# Entropy

We now introduce the following postulate, that will allow us to solve the general problem of thermodynamics:

Given a thermodynamic system there exists a state function calledentropy, depending on the variables and which is extensive, convex, monotonically increasing with respect to the internal energy and such that the equilibrium states of the system are its maxima, compatibly with the constraints put on the system itself.

The assumption that is a monotonically increasing function of the internal energy implies (Dini's theorem) that it is locally invertible; namely we can write at least locally as a function of and the other thermodynamic variables:

*systems*). It can also be shown that vanishes if and only if , and since we have:

^{[1]}, defined as:

*chemical potential*(note that from what we have previously stated, if and only if ). We thus have:

*state equations*, which are the relations we were looking for that bound the thermodynamic variables of a system.

Let us note that once the entropy of a system is known, all its thermodynamics can be straightforwardly derived: it is in fact sufficient to invert and express as and then take some derivatives in order to obtain all the state equations of the system.

Let us also briefly see that, for example, is indeed the temperature of a system, namely that if two systems with different temperatures are allowed to exchange internal energy they will finally reach an equilibrium where both have the same temperature. Let us call 1 and 2 these systems, which can be represented for example by two compartments of a box, each of volume , containing particles of gas (not necessarily of the same kind), at temperature and with internal energy (with ), separated by a wall that allows the exchange of heat (namely internal energy), but not particles (it's impermeable) or volume (it's fixed). Once the systems are in thermal contact, they will reach a new equilibrium, which will be a maximum for the entropy, namely in the final state. However, since the whole system is isolated the total internal energy must remain constant in the process. Thus (remembering that ):

- ↑ Since they are derivatives of a homogeneous function of degree one, they are homogeneous functions of degree zero.