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
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