16.2.1—W = p∆V for the work done when the volume of a gas changes at constant

For constant external pressure, work done by a gas during a volume change is W=p∆V, with expansion positive for work done by the gas under the chosen convention.

State the sign convention and distinguish work by the gas from work on the gas. For changing pressure, use the area under a p–V graph.

A gas expanding by 0.020 m³ against 1.0×10⁵ Pa does 2000 J of work.

The formula is not automatically valid when pressure changes significantly, and sign depends on the convention used.