How Microwaves Produce Heat

Microwaves in themselves are not heat. The materials that absorb microwaves convert the energy to heat. In foods, it is the polar molecules that for the most part interact with microwaves to produce heat. Water is the most common polar molecule and is a component of most foods. In the presence of a microwave electric field, water molecules attempt to line up with the field in much the same manner iron filings line up with the field of a magnet. Because the microwave field is reversing its polarity billions of times each second, the water molecule, because it is constrained by the nature of the food of which it is a part only begins to move in one direction when it must reverse itself and move in the opposite direction. In doing so, considerable kinetic energy is extracted from the microwave field and heating occurs. The phenomenon is similar to the heating of the human body when exposed to the sun or any other heat source. Energy in the form of infrared rays from the sun, is not heat until it is absorbed by the body and the polar molecules in the surface layers of the body convert it into heat.

Space charge polarization is an equally important microwave heating mechanism. Ions, are caused by the microwave field to flow first in one direction then in the opposite direction as the field is reversed. The effect of ionic conduction can be observed in the microwave heating of salted water in that higher temperatures are found at the surface. Ionic conduction has a negative effect on microwave energy penetration, thus foods with a high salt content show greater surface heating.

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