Electrical Appliances Unit 1

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Heating Methods Dr. A. Claude

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Basics of Heat Transfer

In the simplest of terms, the discipline of heat transfer is concerned with only two things:temperature, and the flow of heat. Temperature represents the amount of thermal energyavailable, whereas heat flow represents the movement of thermal energy from place to place.

On a microscopic scale, thermal energy is related to the kinetic energy of molecules. Thegreater a material's temperature, the greater the thermal agitation of its constituentmolecules (manifested both in linear motion and vibrational modes). It is natural for regionscontaining greater molecular kinetic energy to pass this energy to regions with less kineticenergy.Several material properties serve to modulate the heat tranfered between two regions atdiffering temperatures. Examples include thermal conductivities, specific heats, materialdensities, fluid velocities, fluid viscosities, surface emissivities, and more. Taken together,these properties serve to make the solution of many heat transfer problems an involvedprocess.

Heat Transfer Mechanisms

Conduction:Regions with greater molecular kinetic energy will pass their thermal energy to regions withless molecular energy through direct molecular collisions, a process known as conduction. Inmetals, a significant portion of the transported thermal energy is also carried by conduction-band electrons.

Convection:When heat conducts into a static fluid it leads to a local volumetric expansion. As a result ofgravity-induced pressure gradients, the expanded fluid parcel becomes buoyant and

displaces, thereby transporting heat by fluid motion (i.e. convection) in addition toconduction. Such heat-induced fluid motion in initially static fluids is known as freeconvection.

Radiation:All materials radiate thermal energy in amounts determined by their temperature, where theenergy is carried by photons of light in the infrared and visible portions of theelectromagnetic spectrum. When temperatures are uniform, the radiative flux betweenobjects is in equilibrium and no net thermal energy is exchanged. The balance is upset whentemperatures are not uniform, and thermal energy is transported from surfaces of higher tosurfaces of lower temperature.

Question: Introduction to Heat Transfer - How Does Heat Transfer?What is heat? How does heat transfer take place? What are the effects on matter when heattransfers from one body to another?

Answer: Heat transfer is a process by which internal energy from one substance transfers toanother substance. Thermodynamics is the study of heat transfer and the changes thatresult from it. An understanding of heat transfer is crucial to analyzing a thermodynamicprocess, such as those that take place in heat engines and heat pumps.
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Forms of Heat TransferUnder the kinetic theory, the internal energy of a substance is generated from the motion ofindividual atoms or molecules. Heat energyis the form of energy which transfers this energyfrom one body or system to another. This heat transfer can take place in a number of ways:

Effects of Heat Transfer

The basic effect of heat transfer is that the particles of one substance collide with theparticles of another substance. The more energetic substance will typically lose internalenergy (i.e. "cool down") while the less energetic substance will gain internal energy (i.e. "heatup"). The most blatant effect of this in our day-to-day life is a phase transition, where asubstance changes from one state of matter to another, such as ice melting from a solid to aliquid as it absorbs heat. The water contains more internal energy (i.e. the water moleculesare moving around faster) than in the ice. In addition, many substances go through eitherthermal expansionor thermal contraction as they gain and lose internal energy. Water (andother liquids) often expands as it freezes, which anyone who has put a drink with a cap inthe freezer for too long has discovered.

Heat CapacityThe heat capacity of an object helps define how that object's temperature responds toabsorbing or transmitting heat. Heat capacity is defined as the change in heat divided by thechange in temperature.

Laws of ThermodynamicsHeat transfer is guided by some basic principles which have become known as the laws ofthermodynamics, which define how heat transfer relates to work done by a system and placesome limitations on what it is possible for a system to achieve.

Heat ConductionConduction is heat transfer by means of molecular agitation within a material without any

motion of the material as a whole. If one end of a metal rod is at a higher temperature, thenenergy will be transferred down the rod toward the colder end because the higher speedparticles will collide with the slower ones with a net transfer of energy to the slower ones. Forheat transfer between two plane surfaces, such as heat loss through the wall of a house, therate of conduction heat transfer is:

Calculation

= heat transferred in time =

= thermal conductivity of the barrier

= area

= temperature

= thickness of barrier
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Heat ConvectionConvection is heat transfer by mass motion of a fluid such as air or water when the heatedfluid is caused to move away from the source of heat, carrying energy with it. Convectionabove a hot surface occurs because hot air expands, becomes less dense, and rises (see IdealGas Law). Hot water is likewise less dense than cold water and rises, causing convectioncurrents which transport energy.

