Thermodynamics: Thermodynamic Equilibrium, State, Property, Process, Cycle, Reversibility, Quasi static process, Irreversible Process, Causes of Irreversibility, Heat, Work, Point and Path functions

Thermodynamic Equilibrium

A thermodynamic system is said to exist in a 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 the course of time a state of thermodynamic equilibrium and can never depart from it spontaneously.

Therefore, there can be no spontaneous change in any macroscopic property if the system exists in an equilibrium state. A thermodynamic system will be in a state of thermodynamic equilibrium if the system is the state of Mechanical equilibrium, Chemical equilibrium and Thermal equilibrium.

Mechanical equilibrium 

The criteria for Mechanical equilibrium are the equality of pressures.

Chemical equilibrium

The criteria for Chemical equilibrium are the equality of chemical potentials.

Thermal equilibrium

The criterion for Thermal equilibrium is the equality of temperatures.


The thermodynamic state of a system is defined by specifying values of a set of measurable properties sufficient to determine all other properties. For fluid systems, typical properties are pressure, volume and temperature. More complex systems may require the specification of more unusual properties. As an example, the state of an electric battery requires the specification of the amount of electric charge it contains.


Properties may be extensive or intensive.

Intensive properties

The properties which are independent of the mass of thesystem.

For example: Temperature, pressure and density are the intensive properties.

Extensive properties

The properties which depend on the size or extent of the system are called extensive properties.

For example: Total mass, total volume and total momentum.


When the system undergoes change from one thermodynamic state to final state due change in properties like temperature, pressure, volume etc, the system is said to have undergone thermodynamic process.

Various types of thermodynamic processes are: isothermal process, adiabatic process, isochoric process, isobaric process and reversible process.


Thermodynamic cycle refers to any closed system that undergoes various changes due to temperature, pressure, and volume, however, its final and initial state are equal. This cycle is important as it allows for the continuous process of a moving piston seen in heat engines and the expansion/compression of the working fluid in refrigerators, for example. Without the cyclical process, a car wouldn’t be able to continuously move when fuel is added, or a refrigerator would not be able to stay cold. Visually, any thermodynamic cycle will appear as a closed loop on a pressure volume diagram.

Examples: Otto cycle, Diesel Cycle, Brayton Cycle etc.


Reversibility, in thermodynamics, a characteristic of certain processes (changes of a system from an initial state to a final state spontaneously or as a result of interactions with other systems) that can be reversed, and the system restored to its initial state, without leaving net effects in any of the systems involved.

An example of a reversible process would be a single swing of a frictionless pendulum from one of its extreme positions to the other. The swing of a real pendulum is irreversible because a small amount of the mechanical energy of the pendulum would be expended in performing work against frictional forces, and restoration of the pendulum to its exact starting position would require the supply of an equivalent amount of energy from a second system, such as a compressed spring in which an irreversible change of state would occur.

Quasi static process

When a process is processing in such a way that system will be remained infinitesimally close with equilibrium state at each time, such process will be termed as quasi static process or quasi equilibrium process.

In simple words, we can say that if system is going under a thermodynamic process through succession of thermodynamic states and each state is equilibrium state then the process will be termed as quasi static process.

Quasi static process
Quasi static process

We will see one example for understanding the quasi static process, but let us consider one simple example for better understanding of quasi static process. If a person is coming down from roof to ground floor with the help of ladder steps then it could be considered as quasi static process. But if he jumps from roof to ground floor then it will not be a quasi static process.

Weight placed over the piston is just balancing the force which is exerted in upward direction by gas. If we remove the weight from the piston, system will have unbalanced force and piston will move in upward direction due to force acting over the piston in upward direction by the gas.

Irreversible Process

The irreversible process is also called the natural process because all the processes occurring in nature are irreversible processes. The natural process occurs due to the finite gradient between the two states of the system. For instance, heat flow between two bodies occurs due to the temperature gradient between the two bodies; this is in fact the natural flow of heat. Similarly, water flows from high level to low level, current moves from high potential to low potential, etc.

In the irreversible process the initial state of the system and surroundings cannot be restored from the final state.

During the irreversible process the various states of the system on the path of change from initial state to final state are not in equilibrium with each other.

During the irreversible process the entropy of the system increases decisively and it cannot be reduced back to its initial value.

The phenomenon of a system undergoing irreversible process is called as irreversibility.

Causes of Irreversibility

Friction: Friction is invariably present in real systems. It causes irreversibility in the process as work done does not show an equivalent rise in the kinetic or potential energy of the system. The fraction of energy wasted due to frictional effects leads to deviation from reversible states.

Free expansion: Free expansion refers to the expansion of unresisted type such as expansion in a vacuum. During this unresisted expansion the work interaction is zero, and without the expense of any work, it is not possible to restore initial states. Thus, free expansion is irreversible.

Heat transfer through a finite temperature difference: Heat transfer occurs only when there exist temperature difference between bodies undergoing heat transfer. During heat transfer, if heat addition is carried out in a finite number of steps then after every step the new state shall be a non-equilibrium state.

Non-equilibrium during the process: Irreversibilities are introduced due to lack of thermodynamic equilibrium during the process. Non-equilibrium may be due to mechanical inequilibrium, chemical inequilibrium, thermal inequilibrium, electrical inequilibrium, etc. and irreversibility is called mechanical irreversibility, chemical irreversibility, thermal irreversibility, electrical irreversibility respectively. Factors discussed above are also causing non-equilibrium during the process and therefore make process irreversible.


It is the energy in transition between the system and the surroundings by virtue of the difference in temperature Heat is energy transferred from one system to another solely by reason of a temperature difference between the systems. Heat exists only as it crosses the boundary of a system and the direction of heat transfer is from higher temperature to lower temperature. For thermodynamics sign convention, heat transferred to a system is positive; Heat transferred from a system is negative.


Thermodynamic definition of work

Positive work is done by a system when the sole effect external to the system could be reduced to the rise of a weight.

Work done by the system is positive and work done on the system is negative.

Types of work interaction

Expansion and compression work (displacement work)

Work of a reversible chemical cell

Work in stretching of a liquid surface

Work done on elastic solids

Work of polarization and magnetization

Point and Path functions

Point function

Point function does not depend on the history (or path) of the system. It only depends on the state of the system.

Examples of point functions are

temperature, pressure, density, mass, volume, enthalpy, entropy, internal energy etc.

Path function

Path function depends on history of the system (or path by which system arrived at a given state).

Examples for path functions are work and heat.

Path functions are not properties of the system, while point functions are properties of the system.

Change in point function can be obtained by from the initial and final values of the function, whereas path has to defined in order to evaluate path functions.

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