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Name: Jim
Status: educator
Grade: 9-12
Location: FL
Country: N/A
Date: 8/24/2005


Question:
I have told my students that microwaves heat water because the water absorbs the microwave energy. They have asked how and why the water absorbs the energy? And why other materials do not absorb the microwave energy? How do you predict what materials will absorb what frequency energy?


Replies:
A detailed explanation of how molecules interact with microwave radiation is advanced for most high schoolers. The general principle is that any molecule with a permanent electric dipole moment will absorb energy in the microwave region of the electromagnetic spectrum of the proper frequency (wavelength) and change rotational states. The analysis of such "pure rotational absorption spectra" is in fact the most accurate method for determining the molecular structure of molecules, provided they have sufficient vapor pressure (or the temperature is high enough). Water of course has a permanent electric dipole, but in a microwave oven it is present -- at least initially -- as a liquid. However, it too absorbs the microwave radiation, which is "tuned" to a frequency that water absorbs, although the band width is rather wide. How this translates to the operation of a microwave oven in practical terms can be found on the web site:

http://www.colorado.edu/physics/2000/microwaves/water_rotates4.html

Good Cooking

Vince Calder



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