thermodynamics A branch of Physics that deals with transformation of heat into other forms of energy, mainly mechanical, and vice versa. In thermodynamics we deal with variables that characterize the internal state of the system but are independent of its microstructure. The variables such as pressure, volume, temperature, internal energy, entropy, etc. are macroscopic, that is they characterize certain property of all the molecules of the system.

The starting point of thermodynamics is the empirical laws or postulates.

Zeroth law of thermodynamics: Two bodies A and B placed in contact with each other are said to be in thermal equilibrium if no transfer of heat takes place between them. When this happens the bodies are said to be in same temperature. The zeroth law of thermodynamics states that

‘There exists a quantity called temperature which is property of all thermodynamical systems such that if two bodies are at the same temperature they must be in thermal equilibrium. Further if two systems are separately in thermal equilibrium with a third system, then the two systems must be mutually in thermal equilibrium’.

First law of thermodynamics: It is the principle of conservation of energy applied to thermodynamical systems. It can be stated as,

‘The increase in internal energy (D U) of a system is equal to difference of heat added to the system (D Q) and the work done by the system’.

D U = D Q - D W (t4)

The internal energy U is the total energy content of the system, that is sum of kinetic, potential, chemical, electrical, nuclear and all other forms of energy possessed by atoms and molecules of the system.

Second law of thermodynamics: The first law of thermodynamics does not exclude many processes which otherwise do not occur in nature.For example the heat from an ice cube does not flow into the environment which is at higher temperature. Mechanical work and heat are only different forms of energy. Mechanical energy can be fully converted into heat, but it is not possible to convert heat into work completely. The second law of thermodynamics deals with such questions. It is stated as,

‘It is impossible for a self acting machine, unaided by any external agency, to transfer heat from one body to another at a higher temperature’ (Clausius statement).

‘It is impossible to construct a heat engine which operating in a complete cycle will draw heat from a reservoir and convert the whole of it into work’ (Kelvin Planck statement).

It can be shown that the two statements are equivalent.

Third law of thermodynamics: The fundamental feature of all cooling processes is that the lower the temperature is achieved, the more difficult it is to farther lower the temperature. The unattainability of absolute zero is the third law of thermodynamics. It is stated as,

‘By no finite series of processes is the absolute zero attainable.'

An alternative statement of the third law is,

‘The entropy change associated with isothermal reversible process of a condensed system approaches zero as the temperature approaches zero.’

This statement is also called the Nernst heat theorem. See also enthalpy, entropy, Gibbs function, Helmholtz function.