The Equation of State (Relation between P, V and T )

The Equation of State: In order to describe the thermodynamic state of a system, certain variables are required. These variables, in general, are the mechanical variables supplemented by the thermal state of the system.
For example, the thermodynamic state of the gaseous system can be specified by its pressure P, volume V & temperature T. But out of these three variables only two are independent variables & the third one may be considered as their function.
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If we fix two variables out of the three, the third one is automatically fixed, or in other words, only two parameters are necessary & sufficient to specify uniquely the state of the system.
“The equation connecting the pressure, the volume & absolute temperature of a substance is called the Equation of State of the system”.

Thus for a gas forming the system, these three quantities are not independent & connected by a relationship, i.e.

f( P, V, T ) = 0

which is called the equation of state.
For example, the equation of state for an ideal gas in the static condition is represented as:

                   PV = RT

where R = universal gas constant
= 8.314 Joule /gm
= 8314 J/Kg mol – K

V = molar volume

For Vanderwall’s gas, the equation of state is:
                 {P + a / V2 }(V-b) = RT
The Equation of State
Fig. Vanderwall’s equation of state.
where a, b are Vanderwall’s constant.

For an equation of state, it should be noted that:

1. Every thermodynamic system has its own equation of state independent of others.

2. An equation of state is not a theoretical deduction from thermodynamics but is an experimental addition to it.

3. The equation of state is not applicable to systems not in thermodynamic equilibrium.
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