Thermodynamic Potential - The Fundamental Equations

The Fundamental Equations

The definitions of the thermodynamic potentials may be differentiated and, along with the first and second laws of thermodynamics, a set of differential equations known as the fundamental equations follow. (Actually they are all expressions of the same fundamental thermodynamic relation, but are expressed in different variables.) By the first law of thermodynamics, any differential change in the internal energy U of a system can be written as the sum of heat flowing into the system and work done by the system on the environment, along with any change due to the addition of new particles to the system:

where is the infinitesimal heat flow into the system, and is the infinitesimal work done by the system, is the chemical potential of particle type i and is the number of type i particles. (Note that neither nor are exact differentials. Small changes in these variables are, therefore, represented with δ rather than d.)

By the second law of thermodynamics, we can express the internal energy change in terms of state functions and their differentials. In case of quasistatic changes we have:

where

T is temperature,

S is entropy,

p is pressure,

and V is volume, and the equality holds for reversible processes.

This leads to the standard differential form of the internal energy in case of a quasistatic reversible change:

Since U, S and V are thermodynamic functions of state, the above relation holds also for arbitrary non-reversible changes. If the system has more external variables than just the volume that can change, the fundamental thermodynamic relation generalizes to:

Here the are the generalized forces corresponding to the external variables .

Applying Legendre transforms repeatedly, the following differential relations hold for the four potentials:

Note that the infinitesimals on the right-hand side of each of the above equations are of the natural variables of the potential on the left-hand side. Similar equations can be developed for all of the other thermodynamic potentials of the system. There will be one fundamental equation for each thermodynamic potential, resulting in a total of fundamental equations.

The differences between the four thermodynamic potentials can be summarized as follows: