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A PV diagram plots the change in pressure P with respect to volume V for some process or processes. Typically in thermodynamics, the set of processes forms a cycle, so that upon completion of the cycle there has been no net change in state of the system; i.e. the device returns to the starting pressure and volume.
An isochoric thermodynamic quasi-static process is characterized by constant volume, i.e., ΔV = 0. The process does no pressure-volume work, since such work is defined by =, where P is pressure. The sign convention is such that positive work is performed by the system on the environment.
A saturation dome uses the projection of a P–v–T diagram (pressure, specific volume, and temperature) onto the P–v plane. The points that create the left-hand side of the dome represent the saturated liquid states, while the points on the right-hand side represent the saturated vapor states (commonly referred to as the “dry” region).
For a thermally perfect diatomic gas, the molar specific heat capacity at constant pressure (c p) is 7 / 2 R or 29.1006 J mol −1 deg −1. The molar heat capacity at constant volume (c v) is 5 / 2 R or 20.7862 J mol −1 deg −1. The ratio of the two heat capacities is 1.4. [4] The heat Q required to bring the gas from 300 to 600 K is
Cycles composed entirely of quasistatic processes can operate as power or heat pump cycles by controlling the process direction. On a pressure–volume (PV) diagram or temperature–entropy diagram, the clockwise and counterclockwise directions indicate power and heat pump cycles, respectively.
A and C are constant-pressure (isobar) evolutions, B and D are constant-volume (isochoric) evolutions. Français : Un cycle thermodynamique décrit sur un diagramme pression-volume. A et C sont des évolutions à pression constante (isobares), B et D sont des évolutions à volume constant (isochores).
All four diagrams are derived from the physical P–alpha diagram which combines pressure (P) and specific volume (alpha) as its basic coordinates. The P–alpha diagram shows a strong deformation of the grid for atmospheric conditions and is therefore not useful in atmospheric sciences. The three diagrams are constructed from the P–alpha ...
The PV diagram is a particularly useful visualization of a quasi-static process, because the area under the curve of a process is the amount of work done by the system during that process. Thus work is considered to be a process variable , as its exact value depends on the particular path taken between the start and end points of the process.