Now consider the arrangement shown in Fig. 2.6, in which one wall of a system is a
massless, frictionless, rigid, perfectly fitting piston of area A. If the external pressure is
pex, the magnitude of the force acting on the outer face of the piston is F = pexA. When
the system expands through a distance dz against an external pressure pex, it follows
that the work done is dw = −pexAdz.