Thermal equilibrium
Thermal equilibrium is the state two systems in thermal contact reach when no net energy is transferred between them by thermal processes. The condition for it is equal temperature, not equal internal energy.
Essential knowledge 9.3.A.4 words it carefully: thermal equilibrium results when no net energy is transferred by thermal processes between two systems in thermal contact with each other.
"Net" is the load-bearing word. At equilibrium the atoms keep colliding across the boundary and keep handing energy across in both directions. What has stopped is the imbalance: as much crosses one way as the other, so neither system's internal energy changes. This is a dynamic balance, not a halt.
The condition is equality of temperature. While a temperature difference exists, energy transfers spontaneously from the hotter system to the cooler one (9.3.A.3), so there is a net flow and equilibrium has not arrived.
Two distinctions settle most questions here.
- Equal temperature, not equal energy. Systems at equilibrium share a temperature and an average kinetic energy per atom. Their internal energies can differ by any factor, because internal energy also counts how many atoms there are.
- Equilibrium is not insulation. Two systems separated by a perfect insulator exchange no net energy either, but they are not in thermal contact, so 9.3.A.4 does not describe them. Open that boundary between an 800 K system and a 200 K one and energy moves at once.
It is approached rather than snapped into, since the temperature difference is what drives the transfer. Two readings still 5 K apart and still moving have not reached it.