Dielectric heating (also known as
electronic heating,
RF heating,
high-frequency heating) is the process in which
radioRadio is the transmission of signals by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...
wave or
microwaveMicrowaves are electromagnetic waves with wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300MHz and 300 GHz. This is an extremely broad definition including both UHF and EHF , and various sources use different boundaries...
electromagnetic radiationElectromagnetic radiation is a ubiquitous phenomenon that takes the form of self-propagating waves in a vacuum or in matter. It consists of electric and magnetic field components which oscillate in phase perpendicular to each other and perpendicular to the direction of energy propagation...
heats a
dielectricA dielectric is a nonconducting substance, i.e. an insulator. The term was coined by William Whewell in response to a request from Michael Faraday...
material. This heating is caused by
dipole rotation.
Molecular
rotationA rotation is a movement of an object in a circular motion. A two-dimensional object rotates around a center of rotation. A three-dimensional object rotates around a line called an axis. If the axis of rotation is within the body, the body is said to rotate upon itself, or spin—which implies...
occurs in materials containing polar molecules having an electrical dipole moment, which will align themselves in an electromagnetic field.
Dielectric heating (also known as
electronic heating,
RF heating,
high-frequency heating) is the process in which
radioRadio is the transmission of signals by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...
wave or
microwaveMicrowaves are electromagnetic waves with wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300MHz and 300 GHz. This is an extremely broad definition including both UHF and EHF , and various sources use different boundaries...
electromagnetic radiationElectromagnetic radiation is a ubiquitous phenomenon that takes the form of self-propagating waves in a vacuum or in matter. It consists of electric and magnetic field components which oscillate in phase perpendicular to each other and perpendicular to the direction of energy propagation...
heats a
dielectricA dielectric is a nonconducting substance, i.e. an insulator. The term was coined by William Whewell in response to a request from Michael Faraday...
material. This heating is caused by
dipole rotation.
Mechanism
Molecular
rotationA rotation is a movement of an object in a circular motion. A two-dimensional object rotates around a center of rotation. A three-dimensional object rotates around a line called an axis. If the axis of rotation is within the body, the body is said to rotate upon itself, or spin—which implies...
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 electomagnetic wave, these molecules rotate to continuously align with it. This is called dipole rotation. As the field
alternatesIn alternating current the movement of electric charge periodically reverses direction. An electric charge would for instance move forward, then backward, then forward, then backward, over and over again...
, 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 ovenA microwave oven, or a microwave, is a kitchen appliance that cooks or heats food by dielectric heating. This is accomplished by using microwave radiation to heat water and other polarized molecules within the food...
where it operates most efficiently on liquid
waterWater is an ubiquitous chemical substance that is composed of hydrogen and oxygen and is essential for all known forms of life.In typical usage, water refers only to its liquid form or state, but the substance also has a solid state, ice, and a gaseous state, water vapor or steam. Water covers 71%...
, and much less so on
fatFats consist of a wide group of compounds that are generally soluble in organic solvents and largely insoluble in water. Chemically, fats are generally triesters of glycerol and fatty acids. Fats may be either solid or liquid at normal room temperature, depending on their structure and composition...
s,
sugarSugar is a class of edible crystalline substances, mainly sucrose, lactose, and fructose. Human taste buds interpret its flavor as sweet. Sugar as a basic food carbohydrate primarily comes from sugar cane and from sugar beet, but also appears in fruit, honey, sorghum, sugar maple , and in many...
s, and frozen water. 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. On the other hand, frozen water molecules are fixed in place in a crystal lattice and cannot freely rotate, so they cannot accelerate as much in response to the electromagnetic forces they experience from the external electromagnetic waves. Outside of cooking, the effect can be used generally to heat solids, liquids, or gases, provided they contain some electric dipoles.
Power
For dielectric heating the generated power density per volume is calculated by
where ω is the
angular frequencyIn physics , angular frequency ω is a scalar measure of rotation rate. Angular frequency is the magnitude of the vector quantity angular velocity...
, ε
r is the
imaginary partIn mathematics, the imaginary part of a complex number , is the second element of the ordered pair of real numbers representing i.e. if , or equivalently, , then the imaginary part of is . It is denoted by or , where is a capital I in the Fraktur typeface....
of the complex relative
permittivityPermittivity is a physical quantity that describes how an electric field affects, and is affected by, a dielectric medium, and is determined by the ability of a material to polarize in response to the field, and thereby reduce the total electric field inside the material...
, ε
0 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 Communication is a process of transferring information from one entity to another. Communication processes are sign-mediated interactions between at least two agents which share a repertoire of signs and semiotic rules. Communication is commonly defined as "the imparting or interchange of...
microwave frequencies penetrate semi-solid substances like
meatMeat is animal flesh that is used as food. Most often, this means the skeletal muscle and associated fat, but it may also describe other edible tissues such as organs, livers, skin, brains, bone marrow, kidneys, or lungs...
and living tissue to a distance proportional to their power density. 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 is 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.
Heating
In the natural sciences, the term
diathermy means "electrically induced heat" and is commonly used for
muscleMuscle is the contractile tissue of the body and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...
relaxation. It is also a method of heating tissue electromagnetically or ultrasonically for therapeutic purposes in medicine.
Ultrasonic diathermy refers to heating of tissues by ultrasound for the purpose of therapeutic deep heating. No tissue is ordinarily damaged hence it is generally used in biomedical applications.
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. No tissue is ordinarily damaged.