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System and surroundings in thermodynamics

A system of thermodynamics can be defined as a matter or region on which analysis is done. The system is separated from surrounding by the boundary. Everything external to the system is surrounding. System and surroundings in thermodynamics together is called a universe.


System and surroundings in thermodynamics

System and surroundings in thermodynamics


  • System: Any matter or region which is focused.
  • Surrounding: Everything except the system is surrounding


  • Boundary: It separates the system and surrounding. It can be fixed or movable.

Fixed boundary example: a rigid box containing gas

Movable boundary example: a cylinder with a piston


Types of system

System and surroundings in thermodynamics


  • Open system: Both energy and mass transfer across the boundary is possible

Example: Turbine, pump, compressor

  • Closed system: Only energy taser is possible across the boundary

Example: Piston cylinder without valves

  • Isolated system: Energy or mass transfer is not possible across the boundary.

Example: Thermos flask


Property of system


In thermodynamics, properties are point function and are exact differentials

  • Point function: Does not depends on path history

Example: Temperature, pressure, volume

  • Path function: Depends on the path history

Example: Work, heat


  • Intensive properties:¬† Independent of mass

Example: Pressure, temperature, density, specific volume, specific heat

Note: All specific properties are intensive properties.


  • Extensive properties: Dependent on mass

Example: Volume, energy, heat capacity, enthalpy




  • Reversible process

A process when reversed in direction, follows the same path as that of the forward path without leaving any effect on the system and surrounds in thermodynamics is called reversible process.

Example: A frictionless quasi-static process is called a reversible process.


  • Irreversible process

The process which is not reversible means it does not follow the same path when reversed is called irreversible process.

Example: all actual process are irreversible process.


This is all about system and surroundings in thermodynamics.

Posted in: Energy, Thermodynamics

2 Comments on "System and surroundings in thermodynamics"

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