Mean free path

Mean free path is the average distance a particle or wave travels between interactions, such as collisions or scattering events. It characterizes how frequently those interactions interrupt propagation in a medium.

Collision cross section: An effective area that quantifies the likelihood of an interaction between particles. It determines how readily moving particles collide and therefore how far they travel between collisions.
Kinetic theory of gases: A theory that explains gas properties through the motion and collisions of microscopic particles. Mean free path connects molecular collision rates to macroscopic gas behavior.
Molecular collision: An encounter between molecules that changes their velocities or internal states. Such encounters are the interactions whose spacing the gas mean free path averages.
Hydrodynamic limit: A regime in which a material's behavior is described by continuum fields rather than individual particle trajectories. This approximation requires system scales to greatly exceed molecular mean free paths.
Number density: The number of particles per unit volume. It sets how many potential collision partners occupy each unit of a gas.
Gas viscosity: A gas's resistance to flow, arising partly from momentum transfer between molecules. Molecules carry momentum across layers over distances related to their mean free path.
Random walk: A path formed by successive steps whose directions or sizes are random. Repeated free flights and collisions create random-walk transport through matter.
Diffusion length: A characteristic distance over which diffusion spreads a quantity before removal or reaction. It describes net spreading over many steps, not the distance between individual interactions.
Exponential distribution: A probability distribution describing waiting times or distances between independent events occurring at a constant rate. In a uniform medium, free-flight distances commonly follow this distribution.
Thermal conductivity of gases: The transfer of heat through a gas by molecular motion and collisions. Molecules transport energy across temperature gradients during their free flights.

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