# Irodov Solutions → Thermodynamics and Molecular Physics → Transport Phenomena

 2.223. Find the mean free path and the mean time interval between successive collisions of gaseous nitrogen molecules (a) under standard conditions; (b) at temperature t = 0 °C and pressure p = 1.0 nPa (such a pressure can be reached by means of contemporary vacuum pumps). Free solution >> 2.242. Under standard conditions helium fills up the space between two long coaxial cylinders. The mean radius of the cylinders is equal to R, the gap between them is equal to ΔR, with ΔR << R. The outer cylinder rotates with a fairly low angular velocity ω about the stationary inner cylinder. Find the moment of friction forces acting on a unit length of the inner cylinder. Down to what magnitude should the helium pressure be lowered (keeping the temperature constant) to decrease the sought moment of friction forces n = 10 times if ΔR = 6 mm? Free solution >> 2.243. A gas fills up the space between two long coaxial cylinders of radii R1 and R2, with R1 < R2. The outer cylinder rotates with a fairly low angular velocity ω about the stationary inner cylinder. The moment of friction forces acting on a unit length of the inner cylinder is equal to N1. Find the viscosity coefficient η of the gas taking into account that the friction force acting on a unit area of the cylindrical surface of radius r is determined by the formula σ = ηr (∂ω/∂r). Free solution >> 2.244. Two identical parallel discs have a common axis and are located at a distance h from each other. The radius of each disc is equal to a, with a >> h. One disc is rotated with a low angular velocity ω relative to the other, stationary, disc. Find the moment of friction forces acting on the stationary disc if the viscosity coefficient of the gas between the discs is equal to η. Free solution >> 2.247. One end of a rod, enclosed in a thermally insulating sheath, is kept at a temperature T1 while the other, at T2. The rod is composed of two sections whose lengths are l1 and l2 and heat conductivity coefficients χ1 and χ2. Find the temperature of the interface. Free solution >>