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THERMODYNAMIC EQUILIBRIUM AND ITS TYPES

A System is said to exist in state of thermodynamic equilibrium when no change in any macroscopic property is registered, if the system is isolated from its surroundings,

An isolated system always reaches in course of  time a state of thermodynamic equilibrium and can never depart from it spontaneously.

Therefore, there can be no spontaneous change in macroscopic property if the system exists in equilibrium state. Thermodynamics studies mainly the properties of physical systems that are found in equilibrium states. 



A system will be in a state of  thermodynamic equilibrium, if the conditions for the following three types of equilibrium are satisfied :

1. Mechanical equilibrium 

2. Chemical equilibrium

3. Thermal equilibrium 

In the absence of any unbalanced force within the system itself and also between the system and the surroundings, The system is said to be in a state of mechanical equilibrium. If an unbalanced force exists, either the system alone or both the system and the surroundings will undergo a change of state till mechanical equilibrium is attained. 

If there is no chemical reaction or transfer of matter from one part of the system to another, such as diffusion or solution, the system is said to exist in a state of chemical equilibrium.

When a system existing in a mechanical and chemical equilibrium is separated from its surroundings by a diathermic wall ( wall which allows heat to flow ). and if there is no spontaneous change in any property of the system, the system is said to exist in a state of  thermal equilibrium. when this is not satisfied, the system will undergo a change of state till thermal equilibrium is restored.

when the conditions for any one of the three types of equilibrium are not satisfied, a system is said to be in a non-equilibrium state.



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