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:
NEWTON is an electronic community for Science, Math, and Computer Science K-12 Educators, sponsored and operated by Argonne National Laboratory's Educational Programs, Andrew Skipor, Ph.D., Head of Educational Programs.
