Brownian Motion in a Classical Ideal Gas: a Microscopic Approach to Langevin's Equation

Details Brownian Motion in a Classical Ideal Gas: a Microscopic Approach to Langevin's Equation Brownian Motion in a Classical Ideal Gas: a Microscopic Approach to Langevin's Equation

2002-07-10

arxiv.org/abs/cond-mat/0207246v1

Physics

Condensed Matter

Statistical Mechanics

7 pages revtex4 3 eps figures included using psfig

Scientific paper

We present an insightful ``derivation'' of the Langevin equation and the fluctuation dissipation theorem in the specific context of a heavier particle moving through an ideal gas of much lighter particles. The Newton's Law of motion ($m{\ddot x}=F$) for the heavy particle reduces to a Langevin equation (valid on a coarser time scale) with the assumption that the lighter gas particles follow a Boltzmann velocity distribution. Starting from the kinematics of the random collisions we show that (1) the average force $ \propto -{\dot x}$ and (2) the correlation function of the fluctuating force $\eta= F-< F>$ is related to the strength of the average force.

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