Thermodynamics

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Thermodynamics->> THERMODYNAMIC CYCLES

All important thermodynamic relations used in engineering are derived from the first and second laws of thermodynamics. One useful way of discussing thermodynamic processes is in terms of cycles—processes that return a system to its original state after a number of stages, thus restoring the original values for all the relevant thermodynamic variables. In a complete cycle the internal energy of a system depends solely on these variables and cannot change. Thus, the total net heat transferred to the system must equal the total net work delivered from the system.

An ideal cycle would be performed by a perfectly efficient heat engine—that is, all the heat would be converted to mechanical work. The 19th-century French scientist Nicolas Léonard Sadi Carnot, who conceived a thermodynamic cycle that is the basic cycle of all heat engines, showed that such an ideal engine cannot exist. Any heat engine must expend some fraction of its heat input as exhaust. The second law of thermodynamics places an upper limit on the efficiency of engines; that upper limit is less than 100 percent. The limiting case is now known as a Carnot cycle.

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