Convection can also lead to circulation in a liquid,as in the heating of a pot of water over a flame.Heated water expands and becomes more buoyant.Cooler, more dense water near the surfacedescends and patterns of circulation can beformed, though they will not be as regular assuggested in the drawing.

Convection cells are visible in the heated cooking oil inthe pot at left. Heating the oil produces changes in theindex of refraction of the oil, making the cell boundariesvisible. Circulation patterns form, and presumably thewall-like structures visible are the boundaries betweenthe circulation patterns.
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Convection is thought to play a major role intransporting energy from the center of the Sun to thesurface, and in movements of the hot magma beneath

the surface of the earth. The visible surface of the Sun(the photosphere) has a granular appearance with atypical dimension of a granule being 1000 kilometers.The image at right is from the NASA Solar Physicswebsite and is credited to G. Scharmer and theSwedish Vacuum Solar Telescope. The granules aredescribed as convection cells which transport heatfrom the interior of the Sun to the surface.

In ordinary heat transfer on the Earth, it is difficult to quantify the effects of convection since

it inherently depends upon small nonuniformities in an otherwise fairly homogeneousmedium. In modeling things like the cooling of the human body, we usually just lump it inwith conduction.

Heat RadiationRadiation is heat transfer by the emission of electromagnetic waves which carry energy awayfrom the emitting object. For ordinary temperatures (less than red hot"), the radiation is inthe infrared region of the electromagnetic spectrum. The relationship governing radiationfrom hot objects is called the Stefan-Boltzmann law:

Calculation

Perspiration Cooling of Body

When the ambient temperature is above body temperature,then radiation, conduction and convection all transfer heatinto the body rather than out. Since there must be a netoutward heat transfer, the only mechanisms left under thoseconditions are the evaporation of perspiration from the skinand the evaporative cooling from exhaled moisture. Even whenone is unaware of perspiration, physiology texts quote anamount of about 600 grams per day of "insensate loss" ofmoisture from the skin.
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The cooling effect of perspiration evaporation makes use of thevery large heat of vaporization of water. This heat ofvaporization is 540 calories/gm at the boiling point, but is evenlarger, 580 cal/gm, at the normal skin temperature.

Heat Transfer by VaporizationIf part of a liquid evaporates, it cools the liquid remaining behind because it must extract thenecessary heat of vaporization from that liquid in order to make the phase change to thegaseous state. It is therefore an important means ofheat transfer in certain circumstances,such as the cooling of the human bodywhen it is subjected to ambient temperatures abovethe normal body temperature.

Greenhouse EffectThe greenhouse effect refers to circumstances where the short wavelengths of visible lightfrom the sun pass through a transparent medium and are absorbed, but the longerwavelengths of the infrared re-radiation from the heated objects are unable to pass through

that medium. The trapping of the long wavelength radiation leads to more heating and ahigher resultant temperature. Besides the heating of an automobile by sunlight through thewindshield and the namesake example of heating the greenhouse by sunlight passingthrough sealed, transparent windows, the greenhouse effect has been widely used to describethe trapping of excess heat by the rising concentration ofcarbon dioxide in the atmosphere.The carbon dioxide strongly absorbs infrared and does not allow as much of it to escape intospace.

Sunlight warms your car

Increasing atmospheric carbon dioxide

Global warming

Role in the absence of water on Venus?

A major part of the efficiency of the heating of an actual greenhouse is the trapping of the airso that the energy is not lost byconvection. Keeping the hot air from escaping out the top ispart of the practical "greenhouse effect", but it is common usage to refer to the infraredtrapping as the "greenhouse effect" in atmospheric applications where the air trapping is notapplicable.

Greenhouse Effect ExampleBright sunlight will effectively warm your car on a cold, clear day by the greenhouse effect.The longer infrared wavelengths radiated by sun-warmed objects do not pass readily throughthe glass. The entrapment of this energy warms the interior of the vehicle. The trapping of thehot air so that it cannot rise and lose the energy byconvection also plays a major role.
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Short wavelengths ofvisible light are readilytransmitted through the transparentwindshield. (Otherwise you wouldn't be ableto see through it!)

