The postulate of Clausius is a postulate drawn up by Rudolf Clausius (1822-1888). This postulate states that it is impossible for a thermodynamic system to run a cycle process, the only result being that heat is withdrawn from a low temperature heat reservoir and the delivery of the same amount of heat to a higher temperature heat reservoir. There is always a need for work on the system, or else a third heat reservoir with an even higher temperature that supplies heat to the intended system.
Examples of applications of Clausius's principle are refrigerators (refrigerators), heat pumps and thermal refrigerators.
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