Editing Kinetic theory of gases (section)

{{Short description|Understanding of gas properties in terms of molecular motion}}
[[Image:Translational motion.gif|thumb|upright=1.4|The [[temperature]] of the [[ideal gas]] is proportional to the average [[kinetic energy]] of its particles. The [[Bohr radius|size]] of [[helium]] atoms relative to their spacing is shown to scale under 1,950 [[Atmosphere (unit)|atmospheres]] of pressure. The atoms have an average speed relative to their size slowed down here two [[1000000000000 (number)|trillion]] fold from that at room temperature.]]

The '''kinetic theory of gases''' is a simple [[classical mechanics|classical]] model of the [[thermodynamics|thermodynamic]] behavior of [[gas]]es. Its introduction allowed many principal concepts of thermodynamics to be established. It treats a gas as composed of numerous particles, too small to be seen with a microscope, in constant, random motion.<!-- not Brownian motion, so don't link to it --> These particles are now known to be the [[atom]]s or [[molecule]]s of the gas. The kinetic theory of gases uses their collisions with each other and with the walls of their container to explain [[Ideal gas law#Combined gas law|the relationship]] between the [[macroscopic scale|macroscopic]] properties of gases, such as [[volume]], [[pressure]], and [[temperature]], as well as [[Transport phenomena|transport properties]] such as [[viscosity]], [[thermal conductivity]] and [[mass diffusivity]].

The basic version of the model describes an [[ideal gas]]. It treats the collisions as [[elastic collision|perfectly elastic]] and as the only interaction between the particles, which are additionally assumed to be much smaller than their average distance apart.

Due to the [[time reversibility]] of microscopic dynamics ([[microscopic reversibility]]), the kinetic theory is also connected to the principle of [[detailed balance]], in terms of the [[fluctuation-dissipation theorem]] (for [[Brownian motion]]) and the [[Onsager reciprocal relations]].

The theory was historically significant as the first explicit exercise of the ideas of [[statistical mechanics]].