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How do microwave ovens ACTUALLY work?

  1. Mar 21, 2004 #1
    My textbook says that in microwave ovens, the frequency of the microwaves produced matches the natural frequebcy of water. Therefore, food containing water will heat up. But sites I have found on the net say a variety of other things. Such as 'It's a common misconception that the microwaves in a microwave oven excite a natural resonance in water. The frequency of a microwave oven is well below any natural resonance in an isolated water molecule, and in liquid water those resonances are so smeared out that they're barely noticeable anyway. It's kind of like playing a violin under water--the strings won't emit well-defined tones in water because the water impedes their vibrations. Similarly, water molecules don't emit (or absorb) well-defined tones in liquid water because their clinging neighbors impede their vibrations.

    Instead of trying to interact through a natural resonance in water, a microwave oven just exposes the water molecules to the intense electromagnetic fields in strong, non-resonant microwaves. The frequency used in microwave ovens (2,450,000,000 cycles per second or 2.45 GHz) is a sensible but not unique choice. Waves of that frequency penetrate well into foods of reasonable size so that the heating is relatively uniform throughout the foods. Since leakage from these ovens makes the radio spectrum near 2.45 GHz unusable for communications, the frequency was chosen in part because it would not interfere with existing communication systems.'

    Others say things like 'In the case of microwave ovens, the commonly used radio wave frequency is roughly 2,500 megahertz (2.5 gigahertz). Radio waves in this frequency range have aninteresting property: they are absorbed by water, fats and sugars. When theyare absorbed they are converted directly into atomic motion - heat. Microwaves
    in this frequency range have another interesting property: they are notabsorbed by most plastics, glass or ceramics. Metal reflects microwaves, which is why metal pans do not work well in a microwave oven.'

    Which is true, if any?

    Thank you very much.
     
  2. jcsd
  3. Mar 21, 2004 #2
    A microwave oven works by passing microwave radiation, usually at a frequency of 2450 MHz (a wavelength of 12.24 cm), through the food. Water, fat, and sugar molecules in the food absorb energy from the microwave beam in a process called dielectric heating. Most molecules are electric dipole, meaning that they have a positive charge at one end and a negative charge at the other, and is therefore twisted to and fro as it tries to align itself with the alternating electric field induced by the microwave beam. This molecular movement creates heat. Microwave heating is most efficient on liquid water, and much less so on fats, sugars, and frozen water. Microwave heating is sometimes incorrectly explained as resonance of water molecules, but this occurs only at much higher frequencies, in the tens of gigahertz.
     
  4. Mar 22, 2004 #3
    Cool, thanks very much.
     
  5. Mar 24, 2004 #4
    This is interesting. I have read about dielectric heating being used in manufacturing processes, in which the material to be heated is generally flat and is temporarily made to act as the dielectric between two capacitor plates. I never realized that a microwave oven worked on the same basic principle of making the molecules flip back and forth in an attempt to stay aligned with an ever changing polarity.
     
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