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system in the straight direction is called reversible process.
It must be noted, however, that a reversible process is
merely an ideal process, since it is impossible to make a
“frictionless” piston.
A series of changes, which brings a system back to its
initial condition, is called a thermodynamic cycle. If the
changes are all reversible, then the cycle is called a reversible
cycle.

Fig.1.4 - Graf of thermo- 1.2 Thermal Variables of State or Thermodynamic
dynamics process Coordinate of the System and Relation between Them

1.3.1 Volume, Density, Specific Volume, and Specific Weight
Variables of state are volume V, pressure P and temperatures T.
Density (ρ) is defined as the mass of a substance divided by its volume, or the mass
per unit volume

mass m
   (1.1)
volume V

Specific volume (v) is defined as the volume per unit mass or the reciprocal of density

V 1
   (1.2)
m 

Specific weight (γ) is defined as the weight of a substance divided by its volume, or the
weight per unit volume:

weight w
   (1.3)
volume V

1.2.1 Pressure
When a gas is enclosed in a vessel, the molecules of the gas collide with the walls
of the vessel and, in this way, they exert some average force on the walls.
The normal force per unit area on the walls is called pressure of the gas.

F
P   н  ; Pa
, (1.4)
S  2 
 м


where F - force; S – area.

The pressure may vary from place to place on the system boundary, even when the
system is in equilibrium. For example, consider a system consisting of a fluid (either
gas or liquid) in a closed tank. A simple force balance on the fluid shows that the

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