Thermodynamic System

Definition:

The thermodynamics system may be broadly defined as a definite area or space where some thermodynamics process is taking place. It may be noted that a thermodynamics system has its boundaries and anything outside the boundaries is called surroundings.

 

(i) System: 
            A System is a finite quantity of matter or a prescribed region of space.

(ii) Boundary: 
                The actual or imaginary area enclosing the system is called the boundary of the system. The boundary may be fixed or it may move as and when a system containing a gas is compressed or expanded. The boundary may be real or imaginary. It is not difficult to see a real boundary but an example of imaginary boundary would be one drawn around a system consisting of the fresh mixture about to enter the cylinder of an I.C Engine together with the remnants of the last cylinder charge after the exhaust process. 

Classification of Thermodynamic System:

  

       (i) Closed System:

This is a system of fixed mass and identity whose boundary are determined by the space of the matter (working substance) occupied on it. 

A closed system does not permit any mass transfer across its boundary but it permits transfer of energy (work and heat).

         (ii) Open System:

In this system the mass of the working substance crosses the boundary of the system. Heat and work may also cross the boundary.  

The working substance  crosses the boundary of the systems as the low pressure air enters the compressor and leaves the high pressure air. The work crosses the boundary of the system through the driving shaft and the heat is transferred across the boundary from the cylinder.

An open system permits both mass and energy (heat and work) transfer across the boundaries and the mass within the system may not be constant.

        (iii) Isolated System:

• A system which is completely uninfluenced by the surrounding is called an isolated system.

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

Examples: An open system with its surroundings (known as universe) is an example of isolated system