1. Pressure magnitude

1.1 Thermodynamic approach

For an isotropic medium at thermodynamic equilibrium, energy E and entropy S are fundamental additive thermodynamic quantities. This means that the energy or entropy of the whole is equal to the sum of the energies or entropies of the various parts making up the macroscopic body.

Let's apply this principle to a fluid isolated in a volume V: for a given E, S depends only on V or, for a given S, E depends only on V and not on the shape of the body.

This results in the well-known total differential :