In Part 2 we saw a very efficient formulation of Maxwell’s relations, from which we can easily derive their usual form. Now let’s talk more about the meaning of the Maxwell relations—both their physical meaning and their mathematical meaning. For the physical meaning, I’ll draw again from Ritchie’s paper:
• David J. Ritchie, A simple method for deriving Maxwell’s relations, American Journal of Physics 36 (1958), 760–760.
but Emily Roach pointed out that much of this can also be found in the third chapter of Jaynes’ unpublished book:
• E. T. Jaynes, Thermodynamics, Chapter 3: Plausible reasoning.
First I’ll do the case of 2 dimensions, and then the case of n dimensions. In the 2d case I’ll talk like a physicist and use notation from thermodynamics. We’ll learn the Maxwell relations have these meanings, apart from their obvious ones:
• In any thermodynamic cycle, the heat absorbed by a system equals the work it does.
• In any thermodynamic cycle, energy is conserved.
• Any region in the surface of equilibrium states has the same area in pressure-volume coordinates as it does in temperature-entropy coordinates.
In the n-dimensional case I’ll use notation that mathematicians will like better, and also introduce the language of symplectic geometry. This will give a general, abstract statement of the Maxwell relations:
• The manifold of equilibrium states is a Lagrangian submanifold of a symplectic manifold.
Don’t worry—I’ll explain it!
Maxwell’s relations in 2 dimensions
and we call the coordinates on this plane
Next we introduce two more variables called temperature and pressure So, we’ll give coordinates But, there’s a 2-dimensional surface where these extra variables are given by the usual formulas in thermodynamics:
We call this the surface of equilibrium states.
By the equations that and obey on the surface of equilibrium states, the following equation holds on this surface:
So, if is any loop on this surface, we have
where the total change in is zero because the loop ends where it starts! We thus have
In thermodynamics this has a nice meaning: the left side is the heat absorbed by a system as its state moves around the loop while the right side is the work done by this system. So, the equation says the heat absorbed by a system as it carries out a cycle is equal to the work it does.
We’ll soon see that this equation contains, hidden within it, all four Maxwell relations. And it’s a statement of conservation of energy! By the way, it’s perfectly obvious that energy is conserved in a cycle: our loop takes us from a point where has some value back to the same point, where it has the same value.
So how do we get Maxwell’s relations out of this blithering triviality? There’s a slow way and a fast way. Since the slow way provides extra insight I’ll do that first.
Suppose the loop bounds some 2-dimensional region in the surface Then by Green’s theorem we have
That is, the heat absorbed in the cycle is just the area of the region in coordinates. But Green’s theorem also says
That is, the work done in the cycle, which is minus the left hand side, is minus the area of the region in coordinates!
That’s nice to know. But since we’ve seen these are equal,
for every region in the surface
Well, at least I’ve shown it for every region bounded by a loop! But every region can be chopped up into small regions that are bounded by loops, so the equation is really true for any region in the surface
We express this fact by saying that equals when these 2-forms are restricted to the surface We write it like this:
Now, last time we saw how to quickly get from this equation to all four Maxwell relations!
(Back then I didn’t write the symbol because I was implicitly working on this surface without telling you. More precisely, I was working on the plane but we can identify the surface with that plane using the coordinates.)
So, given what we did last time, we are done! The equation
expresses both conservation of energy and all four Maxwell equations—in a very compressed, beautiful form!
Maxwell’s relations in n dimensions
Now we can generalize everything to n dimensions. Suppose we have a smooth function
Write the coordinates on as
and write the corresponding coordinates on the cotangent bundle as
There is a submanifold of where equals the partial derivative of with respect to
Let’s call this submanifold since it’s the same one we’ve seen before (except for that annoying little minus sign in the definition of pressure):
In applications to thermodynamics, this is the manifold of equilibrium states. But we’re doing math here, and this math has many applications. It’s this generality that makes the subject especially interesting to me.
Now, started out life as a function on but it lifts to a function on and we have
By the definition of we have
The 1-form on the right-hand side here:
is called the tautological 1-form on Its exterior derivative
is called the symplectic structure on this cotangent bundle. Both of these are a big deal in classical mechanics, but here we are seeing them in thermodynamics! And the point of all this stuff is that we’ve seen
Taking of both sides and using , we get
or in other words
And this is a very distilled statement of Maxwell’s relations!
Why? Well, in the 2d case we discussed earlier, the tautological 1-form is
thanks to the annoying minus sign in the definition of pressure. Thus, the symplectic structure is
and the fact that the symplectic structure vanishes when restricted to is just our old friend
As we’ve seen, this equation contains all of Maxwell’s relations.
In the n-dimensional case, is an n-dimensional submanifold of the 2n-dimensional symplectic manifold In general, if an n-dimensional submanifold of a 2n-dimensional symplectic manifold has the property that the symplectic structure vanishes when restricted to we call a Lagrangian submanifold.
So, one efficient abstract statement of Maxwell’s relations is:
It takes a while to see the value of this statement, and I won’t try to explain it here. Instead, read Weinstein’s introduction to symplectic geometry:
• Alan Weinstein, Symplectic geometry, Bulletin of the American Mathematical Society 5 (1981), 1–13.
You’ll see here an introduction to Lagrangian submanifolds and an explanation of the “symplectic creed”:
— Alan Weinstein
The restatement of Maxwell’s relations in terms of Lagrangian submanifolds is just another piece of evidence for this!
• Part 1: a proof of Maxwell’s relations using commuting partial derivatives.
• Part 2: a proof of Maxwell’s relations using 2-forms.
• Part 3: the physical meaning of Maxwell’s relations, and their formulation in terms of symplectic geometry.
For how Maxwell’s relations are connected to Hamilton’s equations, see this post: