1.11    Types of Thermodynamic Processes

1.11.1   Isothermal process

If the temperature remains constant during a process undergone by a system, that process is called an Isothermal process. It can be represented on a Pressure-Volume coordinate axes diagram as shown in fig. 1.6.

Isohermal Process

Isohermal Process


1.11.2 Adiabatic process

A process undergone by a system (a gas) is said to be Adiabatic if there is no heat transfer between the system and surroundings, that is, heat flows neither into the system nor out.

For adiabatic process   the property relation is given by;        PV γ = Constant

That is    P1V1 γ =   P2V2 γ

where γ is the adiabatic index which is the ratio Cp/Cv


1.11.3 Poly tropic process

This is the general law for expansion and compression of gases. The property relation for polytropic process is,       PVn = constant

P1V1n=P2V2n {PVn = P1V1n=P2V2n ; hence  P = P1V1n/Vn or  P = P2V2n/Vn


1.11.4 Constant volume process:

Process involving heat transfer during which the volume remains constant. It is also known as isochoric process.

P1V1/T1 = P2V2/T2 since V1 = V2 ;              P1/T1 =  P2/T2 or         P2/P1 = T2/T1


1.11.5 Constant Pressure Process (Isobaric process)

This is the heating or cooling of a gas without any change in pressure.

P1V1/T1 = P2V2/T2 since P1 = P2 ;                V1/T1 =  V2/T2 or         V2/V1 = T2/T1


This is the same as change in enthalpy. Hence, in a constant pressure process, the heat transferred is to increase the enthalpy of the gas.