Investigations of energy conversion processes in macroscopic systems as well as properties of matter that takes part in these processes. Thermodynamics is broadly interpreted to include all aspects of energy transformations, including power generation, refrigeration, and relationships among the properties of matter.

Depending on the method – two general kinds of the thermodynamics may be distinguished:

-phenomenological (technical) thermodynamics, which uses the concepts related to measurements made by any macroscopic or mesoscopic scale.

-statistical (molecular) thermodynamics, which penetrates deeper into the construction of the substance, considering it as a collection of atoms and molecules, where a huge amount of molecules force to describe their behavior methods of mathematical statistics.

System is defined as a quantity of matter or a space chosen for a study

The space outside the system is called the surroundings.

The real or imaginary surface that separates the system from its surroundings is called the boundary: fixed and movable.

There are two types of the system:

•Closed system consists of a fixed amount of substance, and no substance can cross its boundary.

•Open system: substance and energy can cross the system boundary.

An intensive quantity is a physical quantity whose value does not depend on the amount of the substance for which it is measured.

·Examples: temperature, density, velocity, viscosity, pressure and elasticity.

An extensive quantity is a physical quantity whose value is proportional to the amount of substance as well as size of the system it describes. Such a property can be expressed as the sum of the quantities for the separate subsystems that compose the entire system.

·Examples: mass, energy, volume, enthalpy and entropy.

Pressure is defined as a normal force exerted by a fluid per unit area. Pressure is defined for fluids (gas or a liquid). Since pressure is defined as force per unit area, it has the unit of newton per square meter (N/m2), which is called Pascal (Pa). The pressure unit Pascal is too small for pressures encountered in practice, so there is used 1 kPa 103 Pa and 1 MPa 106 Pa.

Density is defined as mass per volume unit.

Specific volume is defined as volume per mass unit.

TEMPERATURE is a measure of the average energy of kinetic energy of particles in matter. When particles of matter, whether in solids, liquids, gases, move faster or have greater mass, they carry more kinetic energy, and the material appears warmer than a material with slower or less massive particles.

Function of state

Consider a system not undergoing any change. At this point, all the properties can be measured or calculated throughout the entire system, which gives us a set of properties that completely describes the condition of the system. At a given state, all the properties of a system have fixed values.

State function is a property of a system that depends only on the current state of the system, not on the way in which the system acquired that state. They describe quantitatively an equilibrium state of thermodynamic systems. Examples: energy, density, enthalpy, temperature, entropy, density, specific volume.


Mechanical energy is the sum of the kinetic energy as well as potential energy of gravity.

Mechanical energy as well as internal energy may be evaluated only if the reference state is established. There is necessary to choose a convenient reference state and assign a value of zero for a convenient property or properties at that state.

Internal energy U

This is energy of atoms, molecules and parts of molecules, which constitute the physical body. The internal energy covers:

Nuclear energy: energy associated with the strong bonds within the nucleus of the atom itself.

Chemical energy: energy associated with the atomic bonds in a molecule.

Thermal energy: is the sum of the kinetic energies of the molecules in translate motion, rotational motion and oscilation as well as potential energies.

Thermodynamic processes.

Any change that a system undergoes from one equilibrium state to another is called a process, and the series of states through which a system passes during a process is called the path of the process. Any change that a system undergoes from one equilibrium state to another is called an equilibrium process, and the series of states through which a system passes during a process is called the path of the process. To describe a process completely, one should specify the initial and final states of the process, as well as the path it follows, and the interactions with the surroundings.

Heat Q is a part of the thermal energy that is transferred through the system boundary without mass transfer as a result of temperature difference. Heat may not be considered as a kind of energy. Heat is a kind of thermal energy transfer. Heat is a process function. For various processes the amount of heat absorbed by the system or transferred to the surroundings is different. Heat delivered to the system is treated is assumed as positive while heat transferred from the system to the surroundings is assumed as negative.

Technical work Lt

The technical work is sum of the three absolute works: work of the delivery of substance inside the open system; work of process that is performed inside the system; work of transfer of substance from the system to the surroundings.

Enthalpy H is sum of the internal energy and the so called transport energy pV.

Enthalpy is a function of state.

The First Law of Thermodynamics states that heat is a form of energy, and thermodynamic processes are therefore subject to the principle of conservation of energy. This means that heat energy cannot be created or destroyed. It can, however, be transferred from one location to another and converted to and from other forms of energy.

“The First Law says that the internal energy of a system has to be equal to the work that is being done on the system, plus or minus the heat that flows in or out of the system and any other work that is done on the system”

Definition of perfect gas: Fulfill gas laws: Clapeyron, Avogadro and Dalton. Constant specific heats.

Semi-perfect gases: Fulfill gas laws: Clapeyron, Avogadro and Dalton. The specific heat depend on temperature for given process and substance.

Equipartition theorem

Internal energy is shared equally among all of its various forms; as a consequence – the average kinetic energy per degree of freedom in the translational motion of a molecule should equal that of its rotational motions.

Isochoric process: V = const; v = const. Heating and cooling of gas (vapour) stored in the closed vessel.

Isothermal process: T = const