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When you heat up food with a
microwave oven, all you do is to jiggle the
water molecules with a frequency where the
dielectric losses are
large. |
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Lets look at the dielectric function of
water: |
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First, we see that the general graph of the
frequency dependence is exactly as we would expect from the
theory. |
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There is a noticeable change of e' and e'' with the
temperature. This is as it should be, we had a temperature dependence for the
polarization . |
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Changing the temperature from about 300 K to 400
K thus should reduce '(300 K) » 80
by about ¾; i.e. to e'(400 K) =
60. The observed reduction is somewhat more severe, because we did not take
into account that the water dipoles interact to some extent; and at low
temperature this interaction is stronger than at high temperatures. |
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The maximum of the dielectric losses occurs
roughly between 5 GHz - 100 GHz., i.e. in the microwave region of the
spectrum |
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Most microwave ovens work at 2.45 GHz, well below the
region of maximum losses. This is intentional to ensure that the radiation is
not totally absorbed by the first layer of water it encounters but may
penetrate further into the foodstuff, heating it more evenly. |
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Radiation passing through the food items is mostly reflected
back, due to the design of the microwave oven, and absorbed on later passes.
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If the water is frozen, i.e. you have ice, you
have problems. The DK of ice at the microwave frequency is only about
3, and little energy is absorbed |
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If your water is salty, the DK goes down,
too. On the other hand, the Na+ and Cl ions
are jiggled by the electrical field; too (without producing dipoles and thus a
DK), producing frictional heat and increasing the heating rate. |
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Thus whereas ice is a very poor microwave absorber, and water
becomes a poorer microwave absorber with rising temperature, a lossy salty food
such as salt meat becomes a better microwave absorber with rising temperature.
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This is particularly noticed on thawing. It may take forever
to thaw your frozen butter or steak, but after it happened it only takes a
short time to turn it liquid or mushy, respectively |
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© H. Föll (Electronic Materials - Script)
To index
3.4.1 Dynamic Properties