There are two principal mechanisms by which a non-conductive material can be warmed in an EM field:
- Electrical conduction: current flow in the material induced by the oscillating electric field generates heat by ohmic losses in the material.
- 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. 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 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, εr'' 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.
Surgical uses
Surgical diathermy is usually better known as electrosurgery, and is also referred to occasionally as electrocautery. Electrosurgery involves the use of high frequency alternating current in surgery as either a cutting modality, or else to cauterize small blood vessels to stop bleeding. This technique induces localized tissue burning and damage, the zone of which is controlled by the frequency and power of the device.[1]
Heating uses
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.
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.
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.
