I read on 27 chapter from Blundell's Thermal Physics 2nd edition. For ideal gases, Joule expansion doesn't cool gases. But for real gases, Joule expansion makes cooling effects. And this book(page 314) says that when gases expand, their potential energy of molecular interactions increases(since V is proprtional to 1/r). And this energy is exactly from kinetic energy of gas.(So makes kinetic energy of gas decrease) What does the last sentence mean? Why does temperature decrease when kinetic energy of gas decrease? I understand that low temperature makes average kinetic energy decrease(because of boltzmann distribution, low temperature makes low average kinetic energy).. Or... Is temperature defined by boltzmann distribution? If a system has energy distribution of a temperature T, then does it said that the temperature of that system T? I am sorry for this absurd question.