Posted by Larry Hoover on February 21, 2006, at 12:47:11 [reposted on February 27, 2006, at 18:06:53 | original URL]
In reply to how hot does it get inside microwaves?, posted by mike99 on February 21, 2006, at 11:43:36
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> Or does this question not really make sense?It gets extremely hot, but only in places where the microwaves are absorbed.
I'll try an analogy. A tuning fork can be made to resonate sound energy if it is itself exposed to the right frequency of sound. A wine glass can resonate to a voice of the correct pitch, causing it to shatter. That's because masses of certain specific sizes and spacing have characteristic resonance frequencies. If you match the masses and the distances and the exciting frequencies properly, you'll get resonance. The energy of the waves will be absorbed and propogated.
In the case of microwave energy, those waves are matched nicely with the masses of, and bonds formed between, atoms which are bonded together into molecules. Water molecules have characteristic resonance frequencies for the bonds between their hydrogen and oxygen atoms. The "temperature" of a water molecule is really the amount of vibrational energy it has in its bonds. If you can match the vibrational frequency of water molecules with an energy wave, the water will absorb that energy, and exhibit the energy as heat. Microwave frequency radio waves do just that. They match the frequency of water molecules' heat vibrations, and the molecules dance with heat.
Individual water molecules that have absorbed all this extra energy vibrate so much that they bump into all the molecules around them, and transfer the vibrational energy, by conduction. That cools the water a bit, but that water molecule is once again free to absorb the next microwave that happens by. Rotating the position of moist food inside a microwave oven allows the greatest opportunity for water molecules to be oriented properly to best absorb microwave energy, because the waves are directional within the oven. Turn the food the right way, and you maximize resonance. And the hot water molecules that are oriented so as to absorb the wave warm the rest of the food up. That's also why it's best to stir the food around, half way through warming it. The heating is a random process, in some respects.
There are other resonance frequencies for common food molecules that also match up nicely with the microwave frequency band used in microwave ovens, but the example with water is probably the most common one we use when cooking. You add some water to some dishes, to provide the absorption capacity to surround the food with heat (as steam, or hot water vapour). Fatty food also absorbs microwave energy. Melting butter in a microwave oven is a snap.
An earlier question asked about styrofoam. Styrene molecules can absorb microwave energy. Zapping styrofoam creates styrene vapour, and you don't want styrene vapour next to your food. Other plastics don't absorb any microwave energy (they act like windows, letting the microwave "light" pass right on through), so they are "microwave safe".
I hope that explains the physical chemistry a little bit. Questions welcome.
Lar
poster:Larry Hoover
thread:613889
URL: http://www.dr-bob.org/babble/health/20060202/msgs/613904.html