Date: Thu, 21 Aug 2003 08:35:06 -0700

Author: Ron Ebert

Subject: RE: Microwaving

Post:

Chuck Britton:

>Wouldn't ice have nearly the same resonant absorption as liquid water?

No. Microwaving works because dipolar molecules like water try to align
with the electric field of the microwave radiation. But there is a time lag
between the maximum E field and the maximum polarization produced by the
alignment of the dipoles. The E field is off before maximum alignment of
the water molecules, and this lag is what induces thermal energy into
liquid water. The microwave frequency of 2450 MHz is close to the resonance
of dipole reorientation and relaxation for liquid water molecules.

But with ice the water molecules are trapped in a crystal lattice and have
little freedom of movement, so ice converts microwaves to heat a lot more
poorly than liquid water does. There is some absorption of energy to be
sure, otherwise you couldn't defrost frozen food in the microwave oven, but
it is far less efficient than with liquid water.


Gary Karshner:

>Ron,
> Is there any difference between the heating time of distilled
> water verses regular water and salted water?

Since non-polar impurities in tap water won't react with microwaves, and
polar impurities would do so less efficiently than water molecules, I would
expect distilled water to heat faster than tap water, though the difference
may be small to measure. Likewise for salt water - since sodium and
chlorine ions won't respond to microwaves at resonance, I'd expect that
heating salt water would be less efficient than distilled water. I don't
have a microwave in my lab. Someone can try it.

Ron Ebert
ron.ebert@ucr.edu
---------------------------
"The facts will eventually test all our theories, and they form, after all,
the only impartial jury to which we can appeal." - Jean Louis Rodolphe Agassiz



From bennett@oakland.edu Thu Aug 21 12:57:16 2003
Mime-Version: 1.0
X-Sender: bennett@pop.oakland.edu

Back