What is system, boundary and surrounding is thermodynamics ?

A system is defined as a region in space containing a specific amount of matter whose behavior is being observed.
The system is separated from its surrounding by a boundary. The boundary may be a real one or some imaginary surface covering  the region. The boundary may be at rest or in motion and may change its size or shape.
The term surrounding is restricted to those portions of matter external to the system which is thermodynamically affected by the changes occurring within the system.
Any thermodynamically analysis begins with the selection of the system, its boundary and the surroundings.  

Open and closed systems

A closed system (sometimes termed as a control mass) is a system for which no masses cross the boundary i.e. quantity of matter within the system remains fixed throughout the investigation. But energy is allowed to cross the boundary (in the form of heat and work).

An open system (sometimes termed as a control volume) on the other hand is a region in the space defined by a boundary across (in/out/through) which matter may flow in addition to energy (in the form of heat and work).
A system is termed as isolated system if neither matter nor energy is allowed to transfer across the boundary. A truly isolated system can only be obtained ideally.

Examples of closed systems

  • Mixtures of water and steam in a closed vessel
  • Gas expanding in a piston- cylinder

 Examples of open systems

  • Water entering boiler and leaving as steam
  • Gases flowing through turbine
  • Gas expanding from pressurized container, through a nozzle.

Example of isolated system: 

Open and Closed systems

Open system, closed system and isolated system
  • A thermo flask may be considered as approaching an isolated system.


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