14.1 Thermal equilibrium

When two regions have different temperatures, thermal energy is transferred from the hotter region to the colder until equilibrium is reached.

Temperature indicates the direction of net thermal transfer; the mechanism may be conduction, convection or radiation.

A hot metal block in cooler water loses thermal energy while the water gains it, even if the block contains less total energy.

Heat is energy in transfer, not a substance stored in an object, and transfer direction is not set simply by which object has more energy.

Two systems are in thermal equilibrium when their temperatures are equal, so there is no net transfer of thermal energy between them in contact.

Microscopic exchanges may still occur, but they balance on average. Thermal equilibrium is not the same as identical internal energy.

A thermometer left long enough in a liquid reaches the liquid’s temperature and is then in thermal equilibrium with it.

Equal temperature does not mean equal mass, energy content or particle number.