1.6

# Resisting pressure-vacuum

The pressure in mechanical work or P-V work

The magnitude of mechanical work or the P-V work is the force F, multiplied by the displacement dx through which the system boundary moved

$\blacklozenge$ Be careful! The pressure $\bar { P }$ here is the “resisting” pressure, NOT the system pressure P.

If your system is a gas with pressure P, and it expands against vacuum by volume ΔV, work done by the system is “ZERO” since the resisting pressure is zero

$$\delta W=-\bar { P }\quad dV=0$$