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What is a Thermodynamic System?

written by: Haresh Khemani • edited by: Swagatam • updated: 5/4/2010

The term thermodynamic system is used frequently in the subject of thermodynamics. Let us see what thermodynamic system is and its various types.

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    The word system is very commonly used in thermodynamics; let us know what it is. Certain quantity of matter or the space which is under thermodynamic study or analysis is called as system. Let us say for example we are studying the engine of the vehicle, in this case engine is called as the system. Similarly, the other examples of system can be complete refrigerator, air-conditioner, washing machine, heat exchange, a utensil with hot water etc.

    Now, let us suppose that we have to analyze the performance of engine in different conditions. Here, we will feed the engine with fuels of different grades and load it with different loads to find out its efficiency. We will also find its performance during idling, acceleration, varying speed, slow speed and high speed. A thorough analysis of the engine is carried out; hence it is called as system.

    The system is covered by the boundary and the area beyond the boundary is called as universe or surroundings. The boundary of the system can be fixed or it can be movable. Between the system and surrounding the exchange of mass or energy or both can occur.

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    Types of Thermodynamic Systems

    There are three mains types of system: open system, closed system and isolated system. All these have been described below:

    1) Open system: The system in which the transfer of mass as well as energy can take place across its boundary is called as an open system. Our previous example of engine is an open system. In this case we provide fuel to engine and it produces power which is given out, thus there is exchange of mass as well as energy. The engine also emits heat which is exchanged with the surroundings. The other example of open system is boiling water in an open vessel, where transfer of heat as well as mass in the form of steam takes place between the vessel and surrounding.

    2) Closed system: The system in which the transfer of energy takes place across its boundary with the surrounding, but no transfer of mass takes place is called as closed system. The closed system is fixed mass system. The fluid like air or gas being compressed in the piston and cylinder arrangement is an example of the closed system. In this case the mass of the gas remains constant but it can get heated or cooled. Another example is the water being heated in the closed vessel, where water will get heated but its mass will remain same.

    3) Isolated system: The system in which neither the transfer of mass nor that of energy takes place across its boundary with the surroundings is called as isolated system. For example if the piston and cylinder arrangement in which the fluid like air or gas is being compressed or expanded is insulated it becomes isolated system. Here there will neither transfer of mass nor that of energy. Similarly hot water, coffee or tea kept in the thermos flask is closed system. However, if we pour this fluid in a cup, it becomes an open system.