Some ideas from thermodynamics

The branch of physics called thermodynamics deals with the laws governing the behavior of physical systems at finite temperature. These laws are phenomenological, in the sense that they represent empirical relations between various observed quantities. These quantities include pressure, temperature, density and other measurable parameters that define the state of a given system. Thus, in discussing magnetic systems, e.g., the variables might include temperature, magnetization and external magnetic field; in discussing elastic systems we might want to use temperature, stress and strain as defining variables. Some useful references for the student who wants to pursue the subject further are