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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.

Contents

Mechanism

Molecular rotation occurs in materials containing polar molecules having an electrical dipole moment, which will align themselves in an electromagnetic field. If the field is oscillating, as in an electomagnetic wave, these molecules rotate to continuously align with it. This is called dipole rotation. As the field alternates, the molecules reverse direction. Rotating molecules push, pull, and collide with other molecules (through electrical forces), distributing the energy to adjacent molecules and atoms in the material. Temperature is the average kinetic energy (energy of motion) of the atoms or molecules in a material, so agitating the molecules in this way by definition increases the temperature of the material. Thus, dipole rotation is a mechanism by which energy in the form of electromagnetic radiation is converted to heat energy in matter. (There are also many other mechanisms by which this conversion occurs.)

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 thus less affected by the forces generated by the alternating electromagnetic fields. On the other hand, frozen water molecules are fixed in place in a crystal lattice and cannot freely rotate, so they cannot accelerate as much in response to the electromagnetic forces they experience from the external electromagnetic waves. Outside of cooking, the effect can be used generally to heat solids, liquids, or gases, provided they contain some electric dipoles.

Power

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

 p = \omega \cdot \varepsilon_r'' \cdot \varepsilon_0 \cdot E^2,

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.

Penetration

Communication microwave frequencies penetrate semi-solid substances like meat and living tissue to a distance proportional to their power density. The penetration stops essentially where all of the penetrating microwave energy has been absorbed as (i.e. converted to) heat in the tissue. For this reason, it is dangerous to stand close to high-power microwave antennas such as those used for broadcasting over long distances (tens of miles); a person in proximity to such antennas may experience severe penetrating burns, which (in the worst cases) may include serious burn injury to internal organs.

Uses

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Heating

The use of high-frequency electric fields for heating dielectric materials had been proposed in the 1930s, for example US patent 2,147,689 (application by Bell Telephone Laboratories, dated 1937) states "This invention relates to heating systems for dielectric materials and the object of the invention is to heat such materials uniformly and substantially simultaneously throughout their mass. ... It has been proposed therefore to heat such materials simultaneously throughout their mass by means of the dielectric loss produced in them when they are subjected to a high voltage, high frequency field." The modern microwave oven makes use of microwave frequency electric fields for highly efficient dielectric 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]

See also

Notes

References

  • Metaxas, A.C. (1996). Foundations of Electroheat, A Unified Approach. John Wiley & Sons. ISBN 0-471-95644-9. 
  • Metaxas, A.C., Meredith, R.J. (1983). Industrial Microwave Heating (IEE Power Engineering Series). Institution of Engineering and Technology. ISBN 0-90604-889-3. 
  • U.S. Patent 2,147,689 - Method and apparatus for heating dielectric materials

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