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| [[File:microwave oven flashon.jpg|thumb|300px|A [[microwave oven]], which uses dielectric heating to cook food]]
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| '''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 == | |
| [[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==
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| 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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There are tons of property brokers in Singapore and it may be onerous to distinguish the nice from the bad. It is a large sum of money invested in property for most Singaporeans, be it business leasing or shopping for a home, so select the proper property agent as we speak by looking out for these signs. 1. Proactiveness
When the time comes so that you can put your own home available on the market, make an effort to pick the suitable agent. Discover an agent who's accustomed to your kind of property. Ask for transaction data. Verify to see if the agent is a member of the Institute of Property Brokers ( IEA ) and if yours is an HDB unit, make sure that the agent is a Listed Housing Agent. Interview the agent to make sure he has the data and the skill required to successfully market your property. Most significantly, it's essential to appoint one unique agent and permit him to conduct his duties in an expert method.
I attempted to assist them and managed to call CHristina and was not awared she is the same agent who turned away my client until when I SMS to ask her when can we view, you had been shocked when she requested for their nationality and then came the ugly fact, even the identify of my customer. Out of the blue she called you and shouted throughout what's the identify of your customer. I then ask her that is my consumer why don't she targeted on making viewing appointment as an alternative of asking me such nonsensical query.she then turned ugly and mentioned my customer is her buyer. I hen requested her how on earth is that my buyer will be hers and in that case why is the deal not close.
As soon as you understand how a lot to invest, the following step is to search out the property that most accurately fits your wants. A CEA licenced real estate agent will have the ability to help you with your property search in Singapore. The skilled real property agent will be able to symbolize and shield your curiosity in addition to to advise you on the properties that you've shortlisted and secure the property at the best possible terms in your interest. The property agent will be capable of be certain that the documentations are in order and you might be dealing with the rightful proprietor of the property.
Good information in Singapore actual property, and I may also help you to choose the "proper" property in Singapore - whether you might be an experienced investor, a first time buyer or a foreigner looking to move into Singapore. My providers cowl you thru the entire course of - finding the property, negotiating the Best Condo In Singapore price, making an offer and closing the deal. I am right here that will help you in the whole lot to make one of the best actual property resolution in your life. Actual Estate Builders' Association of Singapore Singapore Paper Retailers Association Singapore Transport Affiliation Singapore - $7-eight per hour Several types of Landed Residential properties in Singapore which helps you examine Singapore House loans both for brand new property mortgage Singapore Accredited Estate Agencies Ltd actual estate
Due to the rise of property investors in Singapore there may be at all times a seek for somebody to offer dependable information about the properties to take a position. Whether it's for rent, lease or for sale, it is very important be capable to discover a property which is dependable by a source that may vouch for the investment. It is not at all times simple to be able to put a home on sale, or discover a house on lease and even invest in a home with out knowing precisely the kind of folks to focus on. Knowing the audience is a key function n real estate.
Actual estate company for occupiers and buyers, supplies leasing, brokerage, valuation, investment and asset administration, property, project, township and services administration. Building and actual estate consulting. As of present, the first indicator would nonetheless be HDB. With HDB costs tapering and aggressive provides within the pipeline introduced by the federal government coming on-line, this is able to be the median foundation of the fundamentals. It's also typically free to tenants. Nevertheless tenants paying lower than $3,500 per thirty days in rental typically pay a half month commission for annually of rental. For leases above $three,500, tenants don't pay any commission. Come to us for your rental needs , Your curiosity is my priority. Trustworthy, Reliable and nice service is my Motto.