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| [[File:microwave oven flashon.jpg|thumb|300px|A [[microwave oven]], which uses dielectric heating to cook food]]
| | 38 yr old Aircraft Preservation Manufacture (Avionics) Roman Kepp from Kindersley, has lots of passions that include r/c cars, property developers [http://www.nabiljeffri.com/?option=com_k2&view=itemlist&task=user&id=50370 Best Condo In Singapore] singapore and video games. Maintains a tour blog and has lots to write about after visiting Rohtas Fort. |
| '''Dielectric heating''', also known as '''electronic heating''', '''[[Radio frequency|RF]] heating''', '''high-frequency heating''' is the process in which a high-frequency alternating electric field, or [[radio wave]] or [[microwave]] [[electromagnetic radiation]] heats a [[dielectric]] material. At higher frequencies, this heating is caused by [[molecular dipole]] rotation within the dielectric. At lower frequencies in conductive fluids, other mechanisms such as [[ion-drag]] are more important in generating [[thermal energy]].
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| RF dielectric heating at intermediate frequencies, due to its greater penetration over microwave heating, shows greater promise than microwave systems as a method of very rapidly heating and uniformly preparing certain food items, and also killing parasites and pests in certain harvested crops.<ref name="Piyasena-2003-pmid-14669879">{{citation |author=Piyasena P, et al. |title=Radio frequency heating of foods: principles, applications and related properties—a review |journal=Crit Rev Food Sci Nutr. |year=2003 |volume=43 |issue=6 |pages=587–606 |pmid=14669879}}</ref>
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| == Mechanism ==
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| [[Molecular]] rotation occurs in materials containing [[polar molecule]]s having an [[electrical dipole moment]], with the consequence that they will align themselves in an [[electromagnetic field]]. If the field is oscillating, as it is in an electromagnetic wave or in a rapidly oscillating electric field, these [[molecule]]s rotate continuously aligning with it. This is called dipole rotation, or Dipolar Polarisation. As the field [[alternating current|alternates]], the molecules reverse direction. Rotating molecules push, pull, and collide with other molecules (through electrical forces), distributing the energy to adjacent molecules and [[atom]]s in the material. Once distributed, this energy appears as heat.<ref name="homepages.ed.ac.uk">[http://homepages.ed.ac.uk/ah05/basicintro.html ]{{dead link|date=December 2013}}</ref>
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| [[Temperature]] is the average [[kinetic energy]] (energy of motion) of the atoms or molecules in a material, so agitating the molecules in this way increases the temperature of the material. Thus, dipole rotation is a mechanism by which energy in the form of electromagnetic radiation can raise the temperature of an object. There are also many other mechanisms by which this conversion occurs.{{citation needed|date=February 2013}}
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| 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 [[fat]]s and [[sugar]]s. This is because fats and sugar molecules are far less [[chemical polarity|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 [[states of matter|solids, liquids, or gases]], provided they contain some electric dipoles.
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| Dielectric heating involves the heating of electrically insulating materials by dielectric loss. A changing electric field across the material causes energy to be dissipated as the molecules attempt to line up with the continuously changing electric field. This changing electric field may be caused by an electromagnetic wave propagating in free space (as in a microwave oven), or it may be caused by a rapidly alternating electric field inside a capacitor. In the latter case there is no freely propagating electromagnetic wave, and the changing electric field may be seen as analogous to the electric component of an antenna [[near and far field|near field]]. In this case, although the heating is accomplished by changing the electric field inside the capacitive cavity at radio-frequency (RF) frequencies, no actual [[radio wave]]s are either generated or absorbed. In this sense, the effect is the direct electrical analog of [[magnetic induction]] heating, which is also near-field effect (and also does not involve classical radio waves).{{citation needed|date=February 2013}}
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| [[Frequencies]] in the range of 10–100 [[MHz]] are necessary to cause efficient dielectric heating, although higher frequencies work equally well or better, and in some materials (especially liquids) lower frequencies also have significant heating effects, often due to more unusual mechanisms. For example, in conductive liquids such as salt water, "ion-drag" causes heating, as charged ions are "dragged" more slowly back and forth in the liquid under influence of the electric field, striking liquid molecules in the process and transferring kinetic energy to them, which is eventually translated into molecular vibrations and thus into thermal energy.{{citation needed|date=February 2013}}
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| Dielectric heating at low frequencies, as a near-field effect, requires a distance from electromagnetic radiator to absorber of less than about 1/6<sup>th</sup> of a wavelength (λ/2π) of the source frequency. It is thus a contact process or near-contact process, since it usually sandwiches the material to be heated (usually a non-metal) between metal plates that set up to form what is effectively a very large [[capacitor]], with the material to be heated acting as the dielectric inside the capacitor. However, actual electrical contact is not necessary for heating a dielectric inside a capacitor, as the electric fields that form inside a capacitor subjected to a voltage do not require electrical contact of the capacitor plates with the dielectric (non-conducting) material between the plates. Because lower frequency electrical fields penetrate nonconductive materials far more deeply than do microwaves, heating pockets of water and organisms deep inside dry materials like wood, it can be used to rapidly heat and prepare many non-electrically conducting food and agricultural items, so long as they fit between the capacitor plates.{{citation needed|date=February 2013}}
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| At very high frequencies, the wavelength of the electromagnetic field changes begins to be shorter than the distance between the metal walls of the heating cavity, or than the dimensions of the walls themselves. This is the case inside a [[microwave oven]]. In such cases, conventional far-field electromagnetic waves form (the cavity no longer acts as a pure capacitor, but rather as an antenna), and are absorbed to cause heating, but the dipole-rotation mechanism of heat deposition remains the same. However, microwaves are not efficient at causing the heating effects of low frequency fields that depend on slower molecular motion, such as those caused by ion-drag.{{citation needed|date=February 2013}}
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| == Power ==
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| Dielectric heating must be distinguished from [[Joule heating]] of conductive media, which is caused by induced electric currents in the media. For dielectric heating, the generated power density per volume is given by:{{citation needed|date=February 2013}}
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| :<math>p = \omega \cdot \varepsilon_r'' \cdot \varepsilon_0 \cdot E^2,</math>
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| where <math>\omega</math> is the [[angular frequency]] of the exciting radiation, <math>\varepsilon_r''</math> is the [[imaginary part]] of the complex relative [[permittivity]] of the absorbing material, <math>\varepsilon_0</math> is the permittivity of free space and <math>E</math> the [[electric field strength]]. The imaginary part of the (frequency-dependent) relative permittivity is a measure for the ability of a dielectric material to convert electromagnetic field energy into heat.{{citation needed|date=February 2013}}
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| If the [[electrical conductivity|conductivity]] <math>\sigma</math> of the material is small, or the frequency is high, such that <math>\sigma \ll \omega\varepsilon</math> (with <math>\varepsilon=\varepsilon_r'' \cdot \varepsilon_0</math>), then dielectric heating is the dominant mechanism of loss of energy from the electromagnetic field into the medium.{{citation needed|date=February 2013}}
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| == Penetration ==
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| [[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 essentially stops where all the penetrating microwave energy has been converted to heat in the tissue.
