2. Various forms of thermal energy transfer

Thermal energy originates mainly from the vibration of atoms and electrons. It is transmitted from one point in space to another in a variety of ways.

• Atoms interact with their close neighbors and can thus propagate energy by conduction. In the case of solids, where atoms are organized in networks, the collective vibration thus generated is translated by the notion of "phonon", which is the particle carrying conduction energy. The weaker the bonding forces between atoms, the weaker the atomic conduction. For example, conduction through a gas is weaker than through a liquid or solid. In the case of metals, where electrons are delocalized, part of the thermal energy is transferred by conduction through the "electron gas".

• What's more, every body is a source of radiation, and the higher...