Introduction The Nature of Heat and Kinetic Theory

According to the kinetic theory of gases, heat is a form of energy, i.e., the energy of motion of the molecules which constitute a gas and which are in constant agitation and colliding with one another and the walls in a completely random fashion in a closed system. In other words, it is the energy of the disorganized motion of the molecules inside the system that is called heat. This energy is to be distinguished from the kinetic energy of the system of molecules as a whole moving in a particular direction in which the kinetic energy is perceived as that of the centre of gravity of the molecules inside the system moving in that direction. The latter, therefore, is treated as the kinetic energy of organized motion and does not generate any heat.

The distribution of velocities of molecules in a perfect gas is given by the Maxwell-Boltzmann distribution law which states that the number of molecules of mass m having speeds between c and c + dc out of a total number of n molecules per cubic centimetre is given by (see, e.g., Saha and Srivastava, 1931, page 110).

dn = 4nn (m/2nKT)3/2 c2 {exp(—mc2/2KT)}dc (3.1.1)

where k is Boltzmann's constant and T is the absolute temperature.

[For values of k and other important physical constants, see Appendix-8] The theory also predicts that the mean square velocity of the agitating molecules is directly proportional to the absolute temperature of the gas, as stated in Eq. (2.2.1).

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