# What are intuitive definitions of the 4 thermodynamic potentials?

So out of the 4 thermodynamic potentials, only the internal energy seems to make intuitive sense to me.

My understanding of enthalpy is that it is a state function that represents the total heat content at constant pressure.

After checking a few articles they say the Helmholtz free energy is the amount of energy needed to make "space" for a system after accounting for the spontaneous energy.

And the Gibbs free energy is a thermodynamic potential which indicates whether or not a chemical reaction will occur spontaneously at constant pressure and temperature.

Note when I say definitions, I understand their equations but it's quite hard to see what physical interpretations to draw from their differential forms.

• Did you look at the explanations given on the Hyperphysics website? Jan 20, 2020 at 7:59
• Yes thats where I got the definitions in terms of "energy required to make room for the system". Just wanted to know if there were other ways of understanding the potentials. Jan 20, 2020 at 8:53
• For intuition on the free energy see this answer: physics.stackexchange.com/a/369412/226902 Apr 2 at 17:19

There are actually more than 4 thermodynamic potentials - their number depends on the number of degrees of freeedom of the system.

Say your internal energy is a function of the three extensive variables $$S, V, N$$. Then, you can swap out each of these for a corresponding intensive one via Legendre transformation. In principle, this gets you $$2^3 = 8$$ potentials, but one of them is just a constant as its differential is 0 due to the Gibbs-Duhem equation (which follows from internal energy being a homogeneous function).

Now, in order to get an intuitive understanding of a given potential, you hold its intensive parameters fixed and vary the extensive ones, of which there is at least one. The potential is the capacity to release energy under change of those latter variables (eg via expansion work if you vary $$V$$ or heat transfer if you vary $$S$$).

For example, Helmholtz free energy is defined by the transformation $$S\to T$$. If you hold temperature fixed, energy can still be transferred by all other means except heat the change in Helmholtz free energy will be sensitive to all means of energy transfer except heat, so it's the systems capacity to perform (mechanical and non-mechanical) work (at fixed temperature).

Finally, as you alluded to ("gibbs free energy is a thermodynamic potential which indicates whether or not a chemical reaction will occur spontaneously at constant pressure and temperature"), you can use the potentials to analyze systems via the principle of minimum energy.

• What do you mean by "Helmholtz free energy is defined by the transformation S→T". I have only seen the hemholtz defined as U-TS. Jan 20, 2020 at 17:10
• The natural variables of $U$ are $S, V, N$ whereas the natural variables of the Helmholtz free energy $F$ are $T, V, N$. The Legendre transformation changes the functional dependence from $S$ to $T=\partial U/\partial S$. Jan 20, 2020 at 17:22
• Why would you want to swap out intensive and extensive variables? I understand your answer for the mathematical procedure but I'm not sure why someone would want to switch variables. Oct 5, 2020 at 10:39