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Physics · Glossary

What is Carnot cycle?

Definition 24.8 University Physics — Year 1 · Chapter 24 — Heat Engines and Machines

The Carnot cycle is the reversible ditherm cycle: two reversible isotherms, at ThT_h (in contact with the hot source) and at TcT_c (with the cold one), joined by two reversible adiabatic legs (isentropic, with no source in contact). Run clockwise in the (V,P)(V, P) plane it is an engine; counterclockwise, a refrigerator or heat pump. In the entropy diagram it is a rectangle.

The Carnot cycle of a perfect gas. Left: in the Clapeyron diagram two isotherms and two (steeper) adiabats, run clockwise; the enclosed area is the work delivered. Right: the same cycle in the entropy diagram is a rectangle of height T_h - T_c and width Q_h/T_h; its area is again -W, and the ratio of that area to the one under the top side, Q_h, is 1 - T_c/T_h by inspection. The Carnot cycle of a perfect gas. Left: in the Clapeyron diagram two isotherms and two (steeper) adiabats, run clockwise; the enclosed area is the work delivered. Right: the same cycle in the entropy diagram is a rectangle of height T_h - T_c and width Q_h/T_h; its area is again -W, and the ratio of that area to the one under the top side, Q_h, is 1 - T_c/T_h by inspection.
The Carnot cycle of a perfect gas. Left: in the Clapeyron diagram two isotherms and two (steeper) adiabats, run clockwise; the enclosed area is the work delivered. Right: the same cycle in the entropy diagram is a rectangle of height Th−TcT_h - T_c and width Qh/ThQ_h/T_h; its area is again −W-W, and the ratio of that area to the one under the top side, QhQ_h, is 1−Tc/Th1 - T_c/T_h by inspection.
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