In modern thermodynamics the considered thermodynamic systems are conditional concept.

These systems are a set of macroscopic objects (bodies and fields), which exchange energy and matter with each other and with the external environment.

These systems are limited to actual or notional boundaries chosen for the analysis of their internal thermodynamic parameters.

The state of the environment around thermodynamic systems is constantly changing as a result of continuous movement of objects in the space of thermodynamic systems.

Therefore, the thermodynamic processes of these systems also continuously changing - from weakly to strongly non-equilibrium, until the phase transitions, which resulted in creating new flows of energy and matter.

If two thermodynamic systems, there is a temperature gradient between them there energy flux directed from the hotter system to the less heated.

In steady-state thermodynamic process, such as between an infinitely large thermodynamic systems whose state practically does not depend on time, it can be described by Fourier heat equation:

In the formula (1) Jq - vector of heat flux - the amount of energy passing through unit area (energy flux density) per unit time is perpendicular to the axis x, k - thermal conductivity, T - temperature.

In the case of unsteady thermodynamic process: two thermodynamic systems I and II, respectively, with initial temperatures T1 and T2, exchanged between the thermal energy, the temperature of these systems varies, respectively, the latter can be described by the equation

From this equation:

Hence we can formulate the following properties of thermodynamic systems:
1) macroobjects thermodynamic systems continuously strive to thermodynamic equilibrium, but will never reach;
2) thermodynamic system, between which the exchange of energy, always in the unstable non-equilibrium thermodynamic state.

The thermal energy of each of the two thermodynamic systems under consideration can be represented as follows:

Where Q_lim - the quantity of heat energy equilibrium state of a thermodynamic system, to which it seeks, ΔQ (t) - the residual thermal energy of the nonequilibrium state of a thermodynamic system, participating in an exchange of energy.

However, well-known thermodynamic system in which, unlike the above, there is no exchange of energy and the conversion of some types of energy in others. Such systems include systems in which chemical reactions occur, different types of engines: thermal, internal combustion engines, electrical and other, as well as living organisms.

In the process of functioning of these thermodynamic systems of work in accordance with the second law of thermodynamics, accompanied by the release or absorption of thermal energy.

But while the internal energy Q_i of the system does not change:

and their thermodynamic condition Q can be defined as nonequilibrium steady:

where:

Here: W_k - the energy released as a result of transformation, Q_T - the energy delivered to the external environment.

Thus, the above system in the process of their functioning are always in a stable non-equilibrium thermodynamic state.

Based on the foregoing, it can be argued that there are two types of thermodynamic processes:

- Passive, at which there is a reception or feedback of energy. Thus the matter is in a unstable nonequilibrium thermodynamic condition;

- Active, at which the matter carries out transformation of energy accompanying by allocation теполовой of energy. Thus the matter is in a steady nonequilibrium thermodynamic condition.