I am curious about why metals have relatively high ohmic losses; in contrast, dielectrics have low-loss property?
In a metal, the electrons move easily between the ions of the crystal lattice, even under the effect of a very low electric field. They are called the free electrons of the metal. In the presence of an electric field, electrons flow between the atoms and collide with the imperfections of the crystal lattice. The metal resists to the passage of electrons. This results in a heating of the metal. This dissipation of energy is the joule effect. This explains why metals have relatively high ohmic losses.
In contrary, in an insulator, the electrons can not move even under the effect of an intense electric field, one says that they are attached to atoms. So, for insulators, there are no current, then there are no losses.
However, a fluid medium (gaseous or liquid) whose molecules have a permanent electric moment takes a polarization under the action of an electric field. The most common example of a polarization orientation liquid is water. Thus, in a microwave oven, when an outside electric field is allowed to act on a food with a generally large water composition, each water molecule (dipole) tends to orientate parallel to the electric field. If the electric field is variable, alternative variations of the electric field cause the agitation of polar molecules and accentuate their random movement. This dissipation of energy is taken advantage of in microwave ovens for heating food.
In summary, in a dielectric medium with complex permittivity, the energy supplied is not accumulated in free energy, it can only be dissipated in the dielectric. In our example, the permittivity in question is that of water, major constituent of all foods.
Furthermore, the dielectric loss is a disadvantage - however minor - for common capacitors. One may conclude that dielectrics have low loss-property.
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