Shorter wavelengths ofultraviolet light are largely blocked by glass since they have greaterquantum energies which have absorption mechanisms in the glass. Even though you may beuncomfortably warm with bright sunlight streaming through, you will not be sunburned.

Resistance heatingThe generation of heat by electric conductors carrying current. The degree of heating for agiven current is proportional to the electrical resistance of the conductor. If the resistance ishigh, a large amount of heat is generated, and the material is used as a resistor rather thanas a conductor.

In addition to having high resistivity, heating elements must be able to withstand hightemperatures without deteriorating or sagging. Other desirable characteristics are lowtemperature coefficient of resistance, low cost, formability, and availability of materials. Mostcommercial resistance alloys contain chromium or aluminum or both, since a protectivecoating of chrome oxide oraluminum oxide forms on thesurface upon heating and inhibitsor retards further oxidation.

Since heat is transmitted byradiation, convection, orconduction or combinations ofthese, the form of element is

designed for the major mode oftransmission. The simplest form isthe helix, using a round wireresistor, with the pitch of the helixapproximately three wirediameters. This form is adapted toradiation and convection and isgenerally used for room or airheating. It is also used in industrial
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furnaces, utilizing forced convection up to about 1200F (650C). Such helixes are stretchedover grooved high-alumina refractory insulators and are otherwise open and unrestricted.

The electrical resistance of molten salts between immersed electrodes can be used togenerate heat. Limiting temperatures are dependent on decomposition or evaporizationtemperatures of the salt, Parts to be heated are immersed in the salt. Heating is rapid and,

since there is no exposure to air, oxidation is largely prevented. Disadvantages are thepersonnel hazards and discomfort of working close to molten salts.

A major application of resistance heating is in electric home appliances, including electricranges, clothes dryers, water heaters, coffee percolators, portable radiant heaters, and hairdryers. Resistance heating also has application in home or space heating.

If the resistor is located in a thermally insulated chamber, most of the heat generated isconserved and can be applied to a wide variety of heating processes. Such insulatedchambers are called ovens or furnaces, depending on the temperature range and use. Theterm oven is generally applied to units which operate up to approximately 800F (430C).Typical uses are for baking or roasting foods, drying paints and organic enamels, bakingfoundry cores, and low-temperature treatments of metals. The term furnace generally appliesto units operating above 1200F (650C). Typical uses of furnaces are for heat treatment ormelting of metals, for vitrification and glazing of ceramic wares, for annealing of glass, andfor roasting and calcining of ores.

Induction HeatingInduction heating is a method ofproviding fast, consistent heat formanufacturing applications whichinvolve bonding or changing theproperties of metals or other

electrically-conductive materials.The process relies on inducedelectrical currents within thematerial to produce heat.

An RF power supply setsalternating current within thecoil, creating a magnetic field.Your workpiece is placed in thecoil where this field induces eddycurrents in the workpiece,generating precise, clean, non-

contact heat in the workpiece.

The higher the frequency, theshallower the heating in theworkpiece.

Due to hysteresis, magneticmaterials are heated more readilythan non-magnetic, resisting the alternating magnetic field within the induction coil.


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Induced current in the workpiece is mostintense on the surface, diminishing below thesurface; 80% of the heat produced in the partis produced in the outer 'skin'.

The relationship of the current flow in the

workpiece and the distance between theworkpiece and the coil is key; 'close' couplingincreases the flow of current, increasing theamount of heat produced in the workpiece.The size and shape of the water-cooled coppercoil must follow the shape of your workpieceand the variables of your process. The correctheat pattern maximizes the efficiency ofheating.

System output determines the relative speedat which the workpiece is heated (a 5kWsystem heating a workpiece more quicklythan a 3kW system).

Consider the required temperature change, mass, specific heat and electrical properties ofthe workpiece, the coupling efficiency of the coil design and thermal losses due to convection,radiation and conduction into your fixturing.

Eddy current HeatingEddy currents (also called Foucault currents[1]) are currents induced in conductors,opposing the change in flux that generated them. It is caused when a conductor is exposed toa changing magnetic field due to relative motion of the field source and conductor; or due to

variations of the field with time. This can cause a circulating flow of electrons, or a current,within the body of the conductor. These circulating eddies of current create inducedmagnetic fields that oppose the change of the original magnetic field due to Lenz's law,causing repulsive or drag forces between the conductor and the magnet. The stronger theapplied magnetic field, or the greater the electrical conductivityof the conductor, or the fasterthe field that the conductor is exposed to changes, then the greater the currents that aredeveloped and the greater the opposing field.

