Thermodynamics is a field of physics and chemistry. It uses mathematics to describe changes in energy and entropy (disorder) of a material that is subject to mechanical, thermal or chemical action. It is most frequently used to describe the conditions that lead to minimization of energy and thereby predict the equililbrium-state of the material. Sometimes, the difference between the free energies of the current state and the equilibrium state is used to give the temporal rate at which a system approaches equilibrium. In its simplest form, thermodynamics neglect non-hydrostatic stress, surface energy and many other things. Then it is possibole to simply describe the energy of a system to be minimized is the free energy. The enthalpy (or heat content) and the density of a material may also be described with thermodynamics. In many physical processes, it is useful to consider a system to be at equilibrium in small regions, while radients on a larger length scale may still exist.