Tuesday, December 14, 2021

Introducing the Kinetic Theory of Gases

Kinetic theory of gases is a simple, historically significant classical model of the thermodynamic behavior of gases, with which many principal concepts of thermodynamics were established. The model describes a gas as a large number of identical submicroscopic particles (atoms or molecules), all of which are in constant, rapid, random motion.

Their size is assumed to be much smaller than the average distance between the particles. The particles undergo random elastic collisions between themselves and with the enclosing walls of the container. The basic version of the model describes the ideal gas, and considers no other interactions between the particles.

The kinetic theory of gases explains the macroscopic properties of gases, such as volume, pressure, and temperature, as well as transport properties such as viscosity, thermal conductivity and mass diffusivity. The model also accounts for related phenomena, such as Brownian motion.

The kinetic theory of gases deals not only with gases in thermodynamic equilibrium, but also very importantly with gases not in thermodynamic equilibrium. This means using Kinetic Theory to consider what are known as “transport properties”, such as viscosity, thermal conductivity and mass diffusivity.

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In about 50 BCE, the Roman philosopher Lucretius proposed that apparently static macroscopic bodies were composed on a small scale of rapidly moving atoms all bouncing off each other. This Epicurean atomistic point of view was rarely considered in the subsequent centuries, when Aristotlean ideas were dominant.

In 1738 Daniel Bernoulli published Hydrodynamica, which laid the basis for the kinetic theory of gases. In 1856 August Krönig created a simple gas-kinetic model, which only considered the translational motion of the particles.

In 1857 Rudolf Clausius developed a similar, but more sophisticated version of the theory, which included translational and, contrary to Krönig, also rotational and vibrational molecular motions. In this same work he introduced the concept of mean free path of a particle.

At the beginning of the 20th century, however, atoms were considered by many physicists to be purely hypothetical constructs, rather than real objects. An important turning point was Albert Einstein’s (1905) and Marian Smoluchowski’s (1906) papers on Brownian motion, which succeeded in making certain accurate quantitative predictions based on the kinetic theory.

The application of kinetic theory to ideal gases makes the following assumptions:
  • The gas consists of very small particles. This smallness of their size is such that the sum of the volume of the individual gas molecules is negligible compared to the volume of the container of the gas.
  • The number of particles is so large that a statistical treatment of the problem is well justified.
  • The rapidly moving particles constantly collide among themselves and with the walls of the container.
  • Except during collisions, the interactions among molecules are negligible.

Thus, the dynamics of particle motion can be treated classically, and the equations of motion are time-reversible.

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