Diagram of Clapeyron

The diagram of Clapeyron or diagram of Watt is a representation where one indicates the Pression of a system Thermodynamique according to his Volume to follow the evolution of a transformation. It is not possible to plot such a diagram that when the pressure is defined in the system, i.e. in a quasi-static Transformation.

Its study allows a quantitative approach of the behavior of a machine: if a series of transformation forms a thermodynamic Cycle, the direction of course of the cycle in the diagram of Clapeyron corresponds to the sign of the Travail received by the system:

if the course is carried out clockwise, received work is negative: the cycle is a driving Cycle;
if the course is anti-clockwise, received work is positive.

In particular, the surface of the cycle corresponds to the work received (with the sign near) during a cycle.

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