Thermodynamic equilibrium state has the lowest possible energy

any non-equilibrium state is approaching equilibrium state unless existing external interactions and unified state parameters can be assigned only to the equilibrium state. Electromagnetic field can effectively enhance the heat and mass transfer processes

and it also has obvious effects on the equilibrium and stability of dielectric system. Based on the first and the second law of thermodynamics

the electric field energy is introduced into the thermodynamic differential equation

from which a new entropy function of dielectric system with the influence of electric field is deduced. Then

the equilibrium conditions and the stable conditions of dielectric systems under the action of electrostatic field are derived from the entropy criterion. The new equilibrium conditions require that the temperature

effective pressure

effective chemical potential

and electric field are uniform through the whole system. The new stable conditions require that the electric charge on the metal plates is proportional to the electric potential. This effective pressure includes mechanical effect from polarization

and the mechanical equilibrium is determined by real pressure and the electric polarization force. The effective chemical potential includes the effect of polarization potential energy

and the phase equilibrium is determined by concentration and polarization potential energy. The thermodynamic conclusions are consistent with that of electrodynamics and statistical physics

which have been confirmed by experiments.