equilibrium is established. The section of physics that deals with the detailed study of transformations, changes, and conservation of energy is of particular importance. For example, in chemical kinetics, one can not only describe the state of a system, but also calculate the conditions that facilitate its shift in the desired direction.
Hess's law, relating the enthalpy, entropy of the transformation under consideration, makes it possible to identify the possibility of a spontaneous reaction, to calculate the amount of heat released (absorbed) by the thermodynamic system.
Thermochemistry, based on the fundamentals of thermodynamics, is of practical importance. Thanks to this section of chemistry, preliminary calculations of fuel efficiency and the feasibility of introducing certain technologies into real production are carried out in production. Information obtained from thermodynamics makes it possible to apply the phenomena of elasticity, thermoelectricity, viscosity, and magnetization for the industrial production of various materials.
Thermodynamic system- a set of macroscopic bodies that can interact with each other and with other bodies (the external environment) - exchange energy and matter with them. The exchange of energy and matter can occur both within the system itself between its parts, and between the system and the external environment. Depending on the possible ways of isolating the system from the external environment, several types of thermodynamic systems are distinguished.
open system called a thermodynamic system that can exchange matter and energy with the environment. Typical examples of such systems are all living organisms, as well as a liquid, the mass of which is continuously decreasing due to evaporation or boiling.
Thermodynamic system called closed if it cannot exchange either energy or matter with the environment. Closed system we will call a thermodynamic system isolated mechanically, i.e. incapable of exchanging energy with the environment by doing work. An example of such a system is a gas enclosed in a vessel of constant volume. The thermodynamic system is called adiabatic if it cannot exchange energy with other systems by heat exchange.
Thermodynamic parameters (state parameters) called physical quantities that serve to characterize the state of a thermodynamic system.
Examples of thermodynamic parameters are pressure, volume, temperature, concentration. There are two types of thermodynamic parameters: extensive And intense. The former are proportional to the amount of matter in a given thermodynamic system, the latter do not depend on the amount of matter in the system. The simplest extensive parameter is the volume V systems. the value v, equal to the ratio of the volume of the system to its mass, is called the specific volume of the system. The simplest intensive parameters are the pressure R and temperature T.
Pressure is a physical quantity
Where dFn is the modulus of the normal force acting on a small area of the body surface
spare dS.
If pressure and specific volume have a clear and simple physical meaning, then the concept of temperature is much more complex and less obvious. First of all, we note that the concept of temperature, strictly speaking, makes sense only for the equilibrium states of the system.
Equilibrium state of a thermodynamic system- the state of the system, in which all parameters have certain values and in which the system can remain for as long as desired. The temperature in all parts of a thermodynamic system in equilibrium is the same.
In heat exchange between two bodies with different temperatures, heat is transferred from a body with a higher temperature to a body with a lower temperature. This process stops when the temperatures of both bodies equalize.
The temperature of a system in an equilibrium state serves as a measure of the intensity of the thermal motion of atoms, molecules, and other particles that form the system. In a system of particles described by the laws of classical statistical physics and in equilibrium, the average kinetic energy of the thermal motion of particles is directly proportional to the thermodynamic temperature of the system. Therefore, it is sometimes said that temperature characterizes the degree of heating of a body.
When measuring temperature, which can only be done indirectly, the dependence on temperature of a number of physical properties of the body that can be measured directly or indirectly is used. For example, when the body temperature changes, its length and volume, density, elastic properties, electrical resistance, etc. change. A change in any of these properties is the basis for temperature measurements. For this, it is necessary that for one (chosen) body, called a thermometric body, the functional dependence of this property on temperature is known. For practical measurements of temperature, temperature scales are used, established with the help of thermometric bodies. In the International Centigrade Temperature Scale, temperature is expressed in degrees Celsius (°C) [A. Celsius (1701-1744) - Swedish scientist] and is denoted t, and it is assumed that at a normal pressure of 1.01325 × 10 5 Pa, the melting points of ice and boiling points of water are 0 and 100 °C, respectively. In the thermodynamic temperature scale, temperature is expressed in Kelvin (K) [W. Thomson, Lord Kelvin (1821-1907) - English physicist], denoted T and is called the thermodynamic temperature. Relationship between thermodynamic temperature T and temperature on a centigrade scale has the form T = t + 273,15.
Temperature T= 0 K (on a centigrade scale t\u003d -273.15 ° С) is called absolute zero temperature, or zero on the thermodynamic temperature scale.
System state parameters are divided into external and internal. External parameters systems are called physical quantities that depend on the position in space and various properties (for example, electric charges) of bodies that are external to the given system. For example, for a gas, this parameter is the volume V vessel,
in which the gas is located, because the volume depends on the location of external bodies - the walls of the vessel. Atmospheric pressure is an external parameter for a liquid in an open vessel. Internal parameters systems are called physical quantities that depend both on the position of bodies external to the system and on the coordinates and velocities of the particles that form this system. For example, the internal parameters of a gas are its pressure and energy, which depend on the coordinates and velocities of moving molecules and on the density of the gas.
Under thermodynamic process understand any change in the state of the thermodynamic system under consideration, characterized by a change in its thermodynamic parameters. The thermodynamic process is called equilibrium, if in this process the system passes through a continuous series of infinitely close thermodynamically equilibrium states. Real processes of changing the state of the system always occur at a finite rate and therefore cannot be in equilibrium. It is obvious, however, that the real process of changing the state of the system will be the closer to the equilibrium, the slower it takes place, therefore such processes are called quasi-static.
The following processes can serve as examples of the simplest thermodynamic processes:
a) an isothermal process in which the temperature of the system does not change ( T= const);
b) an isochoric process occurring at a constant volume of the system ( V= const);
c) an isobaric process occurring at a constant pressure in the system ( p= const);
d) an adiabatic process that occurs without heat exchange between the system and the environment.
