Thermodynamic System

Basics of Thermodynamics System and Its Properties

Thermodynamics is that branch of science which deals with the energies possessed by gases and vapours . It also includes the conversion of these energies in terms of heat and mechanical work and their relationship with properties of system.


The Thermodynamic system may be defined as a definite area or a space where some thermodynamic process takes place. It may be noted that a thermodynamic system has its boundaries, and anything outside the boundaries called its surroundings. The thermodynamic system may be classified into the following groups.

A system is matter or region on which analysis is done. System is separated from the surrounding by boundary.

Thermodynamic System
Thermodynamic System

Types of System:-

Closed system

This is a system of fixed mass whose boundaries are determined by the space of working substance occupied in it. In a closed system, heat and work cross the boundary of the system , but there is no addition or loss of original mass of the working substance. Thus the mass of the working substance which comprises the system, is fixed.

Open system

In this system, the working substance crosses the boundary of the system. The heat and work may also cross the boundary.

Isolated system

It is a system of fixed mass and no heat or work cross its boundary.

types of system
types of system
Types of System Mass Transfer Energy Transfer Example
Closed No Yes Piston cylineder without valves
Open Yes Yes Turbine, Pump, Compressor , Piston Cylinder with valves
Isolated No No Universe , hot coffee in a perfectly insulated thermos



Everything external to the system is called surroundings.

System and Surroundings together is called Universe.


  • It separates system and surroundings.
  • It can be fixed or movable.
  • It is imaginary.

Fixed boundary eg. rigid box containing gas.

Movable boundary eg. Cylinder with piston.

Properties of a system:-

The state of a system may be identified by certain observable quantities such as volume, temperature, pressure and density etc. All the quantities which identify the state of a system are called Properties.

Properties are point function and exact or perfect differentials.

  • Point function-  Does not depend on path history ( T,P,V)
  • Path function-   depend on path history( work , heat).

The thermodynamic properties are divided into the following two classes.

Extensive properties:-

The properties of the system whose value for the entire system is equal to the sum of their values for the individual parts for the system , are called extensive properties.

  • These properties are dependent on mass.
  • All specific properties are intensive properties.

for example- total volume, total mass, and total energy of a system are extensive properties.

Intensive properties:-

The properties of the system, whose value for the entire system is not equal to the sum of their values for the individual parts of the system, are called intensive properties.

  • These properties are independent of mass.

for example- temperature, pressure and density of system are intensive properties.

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