Imagine two solid iron cubes, each with a mass of 1 kilogram (1 kg). One is at 10ºC and the other at 30ºC.
If we put them in contact, we realize that over the next few minutes, their temperatures change until they reach a situation where they both have the same temperature, 20ºC.
When the two cubes have the same temperature, we say they have reached thermal equilibrium.
In general, two bodies are in thermal equilibrium when they have the same temperature. On the other hand, when the temperature of two bodies is different, they are not in thermal equilibrium. This is the case of the two iron cubes at the beginning of the experiment.
The scientific concept of heat
Why do two bodies at different temperatures reach thermal equilibrium sometime after being brought into contact?
The scientists' explanation for this event is that there is transfer of energy from the hottest to the coldest body. This is a general rule of nature: When two bodies are in contact, energy flows from the higher temperature to the lower temperature.
The energy transferred between two bodies (or parts of the same body) that have different temperatures is called heat. Heat always flows spontaneously from the hottest to the coldest body.
The process is called heat exchange (or transfer) and occurs until thermal equilibrium is established.
Heat exchange processes
Energy transfer from a warmer to a colder body can happen in three distinct ways, which we will talk about later: driving, a convection and the irradiation. In practice, heat exchange between two bodies can even involve one, two or even all three of these processes. However, it is important that you know them in order to better understand some of your day-to-day events.