Proof of dU=TdS-PdV: dw=PdV Not dw=PdV+vdP

[asdf1]

When proving the equation,
dU=TdS-PdV

why is dw= PdV not dw= PdV +vdP?

 

[Cyrus]

I think it is as you wrote, but don't quote me. I'm at school and don't have my book infront of me.

 

[Andrew Mason]

When proving the equation,
dU=TdS-PdV

why is dw= PdV not dw= PdV +vdP?

Work (force x distance) is done only if the pressure (force/area) acts over some change in volume (area x distance moved). If there is no change in volume, the force does not act over any distance - ie. the pressure just builds up but does not move anything (ie. it just increases internal energy). So, VdP is part of the internal energy of the gas, not the work done by the gas.

AM

 

[asdf1]

thank you!