Thermodynamic Equilibrium

The idea of equilibrium is the keystone of classical thermodynamics. As most of the other things in thermodynamics, it is actually an abstraction, as real systems are never severely in symmetry.   

Whenever a system has no unbalanced force inside it and whenever the force it applied on its border is balanced by external forces, the system is supposed to be in mechanical symmetry. Whenever the temperature of a system is uniform during and is equivalent to the temperature of the surroundings, then the system is believed to be in thermal equilibrium. Whenever the chemical composition of a system stays unchanged, the system is said to be in chemical symmetry.  

It might also be seen that whenever two systems are in mechanical symmetry the property which the systems have in general is pressure. Temperature is the property which two systems have in general whenever they are in thermal equilibrium.

There are other types of equilibrium also. The two systems are supposed to be in electrostatic equilibrium if there is no tendency for a net charge flow between them whenever they are brought into contact. The electrostatic potential is a property in which two systems have common whenever they are in electrostatic equilibrium. The two phases of a substance (for illustration, solid & liquid) are said to be in phase equilibrium when there is no tendency for phase transformation (for illustration, melting) whenever they are brought into contact.

The word thermodynamic equilibrium is employed to point out a situation of equilibrium with respect of all probable macroscopic changes in a system. The idea of thermodynamic equilibrium is anxious with the static and time-invariant state of a system where no spontaneous procedures occur and all macroscopic quantities stay unchanged.

It is significant to differentiate among two types of idealized time-independent states. Whenever the assets or state variables measured at any position in the system, and as a function of time on a time-scale that does not solve separate molecular events, stay constant with time all over in the system, the system is said to be in a stable state. Whenever, in addition, no changes emerge in its surroundings, the system is said to be in a state of thermodynamic equilibrium. Therefore, thermodynamic equilibrium is the time-invariant situation of an isolated system; stable state is equivalent time-invariant condition of a system that can interact with its surroundings. In logic, equilibrium is a special situation of the more common steady state, being the limiting case whenever the interaction with the surroundings approaches to zero.

For illustration, when heat is supplied at constant rate at one end of a piece of metal and eliminated at an equivalent rate at the other hand, the temperature at every point will approach a steady value. The system in question stays unchanged in time, though is not in a situation of thermodynamic equilibrium on account of the happening of the dissipative procedure (that is, changes take place in the surroundings).