Isobaric process, a process taking place in a physical system at constant external pressure; on a thermodynamic diagram it is represented by an isobar. The simplest examples of an isobaric process are the heating of water in an open vessel and the expansion of a gas in a cylinder with a freely moving piston. In both cases the pressure equals atmospheric pressure. The volume of an ideal gas in an isobaric process is proportional to the temperature (see Gay-Lussac's law). The heat capacity of a system in an isobaric process is greater than in an isochoric process (at constant volume). For an ideal gas, for example, cp - cv=k, where cp and cv are the heat capacities per particle in the isobaric and the isochoric process. The work done by an ideal gas in an isobaric process is p*ΔV, where p is the pressure and ΔV is the change in the volume of the gas.