Two objects, in thermal contact, are said to be in thermal equilibrium with each other if there is no net heat flow between them.
- In thermal contact, the hotter body becomes cooler while the cooler body becomes hotter until a point is reached where no more change occurs.
- The two objects are said to be at the same temperature if they are in thermal equilibrium.
Note: Objects does not have to be touching one another to be in thermal contact! Two objects are said to be in thermal contact when they can exchange heat energy between them. For instance, Earth is in thermal contact with the Sun, even though the Earth is obviously not touching the surface of the Sun. Hence, real systems (not idealised) are always in thermal contact.
Thermal contact does not mean thermal equilibrium. The Earth is in thermal contact with the Sun, but is definitely not in thermal equilibrium.