- #1
jbunten
- 87
- 0
I understand why
dE=TdS-PdV ...[1]
at constant temperature and pressure
Also, I see that F=E-TS
and therefore
dF=dE-SdT-TdS ...[2]
and how combining [2] and [3] gives
dF=-SdT -PdV
QUESTION: shouldn't [1] be expressed as dE=TdS+SdT-PdV
i.e if you're going to substitute into equation [2] where T isn't kept constant, don't you need to start off with T not being kept constant in your expression for thermal energy? Why are we letting temperature change in equation 2 but not in equation 1?
This seemingly obvious question has me stumped.
dE=TdS-PdV ...[1]
at constant temperature and pressure
Also, I see that F=E-TS
and therefore
dF=dE-SdT-TdS ...[2]
and how combining [2] and [3] gives
dF=-SdT -PdV
QUESTION: shouldn't [1] be expressed as dE=TdS+SdT-PdV
i.e if you're going to substitute into equation [2] where T isn't kept constant, don't you need to start off with T not being kept constant in your expression for thermal energy? Why are we letting temperature change in equation 2 but not in equation 1?
This seemingly obvious question has me stumped.