DiffusionAtomic CollisionMean Free PathRandom WalkConcentrated RegionDiffusion ConstantFick’s LawDiffusionDiffusionAtomic CollisionAtomic CollisionMolecules 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 PathA 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(2222VNrtvrVNtvNLtvrVtvLrARandom WalkRandom WalkA 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 RegionMolecules 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.xCCxC21C1xC2Diffusion ConstantDiffusion ConstantDifferent 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 mediumC1xC2ADO2 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 LawThe 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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