Thermal Equilibrium

Two bodies are said to be in thermal equilibrium with each other when they are at the same temperature, which then does not change with time -- and once thermal equilibrium is reached, there is no further net flow of heat between them. This is illustrated by a hot cup of coffee left on a table: heat flows from the coffee into the surrounding air until eventually the coffee cools down to exactly the same temperature as the room, after which no more net heat flows either way -- the coffee and the surrounding air have reached thermal equilibrium with each other. In a state of full thermodynamic equilibrium more generally, the macroscopic variables of a system (pressure, volume, temperature) settle to fixed values and stop c …