Mean free path in a dilute hard-sphere gas
A collision length depends on number density and cross section.
Key knowledge
In an equilibrium dilute gas of identical hard spheres of collision diameter d, molecular motion gives mean free path λ=1/(√2πd²n), where n is number per volume. A larger density or collision diameter shortens this length. It is a model for binary collisions, distinct from mean interparticle spacing or a guaranteed distance travelled by each molecule.
Values and equations
λ=1/(√2πd²n)
- λ
- Mean molecular path between collisions m
- d
- Hard-sphere collision diameter m
- n
- Molecular number density m⁻³
Conditions and scope
- Dilute classical homogeneous gas with identical spheres, equilibrium random relative velocities and independent binary-collision approximation.
- The collision diameter is an effective model parameter; energy-dependent forces, dense liquids or different species need appropriate cross sections and distributions. A time scale estimated as λ/v_typical is only as precise as the chosen speed model.
Sources · 1
References
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Evidence and review
Read §2.2 Mean Free Path and Mean Free Time derivation2.10–2.11. Convert radius r to diameter d=2r; retain relative-motion √2 and omit the source’s approximate RMS-based collision-time expression as an exact mean time.
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Entry details
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