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2.11. Reversible and Irreversible Processes. Second Law of
Thermodynamics

All thermodynamic processes occurring in a closed system can be
divided into reversible and irreversible ones.

The thermodynamic process is called reversible if it returns the system
in the initial state without external energy expenditures when going in the
opposite direction. Otherwise the thermodynamic process is called
irreversible. Irreversible processes are spontaneously going only in one

direction.
The necessary and sufficient condition for the thermodynamic process
reversibility is the equilibrium of its successive states. The reversibility

means the existence of the symmetry in respect to the operation of time
inversion
The examples of reversible processes are the following: non-damped

vibrations of a pendulum, perfectly elastic collisions, and a Carnot cycle
e.c.
The examples of irreversible processes are the following: damped

vibrations, inelastic collisions, processes with friction, diffusion, thermo
conductivity.
To go in the opposite direction the irreversible process needs the heat
chaotic motion of the body particles spontaneously to be transformed into

the ordered motion of the body. Experimental data do not confirm
existence in the nature of the transformations of such type. To do this it's
necessary to have the additional compensating process.

The first law of thermodynamics cannot describe processes ex-
haustively. The intrinsic restriction (limitation) of the first law of
thermodynamics is the impossibility to predict with its help the direction
of the process going on. Any process during which the law of energy

conservation is not infringed can take place in accordance with the first
law of thermodynamics. In accordance with the first law of
thermodynamics, the process is possible for the spontaneous heat transfer

from a body at lower temperature to the one at a higher temperature. Also
the process is possible the only result of which is receiving some quantity
of heat and its transformation into the equivalent work.

In a Carnot engine, one of the most efficient conceivable, a quantity of
heat AQi is taken from the hot reservoir at the temperature T Some of this
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