Why do I have a big microwave absorption in tissue, when the tissue conductivity is high? I thougt the opposite would be correct. Can somebody explain the reason?
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$\begingroup$ 1st define "microwave"! Ever heard of frequency? 2nd who says/writes that absoption is higher in "high" conductivity tissue? $\endgroup$– GeorgCommented Oct 6, 2011 at 10:51
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$\begingroup$ 1. For a mircowave with 2 GHz. 2. I can't write who said this. $\endgroup$– kameCommented Oct 6, 2011 at 11:05
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$\begingroup$ So its just an assumption of You? You should ask if this is right, not for explanation of a wrong assumption. $\endgroup$– GeorgCommented Oct 6, 2011 at 11:18
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1$\begingroup$ This is wrong. Aqueous ions are too heavy to move at such high frequencies. Only exeption is H+, but protons are rare in tissue, (living) tissue has to be almost neutral (to be alive). Mechanism of microwave (>1 GHz) absorption is by relaxation of orientational polarisation, very specific for water. $\endgroup$– GeorgCommented Oct 6, 2011 at 11:52
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1$\begingroup$ @Georg: Skin is not conductive compared to deep tissue--- you can still electrocute people, because current distributes itself over the whole flesh. The conductivity rises in proportion to the fluid content of the tissue. Also, I find that when I am wrong, it is a good day, because it is when I learn something. $\endgroup$– Ron MaimonCommented Oct 12, 2011 at 4:51
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2 Answers
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If you are referring to the microwave Ovens then this might be the possible reason,
Microwave Oven are basically meant to cook food i.e. water rich food. So anything which has a high content of water effectively absorbs microwave radiation and get hot.
EDIT1: for details on how microwave works..here
Now, since tissue contains water the above statements might hold.
NOTE : The Diagram was useless..I realised it much later... :( deleted for everyone's sake..
answered Oct 7, 2011 at 6:25
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$\begingroup$ Interesting IR/vis/uv spec of ??? (water?) but not relevant at microwave oven frequencies ( about 12 cm wavelength). Did You realize that the spectrum starts at 10 inverse centimeters, which is a frequency measure? $\endgroup$– GeorgCommented Oct 7, 2011 at 12:06
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$\begingroup$ That diagram is not useless! You might use it in case somebody asks why thick layers of water are blue. $\endgroup$– GeorgCommented Oct 10, 2011 at 9:08
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$\begingroup$ @Georg: I mean for the topic in question... $\endgroup$ Commented Oct 10, 2011 at 9:59
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You can read the wikipedia (the link already mentioned), it describes the process called Dielectric heating.
The matter of conductivity of the organic tissue is tricky. For instance, the water is a dielectric, it consists of polarized H2O molecules, all the electrons are tightly bound. However when there are minerals dissolved in the water it's a conductor - due to the ions.
So the the "non-pure" water is being heated by oscillating EM field, although it's a conductor practically.
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$\begingroup$ -1 for rechurning a partly wrong Wiki page. MW and water is not dielectic heating! $\endgroup$– GeorgCommented Oct 10, 2011 at 15:22
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$\begingroup$ @Georg: Well, this is exactly what wiki says, MW targets mostly liquid water. Do you disagree or what? $\endgroup$– valdoCommented Oct 10, 2011 at 15:40
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$\begingroup$ So read Your answer! ""So the the "non-pure" water is being heated by oscillating EM field, although it's a conductor practically."" This shows You did not understand. $\endgroup$– GeorgCommented Oct 10, 2011 at 16:40
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$\begingroup$ @Greg: I'm starting to dislike your tone. Do you think I have to prove you that I realize what I write, hoping to earn your invaluable upvote? Also your comments about down-voting because of "rechurning a partly wrong Wiki page" are inapplicable. Have comments/objections regarding the subject? Fine. Criticism is welcome, when it's justified. I think I do understand (at least in general) how induced dipole interacts with an oscillating EM field. Don't believe me? Fine, I don't insist. But this is not related to the subject. $\endgroup$– valdoCommented Oct 10, 2011 at 20:34