The term eddy current comes from analogous currents seen in water when dragging an oarbreadthwise: localised areas of turbulence known as eddiesgive rise to persistent vortices.Eddy currents, like all electric currents, generate heat as well as electromagnetic forces. Theheat can be harnessed for induction heating. The electromagnetic forces can be used for

levitation, creating movement, or to give a strong braking effect. Eddy currents can also haveundesirable effects, for instance power loss in transformers. In this application, they areminimised with thin plates, bylamination of conductors or other details of conductor shape.Self-induced eddy currents are responsible for the skin effect in conductors.[2]The latter canbe used for non-destructive testing of materials for geometry features, like micro-cracks.[3]Asimilar effect is the proximity effect, which is caused by externally-induced eddy currents.[4]When a conductor moves relative to the field generated by a source, electromotive forces(EMFs) can be generated around loops within the conductor. These EMFs acting on theresistivity of the material generate a current around the loop, in accordance with Faraday's
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law of induction. These currents dissipate energy, and create a magnetic field that tends tooppose the changes in the field.

Eddy currents are created when a conductor experiences changes in the magnetic field. Ifeither the conductor is moving through a steady magnetic field, or the magnetic field ischanging around a stationary conductor, eddy currents will occur in the conductor. Both

effects are present when a conductor moves through a varying magnetic field, as is the caseat the top and bottom edges of the magnetized region shown in the diagram. Eddy currentswill be generated wherever a conducting object experiences a change in the intensity ordirection of the magnetic field at any point within it, and not just at the boundaries.

The swirling current set up in the conductor is due to electrons experiencing a Lorentz forcethat is perpendicular to their motion. Hence, they veer to their right, or left, depending on thedirection of the applied field and whether the strength of the field is increasing or declining.The resistivity of the conductor acts to damp the amplitude of the eddy currents, as well asstraighten their paths. Lenz's law encapsulates the fact that the current swirls in such a wayas to create an induced magnetic field that opposes the phenomenon that created it. In thecase of a varying applied field, the induced field will always be in the opposite direction tothat applied. The same will be true when a varying external field is increasing in strength.However, when a varying field is falling in strength, the induced field will be in the samedirection as that originally applied, in order to oppose the decline.An object or part of an object experiencessteady field intensity and direction wherethere is still relative motion of the field andthe object (for example in the center of thefield in the diagram), or unsteady fields wherethe currents cannot circulate due to thegeometry of the conductor. In these situationscharges collect on or within the object and

these charges then produce static electricpotentials that oppose any further current.Currents may be initially associated with thecreation of static potentials, but these may betransitory and small.

Eddy currents generate resistive losses thattransform some forms of energy, such askinetic energy, into heat. This Joule heatingreduces efficiency of iron-core transformers and electric motors and other devices that usechanging magnetic fields. Eddy currents are minimized in these devices by selectingmagnetic core materials that have low electrical conductivity (e.g., ferrites) or by using thin

sheets of magnetic material, known as laminations. Electrons cannot cross the insulatinggap between the laminations and so are unable to circulate on wide arcs. Charges gather atthe lamination boundaries, in a process analogous to the Hall effect, producing electric fieldsthat oppose any further accumulation of charge and hence suppressing the eddy currents.The shorter the distance between adjacent laminations (i.e., the greater the number oflaminations per unit area, perpendicular to the applied field), the greater the suppression ofeddy currents.
http://en.wikipedia.org/wiki/Faraday%27s_law_of_inductionhttp://en.wikipedia.org/wiki/Lorentz_forcehttp://en.wikipedia.org/wiki/Lenz%27s_lawhttp://en.wikipedia.org/wiki/Joule_heatinghttp://en.wikipedia.org/wiki/Transformerhttp://en.wikipedia.org/wiki/Electric_motorshttp://en.wikipedia.org/wiki/Magnetic_corehttp://en.wikipedia.org/wiki/Ferrite_%28magnet%29http://en.wikipedia.org/wiki/Laminationshttp://en.wikipedia.org/wiki/Hall_effecthttp://en.wikipedia.org/wiki/Hall_effecthttp://en.wikipedia.org/wiki/Laminationshttp://en.wikipedia.org/wiki/Ferrite_%28magnet%29http://en.wikipedia.org/wiki/Magnetic_corehttp://en.wikipedia.org/wiki/Electric_motorshttp://en.wikipedia.org/wiki/Transformerhttp://en.wikipedia.org/wiki/Joule_heatinghttp://en.wikipedia.org/wiki/Lenz%27s_lawhttp://en.wikipedia.org/wiki/Lorentz_forcehttp://en.wikipedia.org/wiki/Faraday%27s_law_of_induction


