Kinetic Theory

Class 11 Physics · Physics Part II · Chapter 5

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Summary

Kinetic theory explains the behaviour of gases by treating them as large numbers of tiny molecules in rapid, random motion, with intermolecular forces negligible except during brief elastic collisions. Matter is made of atoms about an angstrom in size; in gases the molecules are far apart, so the mean free path is large. Gases at low pressure and high temperature obey the ideal-gas equation \(PV=\mu RT=k_B NT\), which embodies Boyle's law, Charles' law, Avogadro's hypothesis and Dalton's law of partial pressures. From the assumption of elastic molecular collisions with the walls, the pressure of an ideal gas is derived as \(P=\tfrac13 nm\overline{v^{2}}\), leading to the kinetic interpretation of temperature: the average translational kinetic energy of a molecule is \(\tfrac32 k_B T\), independent of the gas, and the root-mean-square speed is \(v_{rms}=\sqrt{3k_B T/m}\). The law of equipartition of energy assigns \(\tfrac12 k_B T\) to each degree of freedom, explaining the molar specific heats of monatomic, diatomic and polyatomic gases and of solids. The mean free path \(l=1/(\sqrt2\,\pi n d^{2})\) is the average distance a molecule travels between collisions and accounts for diffusion, viscosity and conduction in gases.

Molecular nature of matterBehaviour of gases and the ideal-gas equationKinetic theory of an ideal gas and pressureKinetic interpretation of temperature and rms speedLaw of equipartition of energySpecific heat capacity of gases and solidsMean free path

Key terms

Ideal gas
A theoretical gas that obeys \(PV=\mu RT\) exactly at all pressures and temperatures; real gases approach it at low pressure and high temperature.
Boltzmann constant
The constant \(k_B=R/N_A=1.38\times10^{-23}\ \text{J K}^{-1}\) linking molecular energy to temperature.
Root-mean-square speed
The square root of the mean of the squared molecular speeds, \(v_{rms}=\sqrt{3k_B T/m}\).
Degree of freedom
An independent way in which a molecule can possess energy — translational, rotational or vibrational.
Law of equipartition of energy
In equilibrium, energy is shared equally among all degrees of freedom, each contributing \(\tfrac12 k_B T\).
Mean free path
The average distance a molecule travels between successive collisions, \(l=1/(\sqrt2\,\pi n d^{2})\).

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In kinetic theory, collisions between gas molecules are assumed to be:
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