How Microwaves Work
Microwaves can be compared to sound. Sound travels in waves by creating very tiny pressure differences in air. Microwaves are different. They travel through space, whether or not air is present, and rather than creating slight pressure differences, microwaves are electromagnetic. Electromagnetic waves are common in our everyday life. Light in the form of color is electromagnetic. AM and FM radio stations receive their signals through electromagnetic waves.
For a simple explanation on how microwaves work you can go to http://www.colorado.edu/physics/2000/microwaves. A more technical overview follows.
Microwaves are largely reflected from metallic conductors, but interact well with dipoles (asymmetrically charged molecules), such as water. Microwaves are an efficient method of heating non-conducting materials know as diaelectrics, which are not heated as efficiently by conventional convective methods. Microwaves generate rapidly changing electric fields and dipoles rapidly change their orientations in response to the changing fields. If the field change is occurring near the natural frequency at which reorientation occurs, then a maximum utilization of energy is realized and optimum heating occurs. The material is then said to be 'well coupled' with the microwaves.
The material properties of greatest importance for insulators, or dielectrics, are the permittivity and loss factor (measures of the materials ability to absorb and store energy) and especially a factor termed tan delta-d (the materials ability to convert stored energy into heat). Hence, for optimum coupling a balanced combination of moderate permittivity and high loss/tan delta-d is required.
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