Thermodynamics - Thermodynamic Equilibrium Thermodynamic Process And It’s Property


Thermodynamics is a branch of science which deals with the relationship between heat and mechanical energy.Scientist Joule proves that mechanical energy can be converted into heat energy.But James watt proved that heat can be converted into mechanical energy. James watt also invented the first steam energy.The relation between heat and mechanical energy is mainly governed by the laws of thermodynamics.When a substance changes from one state to another in the process energy transformation may take place.



A system may contain a substance in a given state. It means that the inner properties are same throughout the entire system and if there is no tendency to change the microscopic properties or inner properties of the system even when the system is isolated from surroundings then it is considered the Thermodynamic equilibrium.

The thermodynamic equilibrium in term equilibrium means, equality of all potential difference, equality of force etc. for which the system can be changed from its original state. Assume two systems are in the same state and they are in equilibrium with each other. Now these systems are changed or vice-versa then surroundings of the systems are changed which means a potential difference of two systems is different or external force acts on the system etc. So, a system is in thermodynamic equilibrium if it can't be changed from its original state. For complete thermodynamic Equilibrium following three types of condition must be satisfied:
 1. Thermal Equilibrium
 2. Mechanical Equilibrium
 3. Chemical Equilibrium

1. Thermal Equilibrium:

Thermal Equilibrium means there is no change of any property of the system. Here property of system contact with its surrounding's Diathermal wall. In Diathermal wall Heat can't flow .So, when a system touch with its surroundings by a diathermal wall and if there is no spontaneous change of its properties then the system exist in thermal equilibrium.

2. Mechanical Equilibrium:

When any unbalanced force is absent in the system then it is called Mechanical Equilibrium. It means unbalanced force is absent force is totally absent in the system or between two systems and its surroundings. If a system and its surroundings exist mechanical equilibrium then its pressure must be same. But pressure can't be the same throughout the system of its gravitational force.

3. Chemical Equilibrium:

A system can't undergo a chemical reaction called chemically equilibrium system. It means there is no chemical reaction or chemical transfer from one part to another.

Any of the above three conditions of equilibrium are not satisfied in any system called non-equilibrium state. In this state, unbalanced force acts between the systems and its surroundings or unbalanced pressure will be different or some chemical reaction may exist between the system and system can't hold its original state.


A thermodynamic system is a region in space containing a quantity of matter whose behavior is being investigated. The actual or hypothetical envelop enclosing the system is the boundary of the system. The region outside the system is called the surrounding.

thermodynamic system

There are three types of systems.See the above fig.
 1. Closed System
 2. Open system
 3. Isolated system

1. Closed System:

Any system in which mass does not cause the system boundary but energy may cross the boundary is called closed system.

2. Open System:

Any system in which both mass and energy may cross the system boundary is called open system.

3. Isolated system :

Any system in which neither mass nor energy cross the system boundary is called isolated system.


Any system's property indicates system's state. System is a comprises or mixture of a substance. System's properties depend on system's state not how the state has been reached .A system holds many properties. Some properties like shape, color etc may not be relevant. A system refers to thermodynamic properties which mean the interaction of energy in the form of heat and work between a system and surroundings. System holds some properties. Some systems are directly measurable thermodynamic properties, Such as specific volume (volume per unit mass), pressure and temperature etc. These represent the outer description of the system. Some properties depend on the existence of molecules, their position, motion, collision etc. and these properties only be calculated. The Thermodynamic property may be divided into following two classes.
 1. Intensive Property
 2. Extensive Property

1. Intensive Property

Any thermodynamic property of a system that is independent of the system mass is called Intensive Property such as pressure,temperature,density,velocity etc.

2. Extensive property

Any thermodynamic property of a system that dependents on the mass of the system is called Extensive Property such as mass,volume,total energy etc.
Most extensive properties can be converted into intensive properties by dividing the extensive property by the system mass.This intensive property created in this way is called specific property.

 Fundamental property

The property which can be measured directly is called fundamental property such as pressure ,temperature etc.

 Thermodynamic Property

The property which can not be measured directly like work,heat etc.


The word state refers to the condition of the system as described by its property.


Any change that a system undergoes from one equilibrium state to another equilibrium state is called a process.The series of state through which a system passes during a process is called path of process.The process may be 1.Constant pressure process 2.Constant volume process3.Constant temperature process(Isothermal Process)4.Constant entropy Process(adiabatic process). The process may be divided another two categories.
 1. Cyclic Process
 2. Open Process

1. Thermodynamic process or Cyclic Process

If the number of processes in sequence bring back the system to its initial state i.e. the initial and final states of the system are the same.

thermodynamics process or cyclic process

In the figure 1-A-2 and 2-B-1 are processes where as 1-A-2-B-1 is a thermodynamic cycle or cyclic process. Cyclic process is considered reversible process if it produces no change anywhere in the universe when traced in the reverse order.Isothermal and adiabatic process is a reversible process.If therer is a change,either in the system or in the universe due to the process,then it is called irreversible process.All natural process produce changes is called irreversible process.Constant pressure,constant volume, and polytropic process are irreversible process.

2. Open system

In case of open process initial and final state point is not the same and the work done by a thermodynamic system can not be resolved.

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