A dielectric is an
electrical insulator that can be polarized by an applied electric
field. When a dielectric is placed in an electric
field, electric charges do not flow through the material as they do in a conductor, but only slightly shift from their average equilibrium
positions causing dielectric polarization. Because of
dielectric polarization, positive charges are displaced toward the field and
negative charges shift in the opposite direction. This creates an internal
electric field which reduces the overall field within the dielectric itself. If a dielectric is composed of weakly
bonded molecules, those molecules not only become polarized, but also reorient
so that their symmetry axis aligns to the field.
The most obvious advantage to using such a dielectric material is that it prevents the conducting plates on which the charges are stored from coming into direct electrical contact. More significant, however, a high permittivity allows a greater charge to be stored at a given voltage.
The most obvious advantage to using such a dielectric material is that it prevents the conducting plates on which the charges are stored from coming into direct electrical contact. More significant, however, a high permittivity allows a greater charge to be stored at a given voltage.
Dielectric
heating, also known as electronic heating, RF heating, high-frequency heating and diathermy, 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.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.
Molecular rotation occurs in materials containing polar molecules 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 molecules rotate to continuously align with it. This is called dipole
rotation. As the field alternates, 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. Once distributed, this energy appears as
heat.
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. 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 fats and sugars. This is because fats and sugar molecules are far less polar than water molecules, and thus less affected by the forces generated by the alternating electromagnetic fields.
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. 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 fats and sugars. This is because fats and sugar molecules are far less polar than water molecules, and thus less affected by the forces generated by the alternating electromagnetic fields.