The Effect Of Applied External Fields On Thermodynamic Equilibrium Criterion And Its Applications

Thermodynamic equilibrium refers to that the macroscopic quantities of a system are invariable, it is able to stay permanently in the state. When the system is in equilibrium with the related environment, the system of thermodynamics parameters must satisfy certain conditions, referred to as the equilibrium cond itions of the system. Thermodynamic equilibrium criterion uses thermodynamic function to judge whether the system achieved the equilibrium. Since it was found that effects of applied external fields on equilibrium is significant, and, plays an important role in the chemical industry, med icine and other production, more and more scientists set out to study the effect of fields on the equilibrium. Some study the effect of the field on the system equilibrium according to the experiment; Others use molecular simulation to study the effect of the field on t the equilibrium. In some way, their work can illustrate some problems, but didn’t get a general conclusion.Since thermodynamics is universal, this paper will use thermodynamics method to study the effect of the external field on the system equilibrium. First of all, according to energy postulate and the second law of thermodynamics, generalized forms of thermodynamic equilibrium criterion under external field are derived. Although the forms of the entropy criterion, the free energy criterion and Gibbs function criterion in the presence of external fields have not changed, due to external field limited, their conditions have changed. On the basis of thermodynamic equilibrium criterion under the external field, the analysis of magnetic refrigeration process and electrochemical process is reasonable. According to thermodynamic equilibrium criterion under external field, phase equilibrium conditions of gravitational field and electric field are derived. According to the phase equilibrium condition under the gravitational field, relationship between pressure of perfect gases and its height is obtained, which is consistent with the conclusions of the classic books; basing on phase equilibrium conditions under the electric field in an isobaric system and an isovolumetric system, two relationships between the saturated vapor pressure of the liquid and the electric field intensity are deduced. The calculated results show that two formulas are equivalent, and the saturated vapor pressure of the liquid decreased slightly because of the electric field, in line with literature values.