# Specific heats

1. Apr 30, 2017

### Silviu

1. The problem statement, all variables and given/known data
Show that $\frac{\partial U}{\partial T}|_{N} = \frac{\partial U}{\partial T}|_{\mu} + \frac{\partial U}{\partial \mu}|_{T} \frac{\partial \mu}{\partial T}|_{N}$ (Pathria, 3rd Edition, pg. 197)

2. Relevant equations
$U=TS + \mu N - pV$

3. The attempt at a solution
I tried to take the derivative of U with respect with T at constant N and then at constant $\mu$ but I can't get the term on the right. I am not sure how can I get a product of 2 derivatives, because in taking derivatives in $U=TS + \mu N - pV$ I always have a derivative times a number (for example $T\frac{\partial S}{\partial T}|_N$). What should I do? Is there another way other than using this formula?

2. Apr 30, 2017

### Charles Link

Just a suggestion=perhaps the book is correct, but they appear to be using $U=U(N,\mu,T)$, so that in taking $\frac{\partial{}}{\partial{T}}$, both $N$ and $\mu$ should be held constant.

3. Apr 30, 2017

### Silviu

OK, But does this leads to having a term with 2 derivatives. I might have stuff like $\frac{\partial U}{\partial T}|_{N,\mu}$ or something similar, but I still don't see how do I get a product of 2 derivatives. Thank you!

4. Apr 30, 2017

### Charles Link

I'm going to need to study it further. It looks like it might take a little work if it is correct.

5. Apr 30, 2017

### Charles Link

With a little further thought, it seems taking a partial with only holding one other variable constant is ambiguous, because on the multi-dimensional surface, you can not precisely define how much the quantity $U$ will change. It allows multiple paths depending on what you do with the 3rd variable. @Chestermiller Might you offer an input here?

6. Apr 30, 2017

### Silviu

But even if you keep 2 constant at each derivation, I am still not sure how to prove it.

7. Apr 30, 2017

### Charles Link

If you look at the term on the left and the first term on the right, if you include the third variable in both of the partial derivatives to be held constant, these two are then the same. IMO the equation is mathematically unsound, but perhaps @Chestermiller can add to that and/or possibly concur.

8. Apr 30, 2017

### Staff: Mentor

U is typically a function of 3 parameters, usually S, V, and N. So two parameters need to be held constant in those partial derivatives. What is the other parameter?

9. Apr 30, 2017

### Charles Link

The author is using $N$ , $\mu$, and $T$ as the 3 parameters, but he's only holding one constant at a time. To me this appears to be mathematically unsound.

10. Apr 30, 2017

### Silviu

I asked my professor and he told me that for $C_n$ N and V must be held constant and for $C_\mu$, $\mu$ and V must be held constant in order to solve the problem. Does this help?

11. Apr 30, 2017

### Charles Link

That is very helpful=I solved it now=I will post it momentarily. $\\$
$U=U(N,T,V)$ and $N=N(\mu,T,V)$ so that $U=U(\mu,T,V)$ and $\mu=\mu(N,T,V)$. $\\$
$(\frac{\partial{U(N,T,V)}}{\partial{T}})_{N,V}=(\frac{\partial{U(\mu,T,V)}}{\partial{T}})_{N,V}=(\frac{\partial{U(\mu,T,V)}}{\partial{\mu}})_{T,V}(\frac{\partial{\mu(N,T,V)}}{\partial{T}})_{N,V}+(\frac{\partial{U(\mu,T,V)}}{\partial{T}})_{\mu,V}$. $\\$
Yes, your professor's input was very helpful. (Note: Ten minutes after posting= I just edited the T, V subscript in the first term after the second "=" sign from an incorrect N,V subscript)

Last edited: Apr 30, 2017
12. Apr 30, 2017

### Charles Link

@Silviu Please see the latest editing on the previous post.

13. Apr 30, 2017

### Silviu

Thank you so much for help. But I am a bit confused about the math behind it. Is this like a chain rule, when you keep some variable constants?

14. Apr 30, 2017

### Charles Link

Very much so=the chain rule. You go through the list of variables and take the appropriate derivatives, and skip over any variables that are held constant.

15. Apr 30, 2017

### Silviu

OK, makes sense. Thank you so so much!

Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted