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It is believed that the heat entering the gas is stored in it as its internal energy, which
is a function of the temperature and pressure of the gas. Thus the internal energy of a
system is also a thermodynamic co-ordinates.
Internal energy is the energy of the thermal motion of the molecules, its sum kinetic
energies. Internal energy is a thermodynamic co-ordinate.
Internal energy does not depend from character of process, but just depend on its
initial and final states of thermodynamic system. For the ideal gas internal energy
depend just on T.
1.5. 1 Enthalpy H
In many thermodynamic analyses the sum of internal energy U and the product of
pressure and volume PV appear. Because this combination (U + PV) occurs so fre-
quently, it has been given a name, enthalpy, and is represented by the symbol H. Since
U, P, and V are all properties, this combination of them is also a property. The defining
relation is
H = U + Pv (1.20)
or, per unit mass h = u + Pv (1.21)

It should be noted that U represents a form of stored energy, but Pv does not;
therefore, their sum is not a form of stored energy. It will be seen later that in certain
applications enthalpy may be treated as energy, but this should not obscure the fact that
enthalpy is simply a useful property defined by an arbitrary combination of other
properties and is not a form of energy.
Since we cannot obtain absolute values of internal energy, we cannot obtain absolute
values of enthalpy. Only changes in enthalpy are of importance to us, however.
Now consider a full energy of the gas under the pressure , which is in equilibrium with
some load (fig.3). It full energy E consists of internal energy U and potential energy
of a load , which is equal:

G  h  p  S  h  p V  (1.22)
hence E  U  p V  (1.23)

Quantity p  V depends on a force acting on the piston and it is called potential
energy of the pressure.
Thus the Enthalpy H is the sum internal energy of gas and potential energy which is
created by the pressure of the environment.
Enthalpy H is the variable of state or co-ordinate of the
system and it does not depend on the character of the process,
but it depends on initial and final states of the system. Enthalpy
just depend on temperature of the ideal gas.

1.5.3 Entropy S.
Entropy is expressed by the equation :
dQ
dS  , (1.24)
Fig.1.6 - Enthalpy T
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