This Demonstration considers the isobaric thermodynamic process of compressing or expanding an ideal gas. Assume that a constant external pressure acts on the piston containing the gas. As you vary the external pressure, the horizontal line in the graph changes accordingly. When the variables for the initial and final volumes and ) are changed, the position of the gridlines on the graph and the initial and final position of the head of the piston move. Work is calculated in the Demonstration with the convention used in chemistry: when the ideal gas expands, the work done is negative (i.e. the ideal gas "loses" energy), and vice versa.