Consider a certain amount of gas in a cylindrical container. The volume (V) can be measured by
measuring the diameter and the height of the cylinder. The pressure (P) of the gas can be
measured by a pressure gauge. The temperature (T) of the gas can be measured using a
thermometer. The state of the gas can be specified by the measured P, V and T . The values of
these variables are space averaged characteristics of the properties of the gas under
consideration. In classical thermodynamics, we often use this macroscopic approach. The
macroscopic approach has the following features.
The structure of the matter is not considered.
A few variables are used to describe the state of the matter under consideration.
The values of these variables are measurable following the available techniques of
On the other hand, the gas can be considered as assemblage of a large number of particles each
of which moves randomly with independent velocity. The state of each particle can be specified
in terms of position coordinates ( xi , yi , zi ) and the momentum components ( pxi , pyi , pzi ). If we
consider a gas occupying a volume of 1 cm3 at ambient temperature and pressure, the number of
particles present in it is of the order of 1020 . The same number of position coordinates and
momentum components are needed to specify the state of the gas. The microscopic approach can
be summarized as:
A knowledge of the molecular structure of matter under consideration is essential.
A large number of variables are needed for a complete specification of the state of the
SI is the abbreviation of Système International d' Unités. The SI units for mass, length, time and
force are kilogram, meter, second and newton respectively. The unit of length is meter, m,
1 650 763.73 wavelengths in vacuum of the radiation corresponding to the orange-red line of the
spectrum of Krypton-86. The unit of time is second, s. The second is defined as the duration of
9 192 631 770 cycles of the radiation associated with a specified transition of the Cesium 133
atom. The unit of mass is kilogram, kg. It is equal to the mass of a particular cylinder of
platinum-iridium alloy kept at the International Bureau of Weights and Measures. The amount of
substance can also be expressed in terms of the mole (mol). One kilomole of a substance is the
amount of that substance in kilograms numerically equal to its molecular weight. The number of