From: Deborah Hunt (dhunt@exploratorium.edu)
Date: Thu Dec 09 1999 - 09:14:51 PST
Date: Thu, 9 Dec 1999 09:14:51 -0800 (PST) From: Deborah Hunt <dhunt@exploratorium.edu> Subject: microwave ovens and a cool site Message-ID: <Pine.GSU.4.10.9912090913030.16624-100000@isaac>
Hi, Pinholers.
We have a Science News listserv for staff here at the Exploratorium that
Larry Shaw sends out. Thought you'd be interested in the message below on
microwave ovens and a website for more science info.
Deb
---------------------------------------------------
Deborah Hunt
Internet Resource Specialist
Exploratorium
3601 Lyon Street
San Francisco, CA 94123
Voice: 415-353-0485
Fax: 415-561-0370
email: dhunt@exploratorium.edu
**********************
http://howthingswork.virginia.edu/home_current.html
November 2, 1999
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).
--------------------------------------------
Most Recent Questions http://howthingswork.virginia.edu/home_current.html
Louis A. Bloomfield, Professor of
Physics, The University of Virginia
Think of this site as a radio call-in program that's being held on the WWW
instead of the radio. If you ask how something works, using the button
below, I'll try to
provide an explanation. You'll find a more comprehensive discussion of many
common objects in my book: How Things Work: the Physics of Everyday Life.
Feel
free to create links to this site; I'm trying to encourage everyone to
learn about the physics and science of the world around them. Please let me
know what you think
about this site. -- Lou Bloomfield
This archive was generated by hypermail 2b29 : Thu Oct 19 2000 - 11:10:03 PDT