Dielectric heating

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.

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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; as the field alternates, the molecules reverse direction and accelerate the motion of individual molecules or atoms. Heat is a form of energy possessed by a substance by virtue of the vibrational movement, i.e. kinetic energy, of its molecules or atoms.

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 forces generated by the alternating electromagnetic fields. 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.

Dielectric heating power

For dielectric heating the generated power density per volume is calculated by

where ω is the angular frequency, ε_r'' is the imaginary part of the complex relative permittivity, ε₀ is the permittivity of free space and E the electric field strength. The imaginary part of the complex relative permittivity is a measure for the ability of dielectric material to convert radio frequency electromagnetic field energy into heat. Dielectric heating is a process, which increases directly the dielectric material's internal energy by externally forced dipole rotation.

References

• Metaxas, A.C. (1996). Foundations of Electroheat, A Unified Approach. ISBN 0-471-95644-9. 

• Metaxas, A.C., Meredith, R.J. (1983). Industrial Microwave Heating (IEE Power Engineering Series). ISBN 0-90604-889-3.