1. The problem statement, all variables and given/known data Consider two chambers of equal volume separated by an insulating wall and containing an ideal gas, maintained at temperatures T1 = 225K and T2 = 400K. Initially the two chambers are connected by a long tube whose diameter is much larger than the mean free path in either chamber and equilibrium is established (while maintaining T1 and T2). Then the tube is removed, the chambers are sealed but a small hole is opened in the insulating wall, with diameter that is much less than the mean free path. In what direction will the gas flow through the small hole? Why? 2. Relevant equations I know that effusion rate (number of particles moving through a small hole of unit area per unit time) is p/sqrt(2πmkT) 3. The attempt at a solution So I figure that when the wide pipe is attached, the pressure in the two tanks is equal. But then when they're sealed and a small hole is punched, then the respective particle fluxes from each tank will be Φ1->2=p/sqrt(2πmkT1) and Φ2->1=p/sqrt(2πmkT2) So if the pressures are equal then the flux from the colder tank to the warmer tank will be higher? This doesn't make much sense to me. Can anybody explain to me why my calculations are wrong?