**3.1 Basic concepts**

2 Will-be-set-by-IN-TECH

The phenomenological thermodynamics is based on six axioms (or postulates if you wish to

For thermodynamic system at unchained external conditions there exists a state of the thermodynamic equilibrium in which its macroscopic parameters remain constant in time. The thermodynamic system at unchained external conditions always reaches the state of

Energy of the thermodynamic system is a sum of energies of its macroscopic parts. This

When two systems are in the thermal equilibrium, *i.e.* no heat flows from one system to the other during their thermal contact, then both systems have the same temperature as an intensive thermodynamic parameter. If system A has the same temperature as system B and system B has the same temperature as system C, then system A also has the same

There is a function of state called internal energy *U*. For its total differential d*U* we write

where the symbols ¯d*Q* and ¯d*W* are not total differentials but represent infinitesimal values

At temperature of 0 K, entropy of a pure substance in its most stable crystalline form is

This postulate supplements the second law of thermodynamics by defining a natural referential value of entropy. The third law of thermodynamics implies that temperature

Phenomenological thermodynamics using its axioms radically reduces an amount of experimental effort necessary for a determination of the values of thermodynamic quantities. For example enthalpy or entropy of a pure fluid need not be measured at each temperature and pressure but they can be calculated from an equation of state and a temperature dependence of the isobaric heat capacity of ideal gas. However, empirical constants in an

lim *T*→0

There is a function of state called entropy *S*. For its total differential d*S* we write

<sup>d</sup>*<sup>S</sup>* <sup>=</sup> d¯*<sup>Q</sup>*

d¯*Q*

of 0 K cannot be attained by any process with a finite number of steps.

equation of state and in the heat capacity must be obtained experimentally.

d*S* >

d*U* =d¯*W* +d¯*Q* , (1)

*<sup>T</sup>* , [reversible process] , (2)

*<sup>T</sup>* , [irreversible process] . (3)

*S* = 0 . (4)

axiom allows to define extensive and intensive thermodynamic quantities.

call them), four of them are called the laws of thermodynamics: • **Axiom of existence of the thermodynamic equilibrium**

temperature as system C (temperature is transitive).

of heat *Q* and work *W* supplied to the system.

the thermodynamic equilibrium.

• **The zeroth law of thermodynamics**

• **The first law of thermodynamics**

• **The second law of thermodynamics**

• **The third law of thermodynamics**

zero

• **Axiom of additivity**

The statistical thermodynamics considers thermodynamic system as an assembly of a very large number (of the order of 1023) of mutually interacting particles (usually molecules). It uses the following concepts:

### • **Microscopic state of system**

The microscopic state of thermodynamic system is given by positions and velocities of all particles in the language of the Newton mechanics, or by the quantum states of the system in the language of quantum mechanics. There is a huge number of microscopic states that correspond to a given thermodynamic (macroscopic) state of the system.
