Question

why do we write work for a closed system as -Pexternal dv rather than as shaft work plus flow work like we do for an open system?

Answer 1

W is a function of pressure and volume . dw= pdv+vdp for closed system pressure is constant so dp = 0 . For closed system work = pdv only . For open system pressure is not constant so work = pdv + vdp.

Example for closed system: cylinder having piston in top . Some gas present inside when we are pushing the top of the cylinder gas is compressed and top pressure above it is constant. Closed system pressure mean outside pressure acting on the system not inside pressure .

Example for open system : pump we are adding shaft work (pdv )using a electric motor but there is another work due the pressure change ( vdp ). Fluid is pressurised from one state to another state.