= Pressure-volume diagram
A <pressure-volume diagram> plots a <thermodynamic process> in the plane of <pressure> $p$ and <volume> $V$. For quasistatic boundary work, work done by the system is $W=\int p\,dV$. Compression and expansion have opposite signs under this convention.
For a closed cycle, the oriented enclosed <area> equals the net boundary work. A clockwise loop gives positive net work by the system. If the process is strongly irreversible, the external <pressure> at the moving boundary, rather than an assumed uniform <equilibrium> system <pressure>, must be used to calculate the actual work.
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