Kinetic theory

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The kinetic theory of gases describes a gas as a large number of small particles (atoms or molecules), all of which are in constant, random motion. The rapidly moving particles constantly collide with each other and with the walls of the container. Kinetic theory explains macroscopic properties of gases, such as pressure, temperature, or volume, by considering their molecular composition and motion. Essentially, the theory posits that pressure is due not to static repulsion between molecules, as was Isaac Newton's conjecture, but due to collisions between molecules moving at different velocities.

While the particles making up a gas are too small to be visible, the jittering motion of pollen grains or dust particles which can be seen under a microscope, known as Brownian motion, results directly from collisions between the particle and gas molecules. As pointed out by Albert Einstein in 1905, this experimental evidence for kinetic theory is generally seen as having confirmed the existence of atoms and molecules.



The theory for ideal gases makes the following assumptions:

  • The gas consists of very small particles, all with non-zero mass.
  • The number of molecules is so large that statistical treatment can be applied.
  • These molecules are in constant, random motion. The rapidly moving particles constantly collide with the walls of the container.
  • The collisions of gas particles with the walls of the container holding them are perfectly elastic.
  • Except during collisions the interactions among molecules are negligible (they exert no forces on one another).
  • The total volume of the individual gas molecules added up is negligible compared to the volume of the container. This is equivalent to stating that the average distance separating the gas particles is large compared to their size.
  • The molecules are perfectly spherical in shape, and elastic in nature.
  • The average kinetic energy of the gas particles depends only on the temperature of the system.
  • Relativistic effects are negligible.
  • Quantum-mechanical effects are negligible. This means that the inter-particle distance is much larger than the thermal de Broglie wavelength and the molecules are treated as classical objects.
  • The time during collision of molecule with the container's wall is negligible as comparable to the time between successive collisions.
  • The equations of motion of the molecules are time-reversible.

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