Just goes to show you can find anything on the net. I have measured leakage from microwave ovens and every one was at 2450 MHz. And that IS a resonant frequency for water and water alone. That's why you can put waterless items in and they won't heat up, and also why you should never run a microwave oven without a water load: with no load you get high vswr and the magnetron can be damaged by reflected energy.
---------- >From: Rich Nute <[email protected]> >To: [email protected] >Cc: [email protected] >Subject: Re: 2. 4 GHz cordless telephone, question of general interest >Date: Fri, Apr 20, 2001, 5:27 PM > > > > > Hi Ken: > > > Here are some quotes: > > http://www.eskimo.com/~billb/weird/microexp.html#demo: > > Q: Aren't these ovens tuned to a special frequency so they only heat > water? > > A: No. The usual operating frequency of a microwave oven is nowhere > near the resonant frequency of water, and the RF energy will heat > other substances. For example, drops of grease on a plastic > microwave dish can be heated far hotter than 100C, and this causes > the mysterious scarring which frequently occurs on plastic utensils. > Any molecule which is "polar" and has positive and negative ends > will be rotated to align with the electric field of the radio waves > in the oven. The vibrating electric field rotates (vibrates) the > water molecules (and any other polar molecules) within the food. > > Microwave ovens have difficulty melting ice, presumably because the > water molecules are bound together and cannot be easily rotated by > the e-fields. If the oven was tuned to the water resonance > frequency, then the water would become far more opaque to the wave > energy. The water in the food's thin surface would absorb all the > energy, and only the outside surface of foods would be heated. The > thin outer surface of meat would become a blast of steam, and the > inside would remain ice cold. But because water does not resonate > with the microwave frequency, the waves can travel an inch or so > into the meat before being absorbed. > > See also: > > http://hypertextbook.com/facts/HowardCheung.shtml > > Here's another quote: > > http://rabi.phys.virginia.edu/HTW//microwave_ovens.html > > My science book said that a microwave oven uses a laser resonating > at the natural frequency of water. Does such a laser exist or was > that a major typo? > > 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. > > As for there being a laser in a microwave oven, there isn't. Lasers > are not the answer to all problems and so the source for microwaves > in a microwave oven is a magnetron. This high-powered vacuum tube > emits a beam of coherent microwaves while a laser emits a beam of > coherent light waves. While microwaves and light waves are both > electromagnetic waves, they have quite different frequencies. A > laser produces much higher frequency waves than the magnetron. And > the techniques these devices use to create their electromagnetic > waves are entirely different. Both are wonderful inventions, but > they work in very different ways. > > The fact that this misleading information appears in a science book, > presumably used in schools, is a bit discouraging. It just goes to > show you that you shouldn't believe everything read in books or on > the web (even this web site, because I make mistakes, too). > > On the other hand: > > http://www.sciencenet.org.uk/database/Physics/EMandLight/p00571b.html > > How does a microwave oven work? > > Everything has what is called a natural frequency. When you hold a > ruler over the edge of a table and ping it, it will bounce up and > down at a certain rate. If the length of ruler is kept the same, > the frequency of the bounce will be the same however hard the ruler > is struck. This frequency is called the natural frequency. A swing > in a children's playground also has a preferred frequency. In fact, > it is extremely difficult to make it swing at any other frequency. > > On a much smaller scale, water molecules also have a natural > frequency at which they prefer to rotate from side to side. > > One way to cook a potato, is to stick it into a hot oven. Heat > energy from the oven is transferred to the potato and the particles > inside the potato absorb this energy and move around more. > Eventually, the water inside the cells of the potato turns to steam > and the potato becomes soft. > > Ordinarily, this process takes about an hour. One way of speeding > up the cooking would be to turn up the temperature of the oven and > therefore transfer heat energy more rapidly. Unfortunately, this > also has the effect of burning the outside of the potato. The > temperature is so high that the outside of the potato reacts with > oxygen and black carbon is formed - not very appetising. > > The microwave oven emits electromagnetic waves of a frequency > exactly the same as the natural frequency of the water molecules. > The water molecules start to resonate, moving with larger and larger > oscillations and absorbing energy from the microwaves very rapidly. > The water turns to steam in a few seconds and the cooking process is > therefore completed much more quickly: for a potato in just a few > minutes. > > > Best regards, > Rich > > > > ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. 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