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The conversion of input energy to heat is not always undesirable, however, as there are somepractical applications. One is in the brakes of some trains known as eddy current brakes.During braking, the metal wheels are exposed to a magnetic field from an electromagnet,generating eddy currents in the wheels. The eddy currents meet resistance as charges flowthrough the metal, thus dissipating energy as heat, and this acts to slow the wheels down.The faster the wheels are spinning, the stronger the effect, meaning that as the train slows

the braking force is reduced, producing a smooth stopping motion. Induction heating makesuse of eddy currents to provide heating of metal objects.

Strength of eddy currentsUnder certain assumptions (uniform material, uniform magnetic field, no skin effect, etc.) thepower lost due to eddy currents can be calculated from the following equations :[6]

For thin sheets:

For thin wires:where: P- power dissipation (W/kg), Bp - peak flux density(T), d- thickness of the sheet ordiameter of the wire (m),f- frequency (Hz), - resistivity (m), D- specific density(kg/m3).It should be borne in mind that these equations are valid only under the so-called "quasi-static" conditions, where the frequency of magnetisation does not result in the skin effect, i.e.the electromagnetic wave fully penetrates the material.Therefore, the following things usually increase the size and effects of eddy currents:

stronger magnetic fields - increases flux densityB faster changing fields (due to faster relative speeds or otherwise) - increases the

frequencyf thicker materials - increases the thickness d

lower resistivitymaterials (aluminium, copper, silver etc.)Some things reduce the effects: weaker magnets - lower B slower changing fields (slower relative speeds) - lowerf thinner materials - lower d slotted materials so that currents cannot circulate - reduced d or coefficient in the

denominator (6, 12, etc.) laminated materials so that currents cannot circulate - reduced d higher resistance materials (silicon rich iron etc.)

Skin effectIn very fast changing fields due to skin effect the equations shown above are not valid

because the magnetic field does not penetrate the material uniformly. However, in any caseincreased frequency of the same value of field will always increase eddy currents, even withnon-uniform field penetration.The penetration depth can be calculated from the following equation:

[7]where: - penetration depth (m)[8],f- frequency (Hz), - magnetic permeability, - electricalconductivity (S/m)
http://en.wikipedia.org/wiki/Eddy_current_brakehttp://en.wikipedia.org/wiki/Induction_heatinghttp://en.wikipedia.org/wiki/Skin_effecthttp://en.wikipedia.org/wiki/Eddy_current#cite_note-5http://en.wikipedia.org/wiki/Eddy_current#cite_note-5http://en.wikipedia.org/wiki/Eddy_current#cite_note-5http://en.wikipedia.org/wiki/Flux_densityhttp://en.wikipedia.org/wiki/Specific_densityhttp://en.wikipedia.org/wiki/Resistivityhttp://en.wikipedia.org/wiki/Skin_effecthttp://en.wikipedia.org/wiki/Eddy_current#cite_note-6http://en.wikipedia.org/wiki/Eddy_current#cite_note-6http://en.wikipedia.org/wiki/Eddy_current#cite_note-7http://en.wikipedia.org/wiki/Eddy_current#cite_note-7http://en.wikipedia.org/wiki/Eddy_current#cite_note-7http://en.wikipedia.org/wiki/Eddy_current#cite_note-7http://en.wikipedia.org/wiki/Eddy_current#cite_note-6http://en.wikipedia.org/wiki/Skin_effecthttp://en.wikipedia.org/wiki/Resistivityhttp://en.wikipedia.org/wiki/Specific_densityhttp://en.wikipedia.org/wiki/Flux_densityhttp://en.wikipedia.org/wiki/Eddy_current#cite_note-5http://en.wikipedia.org/wiki/Skin_effecthttp://en.wikipedia.org/wiki/Induction_heatinghttp://en.wikipedia.org/wiki/Eddy_current_brake


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Dielectric HeatingDielectric heating (also known as electronic heating, RF heating, high-frequencyheating) is the process in which radiowave or microwave electromagnetic radiation heats adielectric material. This heating is caused bydipole rotation.

