✎ Maxwell Relations [The Physics Travel Guide]

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====== Maxwell Relations ======

<note tip>
Explanations in this section should contain no formulas, but instead colloquial things like you would hear them during a coffee break or at a cocktail party.
</note>
  
<tabbox Concrete>

**Derivation**

The Maxwell relations follow directly from the fact that [[https://en.wikipedia.org/wiki/Symmetry_of_second_derivatives|partial derivatives commute]]: $\partial_x \partial_y = \partial_y  \partial_x$.

If we have some function $U(S,V)$ (called the internal energy) that depends on the entropy $S$ and the volume $V$, the total change of it is given by

$$ dU = \frac{\partial U}{\partial S} \big |_V dS + \frac{\partial U}{\partial V} \big |_S dV,  $$

where $\big |_V$ means that we keep $V$ fixed.

Then, we introduce two definitions:

$$ \frac{\partial U}{\partial S} \big |_V \equiv  T , \quad -\frac{\partial U}{\partial V} \big |_S \equiv  P.$$

The minus sign here is just a convention and can be understood as follows: The internal energy usually gets smaller when we increase the volume. Thus, if we want to work with positive pressure $P$ in most situations, we need to include the minus sign here. 
 
Using these definitions equation reads

$$ dU = T dS - P dV,  $$

which is the [[https://en.wikipedia.org/wiki/Fundamental_thermodynamic_relation|fundamental thermodynamic relation]].

Next, we use that partial derivative commute:

$$ \frac{\partial^2 U}{\partial V\partial S} = \frac{\partial^2 U}{\partial S\partial V} $$

and put in our two definitions from above:

$$ \frac{\partial T}{\partial S} \big |_V = -\frac{\partial P}{\partial V}\big |_S $$

This is one of the Maxwell relations.

The other Maxwell relations follow completely analogous, but with different functions instead of the internal energy $U$.

For example, if we start with the Helmholtz free energy $A(T,V)$:

$$ A = U -TS $$

and follow exactly the same steps ($dA= dU-d(TS) = (Tds-PdV)-(SdT+TdS)=-SdT-PdV$), we can derive

$$ \frac{\partial S}{\partial V} \big |_T = \frac{\partial P}{\partial T} \big |_V $$

The other Maxwell relations follow by starting with the enthalpy $H(S,P)$ or the Gibbs free energy $G(T,P)$.

(These notions appear, since:

* a system with fixed entropy and volume will choose the state with minimum internal energy $U$, 
  * a system with fixed temperature and volume will choose the state with minimum enthalpy, 
  * a system with fixed entropy and pressure will choose the state with minimum Helmholtz free energy, 
  * a system with fixed temperature and pressure will choose the state with minimum Gibbs free energy.)

----

For a great explanation, why the Maxwell relations are "just a sneaky way of saying that the mixed partial derivatives of the function U commute". see https://johncarlosbaez.wordpress.com/2012/01/19/classical-mechanics-versus-thermodynamics-part-1/

(The fact that partial derivatives commute is known as Schwarz' theorem (see  https://en.wikipedia.org/wiki/Maxwell_relations . Schwarz' theorem is simply a way of stating "the fact that a function S doesn’t change when we go around a parallelogram" https://johncarlosbaez.wordpress.com/2012/01/23/classical-mechanics-versus-thermodynamics-part-2/)
<tabbox Abstract>

<note tip>
The motto in this section is: //the higher the level of abstraction, the better//.
</note>

<tabbox Why is it interesting?> 
The Maxwell relations encode useful relationships between notions of [[theories:thermodynamics|thermodynamics]].

</tabbox>

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