Dielectric Heating Information
Dielectric heating, also known as electronic heating, RF heating, high-frequency heating and diathermy, is the process in which radio wave or microwave electromagnetic radiation heats a dielectric material. This heating is caused by dipole rotation.
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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 electromagnetic 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 and sugars. 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. Outside of cooking, the effect can be used generally to heat solids, liquids, or gases, provided they contain some electric dipoles.
Power
Dielectric heating should be distinguished from Joule heating of conductive media. If conductivity is poor, or frequency is high, such that , then dielectric heating is the dominant mechanism of loss. 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.
Penetration
Communication microwave frequencies penetrate conductive materials, including semi-solid substances like meat and living tissue, to a distance defined by the skin effect. 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 may be 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
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] In surgery, it is used as a method of electrocauterization, for example in controlling bleeding or cutting through tissues.[2]
Therapeutic ultrasound refers to heating of tissues by ultrasound for the purpose of therapeutic deep heating. If precautions are followed no tissue is ordinarily damaged. It is generally used in physical therapy biomedical applications.[3][4]
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. If precautions are followed no tissue is ordinarily damaged. It is generally used in physical therapy biomedical applications.[5]
See also
Notes
- ^ "Deep Heat: eMedicine Physical Medicine and Rehabilitation". Emedicine.com. 2008-09-25. http://www.emedicine.com/pmr/topic203.htm. Retrieved 2009-06-02.
- ^ Page 777, section: Diathermy (Electrocautery) in: SRBs Manual of Surgery. Jaypee Brothers Medical Publishers (P) Ltd. 2007. ISBN 81-8061-847-1. [1]
- ^ "PT Articles". Medwriting.net. http://medwriting.net/PT_articles.htm. Retrieved 2009-06-02.
- ^ "What We Do : Outpatient Rehabilitation : About Rehabilitation : Therapeutic Modalities". HealthSouth. http://www.healthsouth.com/what_we_do/outpatient_rehabilitation/about_rehabilitation/therapeutic_modalities.asp. Retrieved 2009-06-02.
- ^ Guy, A.W; Lehmann, J.F.; Stonebridge, J.B. (1974). Therapeutic applications of electromagnetic power. http://ieeexplore.ieee.org/Xplore/login.jsp?url=/iel5/5/31176/01451315.pdf?arnumber=1451315.
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
Categories: Electric and magnetic fields in matter | Heating | Medical equipment
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