Thermodynamics : It is a branch of science which deals with exchange of heat energy between bodies and conversion of the heat energy into mechanical energy and vice-versa.

Temperature

If we have a number of gaseous system, whose different states are represented by their volumes and pressures V₁, V₂, V₃…and P₁, P₂, P₃….etc..in thermal equilibrium with one another, we will have φ₁(P₁. V₁) = φ₂(P₂, V₂) = φ₃(P₃, V₃) and so on, where φ is a function of P and V. Hence, despite their different parameters of P and V, the numerical value of the these functions or the temperature of these system is same.

Temperature may be defined as the particular property which determines whether a system is in thermal equilibrium or not with its neighbouring system when they are brought into contact.

Thermodynamic System : A collection of an extremely large number of atoms or molecules confined with in certain boundaries such that it has a certain value of pressure, volume and temperature is called a thermodynamic system. Anything outside the thermodynamic system to which energy or matter is exchanged is called its surroundings.

Thermodynamic system may be of three types

(i) Open system : It exchange both energy and matter with the surrounding.

(ii) Closed system : It exchange only energy (not matter) with the surroundings.

(iii) Isolated system : It exchange neither energy nor matter with the surrounding.

Thermodynamic variables and equation of state : A thermodynamic system can be described by specifying its pressure, volume, temperature, internal energy and the number of moles. These parameters are called thermodynamic variables.

The relation between the thermodynamic variables (P, V, T) of the system is called equation of state.

For m moles of an ideal gas, equation of state is PV = mRT and for 1 mole of an it ideal gas is PV = RT

For m moles of a real gas, equation of state is  and for 1 mole of a real gas it is

Thermodynamic equilibrium : When the thermodynamic variables attain a steady value i.e. they are independent of time, the system is said to be in the state of thermodynamic equilibrium. For a system to be in thermodynamic equilibrium, the following conditions must be fulfilled.

Thermodynamic process : The process of change of state of a system involves change of thermodynamic variables such as pressure P, volume V and temperature T of the system. The process is known as thermodynamic process. Some important processes are

(i) Isothermal process     (ii) Adiabatic process     (iii) Isobaric process      (iv) Isochoric (isovolumic process)        (v) Cyclic and non-cyclic process           (vi) Reversible and irreversible process