Suppose that the free energy of a system is described by where is the temperature, and represents some property of the system, such as its overall magnetization. As rises and falls, but does not cross , the allowed physical value(s) of , change somewhat (see Assumptions).
When crosses , however, there is a qualitative shift in the allowed values of , and the system undergoes a phase transition.
Suppose we're working with the system at a temperature below such that , find .
Assumptions