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The Carnot Cycle and Carnot Theorem: Working and Relation to Second Law of Thermodynamics – Part 1

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

The Carnot cycle comprises two ideal reversible isothermal and two reversible adiabatic processes in heat engine. Carnot theorem and second law of thermodynamics are based on Carnot cycle. The Carnot cycle shows the maximum efficiency that can be achieved by the engine.

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    Carnot cycle is an ideal reversible heat engine cycle, proposed by Sadi Carnot, a young French scientist, in the year 1824. It comprises of two reversible isothermal and two reversible adiabatic processes. Carnot cycle is a hypothetical cycle and it cannot be achieved in the practical situations. However, it gives the guidelines as to what maximum efficiency can be obtained from the engine.

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    Importance of Carnot Cycle

    1) Efficiency of the heat engine depends on the maximum and minimum temperature of the cycle: In Carnot cycle the working medium (steam or air) receives the heat at high temperature and rejects the heat at lower temperature. The efficiency of the Carnot cycle depends on the maximum and minimum temperatures of the cycle. Before Carnot, there was general feeling that the efficiency of heat engine was dependent on the type of fluid used in the cycle. However, in his book, “Reflexions sur la puissance motrice du feu (“Reflections on the Motive Power of Fire")," Carnot says that the efficiency of the engine is not dependent on the type of fluid, rather it depends on the maximum and minimum temperatures between which the heat engine cycle operates. This is also the reason; he stated why the efficiency of the engine is higher with superheated steam.

    The high temperature reservoir from where the heat is absorbed at high temperature is called as source and low temperature reservoir, usually atmosphere, where the heat is rejected is called as sink. Thus the efficiency of Carnot cycle depends on the temperatures of the source and sink, measured in Kelvin i.e. the absolute temperature.

    2) Discovery of the second law of thermodynamics: Carnot’s cycle clearly shows that heat is absorbed from the high temperature reservoir and rejected at low temperature reservoir. This finding was very crucial and it laid the foundation stone for the second law of thermodynamics. The law says that the natural flow of heat is from high temperature reservoir to low temperature reservoir, and to move it from low temperature reservoir to high temperature reservoir external work has to be done. The second law of thermodynamics has been found to be crucial in discovering the important machines like engines, refrigerator and air-conditioning systems, etc.

    Next: Part-2 of this article