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| It is also interesting to note that the microwave ovens used to heat food, are not set to the frequency for optimal absorption by water, so as to achieve the ideal levels of penetration. If that was so, then the piece of food or liquid in question would absorb all microwave radiation in its outer layer, leading to a cool, unheated centre and a superheated outer. This, of course, would not be ideal for use as a cooking utility, though such a function has a handful of uses in industrial chemistry.
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| The frequency of a household microwave oven is 2.45 GHz, while the frequency for optimal absorbency by water is around 10 GHz.
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| <ref name="homepages.ed.ac.uk"/>
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| == Use of RF electric fields in dielectric heating ==
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| The use of high-frequency electric fields for heating dielectric materials had been proposed in the 1930s. For example, {{US patent|2147689}} (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.''" This patent proposed radio frequency (RF) heating at 10 to 20 [[megahertz]] (wavelength 15 to 30 meters).<ref>{{US patent|2147689}}. [http://ip.com/pat/US2147689 Method and apparatus for heating dielectric materials - J.G. Chafee]</ref> Such wavelengths were far longer than the cavity used, and thus made use of near-field effects and not electromagnetic waves. (Commercial microwave ovens use wavelengths only 1% as long).
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| In agriculture, RF dielectric heating has been widely tested and is increasingly used as a way to kill pests in certain food crops after harvest, such as walnuts still in the shell. Because RF heating can heat foods more uniformly than is the case with microwave heating, RF heating holds promise as a way to process foods quickly.<ref name="Piyasena-2003-pmid-14669879"/>
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| In [[medicine]], the RF heating of body tissues is a kind of [[diathermia]] or diathermy.<ref>"Diathermy", [[Collins English Dictionary]] - Complete & Unabridged 10th Edition. Retrieved August 29, 2013, from [http://dictionary.reference.com/browse/diathermy Dictionary.com website]</ref>
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| ==Microwave heating== | |
| Microwave heating, as distinct from RF heating, is a sub-category of dielectric heating at frequencies above 100 MHz, where an electromagnetic wave can be launched from a small dimension emitter and conveyed through space to the target. Modern [[microwave oven]]s make use of electromagnetic waves ([[microwave]]s) with electric fields of much higher frequency and shorter wavelength than RF heaters. Typical domestic microwave ovens operate at 2.45 GHz, but 0.915 GHz ovens also exist. This means that the wavelengths employed in microwave heating are 12 or 33 cm (4.7 or 13 inch). This provides for highly efficient, but less penetrative, dielectric heating.{{citation needed|date=February 2013}}
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| Although a capacitor-like set of plates can be used at microwave frequencies, they are not necessary, since the microwaves are already present as [[near and far field|far field]] type [[Electromagnetism|EM]] radiation, and their absorption does not require the same proximity to a small antenna as does RF heating. The material to be heated (a non-metal) can therefore simply be placed in the path of the waves, and heating takes place in a non-contact process which does not require capacitative conductive plates.{{citation needed|date=February 2013}}
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| == See also ==
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| * [[Specific absorption rate]]
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| * [[Electrosurgery]], which requires direct joule heating of tissue, and thus directly transmitted high frequency currents
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| == References ==
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| {{reflist|30em}}
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| == External links ==
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| {{refbegin}}
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| * {{cite book |last=Metaxas |first=A.C. |title=Foundations of Electroheat, A Unified Approach |year=1996 |publisher=John Wiley & Sons |isbn=0-471-95644-9}}
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| * {{cite book |last=Metaxas, A.C. |first=Meredith, R.J. |title=Industrial Microwave Heating (IEE Power Engineering Series) |year=1983 |publisher=Institution of Engineering and Technology |isbn=0-906048-89-3}}
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| * {{US patent|2147689}} – ''Method and apparatus for heating dielectric materials''
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| {{refend}}
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| {{DEFAULTSORT:Dielectric Heating}}
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| [[Category:Electric and magnetic fields in matter]]
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| [[Category:Heating]]
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| [[Category:Medical equipment]]
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| [[Category:Electromagnetism]]
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38 yr old Aircraft Preservation Manufacture (Avionics) Roman Kepp from Kindersley, has lots of passions that include r/c cars, property developers Best Condo In Singapore singapore and video games. Maintains a tour blog and has lots to write about after visiting Rohtas Fort.