Unformatted text preview: 32 Physics Formulary by ir. J.C.A. Wevers 7.5 Collisions between molecules The collision probability of a particle in a gas that is translated over a distance dx is given by n σ dx , where σ is the cross section . The mean free path is given by = v 1 nu σ with u = v 2 1 + v 2 2 the relative velocity between the particles. If m 1 m 2 holds: u v 1 = 1 + m 1 m 2 , so = 1 n σ . If m 1 = m 2 holds: = 1 n σ √ 2 . This means that the average time between two collisions is given by τ = 1 n σ v . If the molecules are approximated by hard spheres the cross section is: σ = 1 4 π ( D 2 1 + D 2 2 ) . The average distance between two molecules is . 55 n 1 / 3 . Collisions between molecules and small particles in a solution result in the Brownian motion . For the average motion of a particle with radius R can be derived: x 2 i = 1 3 r 2 = kTt/ 3 πη R . A gas is called a Knudsen gas if the dimensions of the gas, something that can easily occur at low pressures. The equilibrium condition for a vessel which has a hole with surfacepressures....
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 Spring '10
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 Physics, Atom, Force, Particle, Van der Waals force

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