dielectric heating

A microwave oven, which uses dielectric heating to cook food

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

Mechanism

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 created from the friction of the molecules rotating against each other.

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. This is caused by fats and sugars being 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 thus cannot create heat from molecular friction). Outside of cooking, the effect can be used to heat solids, liquids, or gases.

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.

Penetration

Communication microwave frequencies penetrate semi-solid substances like meat, and living tissue to a distance proportional to its power density.

Uses

Heating

In the natural sciences, the term diathermy means "electrically induced heat" and is commonly used for muscle relaxation. It is also a method of heating tissue electromagnetically or ultrasonically for therapeutic purposes in medicine.^[1]

Ultrasonic diathermy refers to heating of tissues by ultrasound for the purpose of therapeutic deep heating. No tissue is ordinarily damaged hence it is generally used in biomedical applications. ^[2][3]

Electric diathermy uses high frequency alternating electric or magnetic fields, sometimes with no electrode or device contact to the skin, to induce gentle deep tissue heating by induction or dipole rotation. No tissue is ordinarily damaged.^[4]

Surgery

Surgical diathermy is not a preferred term: better terms are electrosurgery, or electrocautery. Electrosurgery does not involve diathermy, but rather involves the use of directly-applied very high-frequency alternating current in surgery as either a cutting modality, or else to cauterize small blood vessels to stop bleeding. Much higher temperatures are generated in electrosurgery than in any "diathermy" technique. The surgical alternating current technique induces localized tissue burning and damage, the zone of which is controlled by the frequency and power of the device.^[5]

See also

• Electrosurgery
• Specific absorption rate

Notes

References