Thermodynamic equilibrium - Thermodynamics:

Equilibrium is the state of a system in which the state does not undergo any change in itself with the passage of time without the help of any external agent. Equilibrium state of system can be examined by observing weather change in state of system occurs or not. If no change in state of system happens then system is in equilibrium.

Let us consider a steel glass full of hot milk which is kept in open atmosphere. It is obvious that the heat from milk shall be continuously transferred to atmosphere till temperature of milk; glass and atmosphere are not same. During the transfer of heat from milk temperature of milk could be seen to decrease continually. Temperature gets some final value and does not change any more. This is equilibrium state at which the properties stop showing any change in them.

Generally, ensuring mechanical, chemical, thermal and electrical equilibriums of the system may ensure thermodynamic equilibrium of the system.

1. Mechanical Equilibrium: When there is no unbalanced force within system and nor at its boundaries then system is in mechanical equilibrium.

For a system to be in equilibrium there should not be any pressure gradient within the system that is equality of pressure for the entire system.

2. Chemical Equilibrium: When there is no chemical reaction taking place in system it is in chemical equilibrium.

3. Thermal equilibrium: When there is no temperature gradient within the system, system is in thermal equilibrium.

4. Electrical Equilibrium: When there is no electrical potential gradient within system, the system is in electrical equilibrium.

When all the conditions of chemical, thermal, mechanical, electrical equilibrium are fulfilled, the system is in thermodynamic equilibrium.