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

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SUMMARY

The discussion centers on the thermodynamic equation dU=TdS-PdV and clarifies why the differential work done, dw, is expressed as PdV rather than PdV + VdP. It is established that work is only performed when there is a change in volume; thus, if volume remains constant, the pressure does not contribute to work done. The term VdP is identified as part of the internal energy change rather than work, reinforcing the distinction between work and internal energy in thermodynamic processes.

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When proving the equation,
dU=TdS-PdV

why is dw= PdV not dw= PdV +vdP?
 
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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.
 
asdf1 said:
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
 
thank you!
 

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