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In kinetic theory of gases, the mean free path is the average distance traveled by a molecule, or a number of molecules per volume, before they make their first collision. Let σ {\displaystyle \sigma } be the collision cross section of one molecule colliding with another.
The kinetic diameter is not the same as atomic diameter defined in terms of the size of the atom's electron shell, which is generally a lot smaller, depending on the exact definition used. Rather, it is the size of the sphere of influence that can lead to a scattering event. [1] Kinetic diameter is related to the mean free path of molecules in ...
The general equation can then be written as [6] = + + (),. where the "force" term corresponds to the forces exerted on the particles by an external influence (not by the particles themselves), the "diff" term represents the diffusion of particles, and "coll" is the collision term – accounting for the forces acting between particles in collisions.
Kinetic theory may refer to: Kinetic theory of matter: A general account of the properties of matter, including solids liquids and gases, based around the idea that heat or temperature is a manifestation of atoms and molecules in constant agitation. Kinetic theory of gases, an account of gas properties in terms of motion and interaction of ...
μ is the mobility of the particle in the fluid or gas, which can be calculated using the Einstein relation (kinetic theory). m is the mass of the particle. t is time. At long time scales, Einstein's result is recovered, but short time scales, the ballistic regime are also explained.
The motion of a particle located at position can be described by the Smoluchowski's limit of the Langevin equation: [11] [12] = + (), where is the diffusion coefficient of the particle, is the friction coefficient per unit of mass, () the force per unit of mass, and is a Brownian motion.
The kinetic theory of gases applies to the classical ideal gas, which is an idealization of real gases. In real gases, there are various effects (e.g., van der Waals interactions , vortical flow, relativistic speed limits, and quantum exchange interactions ) that can make their speed distribution different from the Maxwell–Boltzmann form.
Schematic drawing of a molecule in a cylindrical pore in the case of Knudsen diffusion; are indicated the pore diameter (d) and the free path of the particle (l).Knudsen diffusion, named after Martin Knudsen, is a means of diffusion that occurs when the scale length of a system is comparable to or smaller than the mean free path of the particles involved.