Why energy flows from one system to another due to difference in temperature but not internal energy? The energy of the atoms of a system is only directly proportional to temperature. Same temperature doesnt mean same internal energy. However, I believed tgat heat is transferred through collisions, in which the atoms exchange their velocities.Due to random motion of atoms, the two systems would eventually have the same average kinetic energy. But same average kinetic energy doesnt mean same temperature,right? Then, why is the temperature of the two system the same eventually?