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Thermodynamic Fundamental Concepts

Thermodynamics is a branch of physics which expresses the equivalence between heat and mechanical work. In thermodynamics, we deal about macroscopic parameters like pressure, volume and temperature but do not go into detail about the microscopic parameters. The theory of thermodynamics was developed by means of experimental facts and empirical data over a long period of time.
Thermodynamics system
Any matter bounded by some closed surface is known as system. The surface is called boundary. Anything outside the system which has direct bearing with it is called surrounding. There are three types of systems:
Open system
It can exchange both mass and energy with the surroundings.
Closed system
It can exchange energy but not mass with the surroundings.
Isolated system
It cannot exchange both mass and energy with the surroundings.
Thermal and thermodynamic equilibrium
A system is in thermal equilibrium with its surroundings if the temperature withi the system is same as that of the surroundings. A system is said to be in thermodynamic equilibrium when it is in mechanical, chemical and thermal equilibrium.
Equation of state
The thermodynamic state of a system can be described by certain variables like volume, temperature and pressure. Among the three variables, two are considered independent and one is dependent variable. The equation relating these thermodynamic variables and expressing the complete behavior of thermodynamic system in equilibrium is called the euation of state. The equation of state for ideal gas is \(PV=nRT\) and for Van der Waal's gas is \((P+\frac{a}{V^2})(V-b)\) where a and b are Van der Waal's constants.
Thermodynamic Processes
Any change in the value of thermodynamic variables to have a new set of values is called a thermodynamic process. A thermodynamic process can be of following three types:
Reversible process
The process carried out in such a way that any of its effects on system and surroundings can be completely restored back to its original is called reversible process.
Irreversible process
The process carried out such that the effects of it cannot be restore back to the original condition is called irreversible process.
Quasi-static process
It is an idealized process in which the changes are so slow that every steps can be considered to be in thermodynamic equilibrium.
Isothermal process
It is a slow process in which the temperature remains constant. Eg: melting of ice into water. Its equation of state is \(PV=constant\) and the work done in such a process is given by, \(W=RT\ln(\frac{P_{1}}{P_{2}})\)
Adiabatic process
It is a rapid process in which no exchange of heat takes place. Eg: bursting of a tyre
Its equation of state is given by \(PV^\gamma=constant\) and the work done is, \(W=\frac{R}{\gamma-1}[T_{1}-T_{2}]\).
Isobaric process
It is a thermodynamical process in which the pressure remains constant throughout.
Isochoric process
It is a process in which volume remains constant. Eg: process carried out in a closed rigid vessel.
External and internal work
If work is done by the thermodynamic system or work is done on it, such work is called external work. Internal work is the work done by one part of the system to other part of the system. Internal work comes into play only when the intermolecular forces beteen the molecules are conisdered.
Internal energy
The sum of internal kinetic energy and internal potential energy constitutes the total internal energy. It is denoted by U and is given by, \(U=U_{k}+U_{p}\). This energy is not visible but is still capable of doing work.


Adhikari, Pitri Bhakta. A Textbook of Physics Volume-I. Kathmandu: Sukunda Pustak Bhawan, 2015.
Young, Hugh D, Roger A Freedman and A Lewis Ford. University Physics. Noida: Dorling Kindersley (India) Pvt. Ltd., 2014.
Singhal, SS, JP Agrawal and Satya Prakash. Heat Thermodynamics and Statistical Physics. Meerut: Pragati Prakashan, 2009.


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