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Ionization and Absorption Effects in the Electric Furnace. Arthur S. King. Proceedings of the National Academy of Sciences Jun , 8 (6) ; DOI: .
Table of contents

Because lower frequency electrical fields penetrate non-conductive 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. At very high frequencies, the wavelength of the electromagnetic field becomes 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. Dielectric heating must be distinguished from Joule heating of conductive media, which is caused by induced electric currents in the media. 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. 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. Microwave ovens used to heat food are not set to the frequency for optimal absorption by water. If they were, 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 surface.

Instead, the frequency selected allows energy to penetrate deeper into the heated food. The frequency of a household microwave oven is 2.

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The use of high-frequency electric fields for heating dielectric materials had been proposed in the s. For example, U. Patent 2,, application by Bell Telephone Laboratories, dated 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.

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. In medicine, the RF heating of body tissues, called diathermy , is used for muscle therapy [6] Heating to higher temperatures, called hyperthermia therapy , is used to kill cancer and tumor tissue. RF Heating is used in the wood industry to cure glues used in plywood manufacturing, fingerjointing, and furniture construction.

RF heating can also be used to speed up drying lumber. Modern microwave ovens make use of electromagnetic waves with electric fields of much higher frequency and shorter wavelength than RF heaters. Typical domestic microwave ovens operate at 2. This means that the wavelengths employed in microwave heating are 0. Although a capacitor-like set of plates can be used at microwave frequencies, they are not necessary, since the microwaves are already present as far field type 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. Microwave volumetric heating is a commercially available method of heating liquids, suspensions, or solids in a continuous flow on an industrial scale. Microwave volumetric heating has a greater penetration depth, of up to 42 millimetres 1.

Magnetic and Electric Effects on Water

In drying of foods, dielectric heating is usually combined with conventional heating. It may be used to preheat the feed to a hot-air drier. By raising the temperature of the feed quickly and causing moisture to move to the surface, it can decrease the overall drying time. Dielectric heating may be applied part-way through the drying cycle, when the food enters the falling rate period. This can boost the rate of drying.

If dielectric heating is applied near the end of hot-air drying it can also shorten the drying time significantly and hence increase the throughput of the drier.

It is more usual to use dielectric heating in the later stages of drying. One of the major applications of RF heating is in the postbaking of biscuits.

The objectives in baking biscuits are to produce a product of the right size, shape, color, and moisture content. In a conventional oven, reducing the moisture content to the desired level can take up a large part of the total baking time. The application of RF heating can shorten the baking time. The oven is set to produce biscuits of the right size, shape, and color, but the RF heating is used to remove the remaining moisture, without excessive heating of the already dry sections of the biscuit.

Postbaking by RF heating has also been applied to breakfast cereals and cereal-based baby foods. Food quality is maximized and better retained using electromagnetic energy than conventional heating. Conventional heating results in large disparity in temperature and longer processing times which can cause overprocessing on the food surface and impairment of the overall quality of the product.

From Wikipedia, the free encyclopedia. This section does not cite any sources.


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Please help improve this section by adding citations to reliable sources. 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 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 waves 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 thus not involving radio waves. 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.

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 taking the place of the dielectric in what is effectively a very large 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 non-conducting dielectric material between the plates.

Because lower frequency electrical fields penetrate non-conductive 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.

Photoelectric effect - Wikipedia

At very high frequencies, the wavelength of the electromagnetic field becomes 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.

Dielectric heating must be distinguished from Joule heating of conductive media, which is caused by induced electric currents in the media. 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. 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. Microwave ovens used to heat food are not set to the frequency for optimal absorption by water. If they were, 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 surface. Instead, the frequency selected allows energy to penetrate deeper into the heated food. The frequency of a household microwave oven is 2.

The use of high-frequency electric fields for heating dielectric materials had been proposed in the s. For example, U. Patent 2,, application by Bell Telephone Laboratories, dated 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. 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.


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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. In medicine, the RF heating of body tissues, called diathermy , is used for muscle therapy [6] Heating to higher temperatures, called hyperthermia therapy , is used to kill cancer and tumor tissue. RF Heating is used in the wood industry to cure glues used in plywood manufacturing, fingerjointing, and furniture construction.