The conductor becomes very hot as the current passes through it. The effect is used in various heaters. (Source: © Nivellen77 / stock.adobe.com)

The process by which an electric current passing through a conductor generates heat. The more resistance the conductor presents, the higher the heat. The electric potential between the beginning and end of the conductor accelerates charged particles (e.g. electrons). As the charged particles travel through the conductor, they collide with the conductor particles and transfer energy to them. The kinetic energy of the charge carriers is thus converted into thermal energy. The Joule heating effect is used in many types of heaters. In a tokamak, Joule heat is used for the initial plasma heating. This is because the partially ionized gas conducts current poorly and then heats up, but as ionization proceeds, its resistance decreases and once the plasma is sufficiently ionized, Joule heating is no longer effective and other, additional heating is needed to reach fusion temperatures.