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Published byElwin Wilkinson Modified over 9 years ago
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2. Brownian Motion 1.Historical Background 2.Characteristic Scales Of Brownian Motion 3.Random Walk 4.Brownian Motion, Random Force And Friction: The Langevin Equation 5.Solving The Langevin Equation: Approximations And Orders Of Magnitude 6.Applications And Implications
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2.1. Historical Background Continuum vs Atomism –Mach vs Boltzmann Einstein’s kinetic theory on Brownian Motion –atomism Fluctuation-dissipation theorem –Transport, Noises, …
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2.2. Characteristic Scales Of Brownian Motion Brownian motion: Random motion of particles (size ~ m) hovering in gas or on surface of fluid.
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Thermal energy of a gas particle at room temperature: Density of gas: Mass density of each hovering particle: Mass of hovering particle: In equilibrium: Speed: Mass of the gas molecule: Speed:
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L = mean free path of hovering particle S = l 2 = its cross section n = number density of gas = mean free time
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2.3. Random Walk Brownian motion: Major achievement of Einstein’s theory : Evaluate using kinetic theory.
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Random walk of a drunk
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Average over behavior of many drunks = Average over different starting points of one drunk ( Average over Ensemble ) ( Average over Initial Conditions ) See Ex.2.2
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