# Kinetic gas theory

The kinetic gas theory (formerly also dynamic gas theory ) is a branch of statistical mechanics .

The kinetic gas theory explains the properties of gases , in particular the gas laws , by the idea that gases consist of a very large number of small particles ( atoms or molecules ) that are in constant motion ( Greek κίνησις kínesis "movement"). The theory leads to a microscopic explanation of the properties of temperature and heat , which in thermodynamics are defined by their macroscopic properties.

As early as the 17th century , physicists like Francis Bacon suspected that heat was a form of motion. The first to come up with a more complete theory was Daniel Bernoulli in 1738. a. Mikhail Wassiljewitsch Lomonossow , Georges-Louis Le Sage , John Herapath and John James Waterston , but their considerations were largely ignored. It was not until 1860 that the kinetic gas theory found wider recognition through the work of physicists such as Rudolf Clausius , James Clerk Maxwell and Ludwig Boltzmann . At the same time, however, the kinetic gas theory was also heavily disputed, even into the 20th century and the like. a. by Ernst Mach and Wilhelm Ostwald , since it depends entirely on the existence of atoms or molecules, which was then regarded as a hypothesis.

The most important basic assumptions of the theory are:

1. The particles of a gas ( atoms , molecules ) are of negligible size and are constantly in disorderly but statistically tangible motion.
2. Between their collisions , they move uniformly and independently of each other, with no preference for any direction.
3. The particles do not exert any forces on one another as long as they do not touch one another.
4. Collisions of the particles with one another and with the vessel wall obey the law of elastic collision . Only two particles are involved in the collisions.

From these assumptions, the kinetic gas theory develops formulas that predict the parameters pressure , specific heat , speed of sound , diffusion , heat conduction and internal friction for an ideal gas . The formulas reproduce the observations on many real gases well and led, for example, to the first determinations of the size, number and mass of atoms or molecules. Through additional additions to assumptions no. 3 and 4, the deviating behavior of real gases was also included in the kinetic gas theory, as is the case e.g. B. is described in the van der Waals equation of state .