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kostikas2002
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In my chemical thermodynamics class/notes (and other references I've used) it is stated throughout that internal energy U is a function of entropy and volume , i.e. it's "natural" variables are S and V:
Since U is a state function, it's exact differential will be:
Then we are introduced to the "fundamental thermodynamic equation" :
Comparing these two forms for dU, we readily see that:
But then later on in the course I find numerous expressions like (dU/dT) and others where it is implied that internal energy is a function of temperature (and pressure). What am I to make of these expressions? Sure, I understand that temperature actually DOES affect the internal energy of a system, but U has been defined in a way that "excludes" it. I mean there must be a reason that they don't state the dependence explicitly (i.e. U = U(S,V,T,p) ) but I just don't understand it.
Maybe it has to do with the fact that (dU/dS) = T but then what am I to understand about the form of U from (dU/dT) ? It must mean that whatever the function for U is, it contains the expression for T, so it is a differential equation. For example: If we have
What's going on? It's things like these that make me hate thermodynamics with a PASSION...
- U = U(S,V)
Since U is a state function, it's exact differential will be:
- dU = (dU/dS) dS + (dU/dV) dV
Then we are introduced to the "fundamental thermodynamic equation" :
- dU = TdS - pdV
Comparing these two forms for dU, we readily see that:
- (dU/dS) = T
- (dU/dV) = - p
But then later on in the course I find numerous expressions like (dU/dT) and others where it is implied that internal energy is a function of temperature (and pressure). What am I to make of these expressions? Sure, I understand that temperature actually DOES affect the internal energy of a system, but U has been defined in a way that "excludes" it. I mean there must be a reason that they don't state the dependence explicitly (i.e. U = U(S,V,T,p) ) but I just don't understand it.
Maybe it has to do with the fact that (dU/dS) = T but then what am I to understand about the form of U from (dU/dT) ? It must mean that whatever the function for U is, it contains the expression for T, so it is a differential equation. For example: If we have
- U(S,V) = exp(S) + exp(V)
- T = (dU/dS) = exp(s)
- p = - (dU/dV) = - exp(V)
- U(S,V) = exp(S) + exp(V) = T - p = U(T,p)
- U(S,V) = exp(S) + exp(V) = T + exp(V) = U(T,V)
- U(S,V) = exp(S) + exp(V) = exp(S) - p = U(S,p)
- (dU/dT) = exp(V) ( = (dU/dS) )
- (dU/dp) = exp(S) ( = (dU/dV) )
What's going on? It's things like these that make me hate thermodynamics with a PASSION...
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