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where S denotes some function of the thermodynamic co-ordinate.

The function S is called entropy. This equation enables us to calculate the change
of the entropy dS for a reversible process.

1.6 Heat and Work
One of the main conceptions of the thermodynamic are Heat and Work. Before it
was mentioned, that the thermodynamic system interacts with environment by
performing some work or the transferring some heat. Thus we have two ways energy
exchange. Work is a measure of the energy transfer by mechanical means, such as
gravitational, electric or magnetic forces.
Heat is a measure of energy transfer by means of temperature differences. Heat is
transferred by thermal motion of the molecules. The sense of heat …. is motion and
nothing else. We describe many common temperatures changes as the transfer of
“something” from one body at higher temperature to another at the lower temperature,
and this “something” we call heat.
Work and heat are energy characteristics of the thermodynamic process and they
value depends on what kind of process is going. We can speak about heat and work
only as they enter or leave a thermodynamic system.
1.6.1 Theory of Heat
Caloric Theory of Heat According to this theory, heat is a sort of invisible fluid,
called the caloric. It is found that, when a body is heated, its weight does not increase.
This is explained by supposing that the caloric is weightless fluid. The caloric is also
supposed to be a fluid which can neither be created or destroyed. Thus, when heat flows
from one body to another, we must have the relation: Heat Lost is equal Heat Gained.
This relation has been verified by a large number of experiments.
th
Hence, throughout the 18 century, almost all the physicists believed in the Caloric
Theory.

Mechanical Theory of Heat It must be noted that according to the Caloric Theory
th
we can not create or produce heat. But several experiments performed during the 19
century clearly showed that it is possible to produce heat. For example, when we rub
our hands together, they become warm. This shows that heat can be produced by the
process of rubbing. This fact could not be explained on the Caloric Theory, so this
theory was rejected.
Now we believe in the Mechanical Theory of Heat. It is supposed that some
mechanical work is converted into heat during the process of rubbing. Thus heat is not
a fluid, but only a form of energy. The “heat” in a body is believed to be the kinetic
energy of its molecules. The molecules of liquids and gases also move from place to
place. When two bodies are rubbed together, their molecules are violently shaken by
the process of rubbing. This increase their kinetic energy, and so heat is produced. The
source of this heat is the mechanical work, which is performed during the process of
rubbing.

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