Mechanism

Molecular rotation occurs in materials containing polar molecules having an electrical dipolemoment, which will align themselves in an electromagnetic field. If the field is oscillating, asin an electomagnetic wave, these molecules rotate to continuously align with it. This is calleddipole rotation. As the field alternates, the molecules reverse direction. Rotating moleculespush, pull, and collide with other molecules (through electrical forces), distributing theenergy to adjacent molecules and atoms in the material. Temperature is the average kineticenergy (energy of motion) of the atoms or molecules in a material, so agitating the moleculesin this way by definition increases the temperature of the material. Thus, dipole rotation is amechanism by which energy in the form of electromagnetic radiation is converted to heatenergy 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 mostwidely observable in the microwave oven where it operates most efficiently on liquid water,and much less so on fats, sugars, and frozen water.[citation needed] This is caused by fats andsugars being far less polar than water molecules, and thus less affected by the forcesgenerated by the alternating electromagnetic fields. On the other hand, frozen watermolecules are fixed in place and cannot freely rotate, so they cannot accelerate as much inresponse to the electromagnetic forces they experience from the external electromagneticwaves. Outside of cooking, the effect can be used generally to heat solids, liquids, or gases,provided they contain some electric dipoles.

PowerDielectric heating should be distinguished from Joule heating of conductive media. Ifconductivity 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 volumeis calculated by

where is the angular frequency, r'' is the imaginary part of the complex relativepermittivity, 0 is the permittivity of free space and Ethe electric field strength. The imaginarypart of the complex relative permittivity is a measure for the ability of dielectric material toconvert radio frequency electromagnetic field energy into heat.

PenetrationCommunication microwave frequencies penetrate conductive materials, including semi-solidsubstances like meat and living tissue, to a distance defined by the skin effect. Thepenetration 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 tostand close to high-power microwave antennas such as those used for broadcasting over longdistances (tens of miles); a person in proximity to such antennas may experience severepenetrating burns, which (in the worst cases) may include serious burn injury to internalorgans.
http://en.wikipedia.org/wiki/Radiohttp://en.wikipedia.org/wiki/Microwavehttp://en.wikipedia.org/wiki/Electromagnetic_radiationhttp://en.wikipedia.org/wiki/Dielectrichttp://en.wikipedia.org/wiki/Molecularhttp://en.wikipedia.org/wiki/Rotationhttp://en.wikipedia.org/wiki/Polar_moleculehttp://en.wikipedia.org/wiki/Electrical_dipole_momenthttp://en.wikipedia.org/wiki/Electrical_dipole_momenthttp://en.wikipedia.org/wiki/Electromagnetic_fieldhttp://en.wikipedia.org/wiki/Alternating_currenthttp://en.wikipedia.org/wiki/Microwave_ovenhttp://en.wikipedia.org/wiki/Waterhttp://en.wikipedia.org/wiki/Fathttp://en.wikipedia.org/wiki/Sugarhttp://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/States_of_matterhttp://en.wikipedia.org/wiki/Joule_heatinghttp://en.wikipedia.org/wiki/Angular_frequencyhttp://en.wikipedia.org/wiki/Angular_frequencyhttp://en.wikipedia.org/wiki/Imaginary_parthttp://en.wikipedia.org/wiki/Permittivityhttp://en.wikipedia.org/wiki/Permittivityhttp://en.wikipedia.org/wiki/Electric_field_strengthhttp://en.wikipedia.org/wiki/Communicationhttp://en.wikipedia.org/wiki/Meathttp://en.wikipedia.org/wiki/Skin_effecthttp://en.wikipedia.org/wiki/Skin_effecthttp://en.wikipedia.org/wiki/Meathttp://en.wikipedia.org/wiki/Communicationhttp://en.wikipedia.org/wiki/Electric_field_strengthhttp://en.wikipedia.org/wiki/Permittivityhttp://en.wikipedia.org/wiki/Imaginary_parthttp://en.wikipedia.org/wiki/Angular_frequencyhttp://en.wikipedia.org/wiki/Joule_heatinghttp://en.wikipedia.org/wiki/States_of_matterhttp://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Sugarhttp://en.wikipedia.org/wiki/Fathttp://en.wikipedia.org/wiki/Waterhttp://en.wikipedia.org/wiki/Microwave_ovenhttp://en.wikipedia.org/wiki/Alternating_currenthttp://en.wikipedia.org/wiki/Electromagnetic_fieldhttp://en.wikipedia.org/wiki/Electrical_dipole_momenthttp://en.wikipedia.org/wiki/Electrical_dipole_momenthttp://en.wikipedia.org/wiki/Polar_moleculehttp://en.wikipedia.org/wiki/Rotationhttp://en.wikipedia.org/wiki/Molecularhttp://en.wikipedia.org/wiki/Dielectrichttp://en.wikipedia.org/wiki/Electromagnetic_radiationhttp://en.wikipedia.org/wiki/Microwavehttp://en.wikipedia.org/wiki/Radio


