Isobaric process

An isobaric process is one in which the pressure of the system is held constant while its volume and temperature change. On a pressure–volume diagram it is a horizontal line, and the work done is just the rectangle beneath it, W = P ΔV. A gas heated under a freely sliding, weighted piston expands isobarically, lifting the piston as it warms.

Because the gas does work as it expands, the heat supplied at constant pressure exceeds what would be needed at constant volume for the same temperature rise; the difference is captured by the constant-pressure heat capacity C_p, which for an ideal gas exceeds C_v by exactly R. The everyday heating of air at atmospheric pressure is very nearly isobaric.

The isobaric leg is one of the four canonical building blocks of thermodynamic cycles, appearing in the Brayton and Diesel cycles among others, where constant-pressure heat addition or rejection alternates with compression and expansion strokes.

Recognised among the canonical processes once Clapeyron's PV diagram (1834) let each be drawn and analysed separately; the constant-pressure heat capacity it requires was already being measured in the early nineteenth century.