- #1
AntiElephant
- 25
- 0
dE = [STRIKE]d[/STRIKE]Q + [STRIKE]d[/STRIKE]W = [STRIKE]d[/STRIKE]Qrev + [STRIKE]d[/STRIKE]Wrev = [STRIKE]d[/STRIKE]Qirev + [STRIKE]d[/STRIKE]Wirev.
We have for an reversible process, [STRIKE]d[/STRIKE]Qrev = TdS and [STRIKE]d[/STRIKE]Wrev = -PdV. So;
dE = TdS - PdV
So this relation is for all changes (irreversible or reversible) since dS and dV are state functions. What doesn't make sense to me is the next part when Helmholtz free energy is defined;
F = E - TS
Then dF = -PdV - SdT.
Another relation for all changes. I'm told and shown that for a system at constant temperature, then ΔW ≥ ΔF, with equality for reversible processes. However how is this equality true ONLY for reversible processes? If it's at constant temperature, then dT = 0. So dF = -PdV = dW no matter what process is?
We have for an reversible process, [STRIKE]d[/STRIKE]Qrev = TdS and [STRIKE]d[/STRIKE]Wrev = -PdV. So;
dE = TdS - PdV
So this relation is for all changes (irreversible or reversible) since dS and dV are state functions. What doesn't make sense to me is the next part when Helmholtz free energy is defined;
F = E - TS
Then dF = -PdV - SdT.
Another relation for all changes. I'm told and shown that for a system at constant temperature, then ΔW ≥ ΔF, with equality for reversible processes. However how is this equality true ONLY for reversible processes? If it's at constant temperature, then dT = 0. So dF = -PdV = dW no matter what process is?