From: crb7q@kelvin.seas.Virginia.EDU (Cameron Randale Bass)
Subject: Re: The dangers in microwaved food...
Organization: University of Virginia
Date: Wed, 26 Aug 1992 04:24:14 GMT

In article <1992Aug26.030207.4547@zia.aoc.nrao.edu> gmartin@zia.aoc.nrao.edu (George Martin) writes:
>In article <1992Aug22.075458.14172@murdoch.acc.Virginia.EDU> crb7q@kelvin.seas.Virginia.EDU (Cameron Randale Bass) writes:
>>In article <22AUG199201293340@rigel.tamu.edu> elo9246@rigel.tamu.edu (LaNelle Ohlhausen) writes:
>>>In article <1992Aug21.015046.12425@cs.mun.ca>, paulk@garfield.cs.mun.ca (Paul E. King) writes...
>>>>I believe that theory has it that microwaves act on food by heating its
>>>>water content. The wavelength of these waves are short enough that they
>>>>are said to cause vibrations in the water molecules. This vibration
>>>>causes the heat. 
>>>
>>>Small correction:
>>>
>>>The wavelengths of microwave radiation are appropriate to cause *rotation*, 
>>>not vibration, of water molecules.  And these are in fact quite long
>>>wavelengths in comparison with most familiar radiation.  The rotational
>>>frequency for water is somewhere around 10 cm^{-1} (I don't have the exact 
>                                             ^^^^^^^
>>>value here, but can look it up on request), which implies a
>>>wavelength of about 0.1 cm.  For comparison, infrared (heat)
>>
>>    Small correction:  The wavelength of your typical microwave oven
>>    is about 12 cm.  Also, we are exciting no rotational resonance in 
>>    water.  It is simply the field wacking the dipole moment around
>>    (how's that for a rigourous argument?).  Also, frequency is 
>>    measured in Hertz or 1/sec or cycles per second, rarely in cm.  
>>    For the typical microwave, the relevant frequency is 2.45 GHz.
>
>One more small correction or nit pick as you please.  A standard unit
>in spectroscopy is inverse centimeters often refered to as wave
>number.  The formal unit name, I think, is Kaysers.  Multiplying the wave
>number by c in cm/sec gives you Hz of course.

     Another small nit, a wavenumber is not strictly a frequency, it
     is (of course) a wavenumber.  Us fluids people use wavenumbers
     all the time, but we rarely confuse them with frequencies
     unless we have a velocity to multiply them by.

     My feeling about this is a bit pedantic (so what else is 
     new?).  After all if you divide c by a frequency, you get a 
     wavelength, but I would hesitate to call 1/f a wavelength.

>>    You are correct if you are saying that water has a resonance at
>>    around 0.75 cm (somewhere around 40 GHz, I'm too lazy to look 
>>    it up right now).  However, that is wavelength
>>    and this particular one (0.1 cm) is not relevant to discussion of 
>>    commercial or home microwave ovens.
>
>There is a very well known transition, I think it is rotational, which
>makes a maser in astronomical conditions at about 22.2233345 GHz.  The
>transition, whether or not conditions causes it to be a maser, has nothing
>to do with resonance.

     I'm not sure what the distinction between a resonance and 
     a transition would be in this context. 

     Please enlighten me.

>For those wondering about the terms rotation and vibration connected to
>spectroscopy.  If you remember the tinker toy models of molecules,
>picture the three orthonganal planes in which such a model can rotate.
>Real molecules in free space do rotate.  But the angular momentum of the
>rotations is quantized with all sort of selection rules which give rise
>to spectral lines which can be observed in radio astronomy if the density
>of the molecule is great enough.

     Not when we are talking about a liquid or a solid.  'Quantized
     rotational spectra disappear' in the words of some great man or
     another.  I assumed that we were discussing liquid water.

>Now instead of the "rigid rods" connecting the tinker toy balls making
>molecules, think of springs, in particular quantum mechanical springs.
>Thus the atoms which make up a molecule can be thought of as a bunch
>of coupled harmonic oscillators.  And again things are quantized with
>selction rules.  Vibrational transitions are typically in the infrared.
>Of course there is also coupling between the rotational and vibrational
>states of a molecule.

     However, once again, things are not quite as neat in solids
     and liquids as y'all have it in the far reaches of space.
     The 'lines' are spread all over the place (except possibly
     in a perfect crystal) due to the Stark effect splitting the
     lines locally all over the place.  I think that they are 
     broadened into a true continuum, but I cannot seem to find
     confirmation within arms' reach.  In any case, 'line' spectra
     are much more complicated when you consider microwaving
     fluids and solids.

                               dale bass

-- 
C. R. Bass                                           crb7q@virginia.edu
Department of Mechanical and Aerospace Engineering
University of Virginia
Charlottesville, Virginia                            (804) 924-7926
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