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NIU PHYS 210 - Diffusion

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DiffusionAtomic CollisionMean Free PathRandom WalkConcentrated RegionDiffusion ConstantFick’s LawDiffusionDiffusionAtomic CollisionAtomic CollisionMolecules in a gas make collisions with each other as Molecules in a gas make collisions with each other as well as the wall.well as the wall.There is an average time and average distance There is an average time and average distance between collisions.between collisions.Mean Free PathMean Free PathA particle of radius A particle of radius rr must must avoid other particles so it avoid other particles so it traces a cylinder of radius 2traces a cylinder of radius 2rr..The length of the cylinder is The length of the cylinder is the speed times the time.the speed times the time.There are N/V particles in There are N/V particles in the volume, but they move the volume, but they move reducing the path.reducing the path.The combination gives the The combination gives the mean free pathmean free path..)/(2414)/(24)2(2222VNrtvrVNtvNLtvrVtvLrARandom WalkRandom WalkA molecule will move in A molecule will move in random directions after each random directions after each collision.collision.If it starts in one place it will If it starts in one place it will eventually move away.eventually move away.The motion is called a The motion is called a random walk.random walk.Concentrated RegionConcentrated RegionMolecules in a concentrated Molecules in a concentrated region will disperse into the region will disperse into the rest of the medium. rest of the medium. The difference in The difference in concentrations is the concentrations is the concentration gradient.concentration gradient.xCCxC21C1xC2Diffusion ConstantDiffusion ConstantDifferent materials pass Different materials pass through an area at different through an area at different rates.rates.•Depends on concentrateDepends on concentrate•Depends on mediumDepends on mediumC1xC2ADO2 in air: D = 1.8 x 10-5 m2/sO2 in water: D = 1.0 x 10-9 m2/sDNA in water: D = 1.3 x 10-12 m2/sFick’s LawFick’s LawThe rate of diffusion depends on the concentration The rate of diffusion depends on the concentration gradient, area, and diffusion constant. gradient, area, and diffusion constant. JJ measures mol/m measures mol/m33..There is a diffusion time There is a diffusion time tt for diffusion in one direction for diffusion in one direction over a distance over a distance


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NIU PHYS 210 - Diffusion

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