Global and Local Thermodynamic Equilibrium
The system is said to be in global thermodynamic equilibrium when the intensive properties like temperature, pressure and volume are homogenous throughout the system. The system is also said to be local thermodynamic equilibrium when the intensive properties are changing in space and time, but they are changing so slowly that at any time one can assume that thermodynamic equilibrium exists in some neighborhood about that point.
Causes of the Non-Equilibrium of the System
When the system does not satisfy the condition for any one of the equilibrium viz. mechanical, chemical, and thermal the system is said to be in non-equilibrium state. Here are the causes of the non-equilibrium state of the system:
The intensive properties within the system changing to a large extent is the major cause of the non-equilibrium of the system.
The unbalanced forces within the system or between the system and the surroundings cause non-equilibrium state of the system. The pressure in such system changes from one point to the other and there is no single pressure of the system.
The temperature varying at various points inside the system as well as in the surroundings is another cause of the non-equilibrium state of the system. In such system there is no single uniform temperature throughout the system.
The different chemical reactions taking place within the system and the surroundings is another of the non-equilibrium state of the system.
For the system to be in thermodynamic equilibrium, it should be in mechanical, chemical and thermal equilibrium.
These conditions of the thermodynamic equilibrium have led to the invention of Zeroth law of the thermodynamics. It states that when two bodies are separately in thermodynamic equilibrium with the third body, then they are also in thermodynamic equilibrium with each other. For example let us say body A and body B are separately in thermodynamic equilibrium with body C, then as per Zeroth law of thermodynamics bodies A and B are also in thermodynamic equilibrium with each other. Zeroth law of thermodynamics is considered to be very important law of thermodynamics.
Book: Engineering Thermodynamics by P K Nag