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

Be careful! The pressure 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

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

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This article is from the free online course:

Thermodynamics in Energy Engineering

Hanyang University