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UsesThe use of high-frequency electric fields for heating dielectric materials had been proposed inthe 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 theobject of the invention is to heat such materials uniformly and substantially simultaneouslythroughout their mass. ... It has been proposed therefore to heat such materials simultaneouslythroughout their mass by means of the dielectric loss produced in them when they aresubjected 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 iscommonly used for muscle relaxation. It is also a method of heating tissueelectromagnetically or ultrasonically for therapeutic purposes in medicine.[1]In surgery, it isused as a method of electrocauterization, for example in controlling bleeding or cuttingthrough tissues.[2]

Therapeutic ultrasound refers to heating of tissues by ultrasound for the purpose oftherapeutic deep heating. If precautions are followed no tissue is ordinarily damaged. It isgenerally used in physical therapybiomedical 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 byinduction or dipole rotation. If precautions are followed no tissue is ordinarily damaged. It isgenerally used in physical therapy biomedical applications.[5]
http://en.wikipedia.org/wiki/Microwave_ovenhttp://en.wikipedia.org/wiki/Microwavehttp://en.wikipedia.org/wiki/Natural_scienceshttp://en.wikipedia.org/wiki/Musclehttp://en.wikipedia.org/wiki/Dielectric_heating#cite_note-0http://en.wikipedia.org/wiki/Dielectric_heating#cite_note-0http://en.wikipedia.org/wiki/Dielectric_heating#cite_note-0http://en.wikipedia.org/wiki/Electrocauterizationhttp://en.wikipedia.org/wiki/Dielectric_heating#cite_note-1http://en.wikipedia.org/wiki/Dielectric_heating#cite_note-1http://en.wikipedia.org/wiki/Dielectric_heating#cite_note-1http://en.wikipedia.org/wiki/Therapeutic_ultrasoundhttp://en.wikipedia.org/wiki/Ultrasoundhttp://en.wikipedia.org/wiki/Physical_therapyhttp://en.wikipedia.org/wiki/Dielectric_heating#cite_note-2http://en.wikipedia.org/wiki/Dielectric_heating#cite_note-2http://en.wikipedia.org/wiki/Dielectric_heating#cite_note-2http://en.wikipedia.org/wiki/Dielectric_heating#cite_note-4http://en.wikipedia.org/wiki/Dielectric_heating#cite_note-4http://en.wikipedia.org/wiki/Dielectric_heating#cite_note-4http://en.wikipedia.org/wiki/Dielectric_heating#cite_note-4http://en.wikipedia.org/wiki/Dielectric_heating#cite_note-2http://en.wikipedia.org/wiki/Dielectric_heating#cite_note-2http://en.wikipedia.org/wiki/Physical_therapyhttp://en.wikipedia.org/wiki/Ultrasoundhttp://en.wikipedia.org/wiki/Therapeutic_ultrasoundhttp://en.wikipedia.org/wiki/Dielectric_heating#cite_note-1http://en.wikipedia.org/wiki/Electrocauterizationhttp://en.wikipedia.org/wiki/Dielectric_heating#cite_note-0http://en.wikipedia.org/wiki/Musclehttp://en.wikipedia.org/wiki/Natural_scienceshttp://en.wikipedia.org/wiki/Microwavehttp://en.wikipedia.org/wiki/Microwave_oven

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Electrical Appliances Unit 1