Dielectric heating

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Dielectric heating (also known as electronic heating, RF heating, high-frequency heating) is the phenomenon in which radiowave or microwave electromagnetic radiation heats a dielectric material, especially as caused by dipole rotation.

There are two principal mechanisms by which a non-conductive material can be warmed in an EM field:

1. Electrical conduction: current flow in the oscillating electric field allows the material to absorb energy as heat. Since current flow implies that the material is not an electrical insulator this is rarely considered true dielectric heating.
2. Dipole rotation: Molecular rotation occurs in materials containing polar molecules having an electrical dipole moment, which will align themselves in the field by rotating in place; as the field alternates the molecules reverse direction, and the successive rotations causes heat through friction at the molecular level.

Dipole rotation is the mechanism normally referred to as dielectric heating, and is most widely observable in the microwave oven where it operates most efficiently on liquid water, and much less so on fats, sugars, and frozen water. The reason is that fats and sugars are far less polar than water molecules, and are thus less affected by the electromagnetic forces generated by the alternating current. Meanwhile, frozen water molecules are fixed in place in a crystal lattice, and cannot freely rotate and absorb heat from molecular friction. Outside of cooking, the effect can be used to heat solids, liquids, or gases (see states of matter).

Communication microwave frequencies penetrate semi-solid substances like meat, and living tissue to a distance proportional to its power density. Some environmentalists are concerned that the widespread adoption of microwave-emitting mobile phones could harm human and animal health through dielectric heating.