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Re: microwave, RF heating



John Denker wrote:
Con: If you look again at Jackson's data,
http://www.deas.harvard.edu/courses/es151/pages/gallery/images/water_spec
http://www.deas.harvard.edu/courses/es151/pages/gallery/images/water_spec.html
you see that the curve for sea water cruises along with a slope of 3dB per
octave for many, many octaves; it is probably safe to extrapolate this to
oven frequency (2.45 GHz). OTOH the curve for pure water has a slope of
6dB per octave 10^8.5 Hz (and maybe lower) to 10^10.5 Hz, which definitely
covers oven frequency. So if the sea water curve is due to conductivity,
why is the pure water curve different?

What would be the charge carrier in each case? For pure water H+ and
OH-, for salt water Na+ and Cl-, and for other possibilities it will be
other ions. With different mobilities the characteristics will be
different, but as to whether that change affects the
absorption-frequency curve, we need a model. I was tempted to start
with the standard equations for free absorption and opened up a book I
had handy (Field and Wave Electromagnetics - 2nd Ed - by Cheng) and
opened to the part on skin depth (p. 371). There I found a table
listing skin depths at various frequencies for various substances -
including seawater.

Seawater:
conductivity: 4 S/m
skin depth at 60 Hz: 32 m
skin depth at 1 MHz: 0.25 m
skin depth at 1 GHz: not given because at 1 GHz "seawater is not a good
conductor" so the model breaks down.

*****************

Some further comment on my earlier post regarding absorption spectra of
water. Microwave absorption spectral lines are mainly due to molecular
rotation not vibrations (the latter are IR) and are typically 20-30
GHz. The microwave oven (2.5 GHz) is far below this - but the spectra
lines are very smeared due to neighbor interactions so I can imagine
that there is
weak absorption due to rotations at 2.5 GHz. Weak absorption is what we
require so that the microwaves penetrate deeply. It would be hard to
cook food if exciting with a frequency near a spectral line.




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Doug Craigen
http://www.dctech.com/physics/