# Thread: Partial Derivative Proof (thermodynamics notation)

1. ## Partial Derivative Proof (thermodynamics notation)

Show that: $\displaystyle \left(\frac{\partial z}{\partial y}\right)_{u} = \left(\frac{\partial z}{\partial x}\right)_{y} \left[ \left(\frac{\partial x}{\partial y}\right)_{u} - \left(\frac{\partial x}{\partial y}\right)_{z} \right]$

I have Euler's chain rule and "the splitter." Also the property, called the "inverter" where you can reciprocate a partial derivative.

If I write Euler's chain rule, I only know how to write it when there are 3 variables, I usually write it in the form:
$\displaystyle \left(\frac{\partial x}{\partial y}\right)_{z} \left(\frac{\partial y}{\partial z}\right)_{x} \left(\frac{\partial z}{\partial x}\right)_{y} = -1$

Where I can write x,y,z in any order as long as each variable is used in every spot. However, I do not know how to work this chain rule if I have an extra variable (u in this case).

I also tried using the "splitter" to do something like writing:
$\displaystyle \left(\frac{\partial z}{\partial y} \right)_{u} = \left(\frac{\partial z}{\partial x} \right)_{u} \left(\frac{\partial x}{\partial y}\right)_{u}$

However, I do not know what to do with this because I have the term
$\displaystyle \left(\frac{\partial z}{\partial x} \right)_{u}$ , which doesn't appear in the original problem.

Any help would be appreciated.

(This is for a thermodynamics course, but we are still in the mathematics introduction.)

2. i too am an engineer but i only took it to the undergrad lvl and am not familiar with this notation... can you clarify?

3. Sure thing.

If I say something like $\displaystyle \left(\frac{\partial z}{\partial x}\right)_{y}$.

This means... The partial derivative of z with respect to x, holding y constant.

4. Originally Posted by Jacobpm64
Sure thing.

If I say something like $\displaystyle \left(\frac{\partial z}{\partial x}\right)_{y}$.

This means... The partial derivative of z with respect to x, holding y constant.
There is no need to write,
$\displaystyle \left(\frac{\partial z}{\partial x}\right)_{y}$

Because the notation,
$\displaystyle \frac{\partial z}{\partial x}$
Means exactly that i.e. "you hold all variables constant except x".

5. Oh, i thought it was different in thermodynamics, Please excuse me.

6. ^ im not sure i agree with your take on this, as this would simply cancel out the RHS of his eqn

i got the gerbils runnin, ill let u know if i come up with anything

Scroll down to neutrino's post.

Partial derivative notation

I think that should clear things up about the notation.