I'm trying to understand the Joule-Thomson effect in a more qualitative way. Here's my attempt at an explanation: Real gases experience intermolecular forces. If we expand a gas whose attractive interactions dominate, it'll cool down. This is due to the potential energy increasing and the kinetic energy decreasing. If we do the same with a gas whose repulsive interactions dominate, it'll heat up. This is due to the kinetic energy increasing and potential energy decreasing. This is where I'm kinda stuck. Why exactly does the kinetic energy increase? I know potential energy is a function of separation (I'm assuming these are just coloumbic attractions), but what about the kinetic energy? Does expansion cause the molecules to speed up?