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Diffusion
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Atomic Collision Molecules in a gas make collisions with each other as well as the wall. There is an average time and average distance between collisions.
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Mean Free Path A particle of radius r must avoid other particles so it traces a cylinder of radius 2r. The length of the cylinder is the speed times the time. There are N/V particles in the volume, but they move reducing the path. The combination gives the mean free path.
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Random Walk A molecule will move in random directions after each collision. If it starts in one place it will eventually move away. The motion is called a random walk.
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Concentrated Region Molecules in a concentrated region will disperse into the rest of the medium. The difference in concentrations is the concentration gradient. C1C1 xx C2C2
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Diffusion Constant Different materials pass through an area at different rates. Depends on concentrateDepends on concentrate Depends on mediumDepends on medium C1C1 xx C2C2 A D O 2 in air: D = 1.8 x 10 -5 m 2 /s O 2 in water: D = 1.0 x 10 -9 m 2 /s DNA in water: D = 1.3 x 10 -12 m 2 /s
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Fick’s Law The rate of diffusion depends on the concentration gradient, area, and diffusion constant. J measures mol/m 3. There is a diffusion time t for diffusion in one direction over a distance x.
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