Thermodynamic System And Surroundings

“A prescribed, identifiable and fixed collection of matter which is completely enclosed with in a definite region, and whose behaviour is being investigated” 

The system may be a quantity of steam, a mixture of vapour and gas, or an internal combustion engine and its components. The term surroundings represent the environments (a combination of matter and space external to the system) which are affected by changes occurring within the system. The boundary separating a system from its surrounding may be real or imaginary, rigid and confined in the space or flexible, i.e., it may change in shape, volume, position and / or orientation.

 Fig. Shows the conventional representation of a system, its boundary and surroundings. The surroundings can also be regarded as a system and hence we are largely concerned with the interactions between systems. 

When the system executes a process (undergoes a change from one state to another), there are mass and energy interactions between the system and the surroundings. For a closed system the same mass matter remains within the system and only work and heat energies cross the systemboundaries. The physical nature and chemical composition of the matter can, however, change. Thus a liquid may evaporate, a gas may condense or a chemical reaction may take place between two or more constituents of a system. Examples of closed system are :

(1) gas expanding in a cylinder by displacing a piston

(2) mixture of water and steam in a closed vessel 

(3) Bomb calorimeter 

When the mass flow of substance also takes place simultaneously with the transfer of energy, due system is known as an open system. Examples of an open system are : 

(1) steam flowing through a turbine 

(2) water entering a boiler and leaving as steam

(3) water wheel suspended in a water stream


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