Talk:Microwave oven/Archive 2

message: The "proof" offered on the main page that the heating is mainly conductive due to that a drop of water is less heated than a glass takes not into account the difference of surface area. The proof should also be supported by calculations, and not only by anectodical stories.  I will check the theories by calculations, and change the text accordingly. —Preceding unsigned comment added by Mossig (talk • contribs) 12:48, 9 April 2006
: After having had a look at the curves at http://www.lsbu.ac.uk/water/microwave.html I can not say that the new statement in the article is correct. I will remove it. Before reinsering it, plese argue with references to the mentioned curves as to way this interpretation is correct. —Preceding unsigned comment added by Mossig (talk • contribs) 13:04, 9 April 2006
:: I was one of the people who did the little "anecdotal" home experiments back in April allegedly proving that electrical conduction was the dominant factor in heating of microwave ovens.  I'm going to include our original comments below for reference:
:: It is interesting to note that while most references refer to the oscillation of polar molecules as being the primary heating mechanism of water in a microwave oven a few simple home experiments illustrate this is in fact not the case. Dielectric heating can be caused either by electrical conduction or molecular rotation. The dominant heating mechanism in a microwave oven is actually electrical conduction. This is illustrated by two facts. First, a drop of water will heat poorly if at all in a microwave while a glass of water adjacent to it will heat rapidly. This demonstrates that the heating mechanism is not molecular which would be independent of scale but rather conductive which requires the food item to be on the same scale as the wavelength for efficient coupling. Second, distilled water will heat much more slowly than regular tap water. This demonstrates that the heating is dependant on the conductivity of the food item rather than the presence of polar water molecules.
:: Thank you for pointing me to the interesting article cited above.  We ran our little experiments in Tucson, which has anywhere between 160 to 760 ppm (see http://www.ci.tucson.az.us/water/clearwater_faqs.htm).  So if I'm interpreting the data from the lsbu.ac.uk article above (I'm referring to the un-labelled plot about two-thirds the way down the article, where they plot the loss coefficient against temperature and salt content), even the minimum level of 160 ppm would produce many times greater heating than pure deionized water.  I don't know how pure the off-the-shelf DI water we were using was, but assuming it was, say 10 ppm, even 35 ppm would produce twice the rate of heating as the deionized water.  At low ppm it appears essentially linear with ion concentration, so 160 would yield a "dielectric loss" of approx 140 (I just eye-balled it),
compared to the 20 of my hypothetical deionized water -- that means tap water would heat 7 times as fast as deionized water!  Those poor souls with 760 ppm tap water might see their tap water heat up as much as 30 times faster than deionized water.  To be a little more clear about what's actually going on here: the molecules in deionized water are simply spinning around really really fast, but not actually jostling and bumping into each other, thus not producing heat.  On the other hand, a much larger proportion of the input power of the microwaves gets turned into heat in hard water.  Thus I conclude that salt contet is more than just important -- it seems critical.  Yet, this is not even mentioned in the main article.
:: Secondly, the lsbu.ac.uk article you cite only considers dipole rotation.  Have you seen literature showing that current induced directly in the salt ions (presumably due to gradient in the EM field) doesn't contribute significantly?  (I can't find any data supporting either theory.)  Our experiment showing that tiny objects don't heat as readily as large objects was designed to show that this effect was at least important if not outright dominant.  However, thank you for pointing out that we neglected to consider the greater relative cooling of the small drop of water due to its much greater surface-to-volume ratio.  I'll have to try to come up with a more sophisticated experiment!  Until then  I have to concede that our experiment, while suggestive, is inconclusive.
:: 63.194.18.133 23:40, 16 September 2006 (UTC)
replies: text:  The reason that your waterdroplets dont heat up is due to their small size. As they are much smaller than the wavelength (12 cm) they will be in effect invicible for the electromagnetic waves, and thus not absorb any power. This is the same effect that is responsible for the power being contained in the oven by the metallic mesh in the window. A first order approximation of a microwve oven gives that all power is absored by the food in the oven. Thus heating times for homogenous substances in the oven will not vary much. (But for different substances in teh same oven there will be differences.) Your resoning about water molecules spinning around themselves without causing any heating has no basis in physics. The link above do take ionic conduction, and the corresponding loss, into account. Mossig 18:34, 17 September 2006 (UTC)
replies: text:  It's certainly not true that particles smaller than a wavelength are somehow invisible. If that were so, atoms could not interact with visible light which is thousands of times their size, wavelength-wise. Yet of course they do. They even undergo elastic scattering, which is why the sky is blue. And droplets scatter by Rayleigh and Mie type mechanisms in the radar range as well, which is why clouds and rain are certainly not invisible to radar with wavelengths many times the size of the droplets being seen. Effects analogous to Raman scattering cause absorption to actually predominate over scattering for very small droplets, and the mechanism is the very sort of Debye relaxation we're talking about (where water molecules rotate in an E field and cause friction while doing it). That runs best at about 30 GHz.  I don't know where anybody got the idea that decreasing absorption with thickness means a non-molecular mechanism. Think of a manifestly molecular mechanism-- a thin layer of dye won't absorb. So lose this notion.  A final comment: while it may be true that conductive solutions absorb microwaves several times better due to solvent drag effects (ions beging dragged through solutions, causing heating), that doesn't mean foods with salt in them heat that much faster. All microwaves introduced into an oven bounce around until they're absorbed, whether there's saltwater in there or distilled water. The only difference is the SKIN thickness the absorption happens in. SBHarris 01:37, 18 September 2006 (UTC)
thread_title:  Heating mechanisms 

message: I'm a little confused, it says earlier that the casing of the microwave and the mesh on the door stop any microwaves from escaping, then later the article says that microwaves can disrupt wireless networks.  If the microwaves can't escape the microwave oven how are they able to disrupt wireless networks? Kniesten 18:56, 5 September 2006 (UTC)
replies: text: Well, sometimes if the oven is badly constructed, enough microwave radiation gets out to disrupt a radio network, but still far less than would be biologically dangerous. No screen stops EVERYTHING. SBHarris 19:34, 5 September 2006 (UTC)
thread_title:  Wireless Disruption 

message: The "Dangers" section mentions dielectric breakdowns in air forms ozone and nitrogen oxides, then claims both are toxic. Whie nitrogen oxides are toxic, I don't belive ozone poses any heath hazards in low exposures such as those one would experiance from the dielectric breakdown of air inside a microwave. Does anyone know enough about ozone health hazards to confirm this is a mistake?--LaughingMan42 06:57, 8 September 2006 (UTC)
replies: text: Ozone production in a microwave shouldn't occur unless something is causing arcing. Absent the arcing, there shouldn't be any detectable ozone so no health risk there. Atlant 12:45, 8 September 2006 (UTC)
thread_title:  Dangers: Toxic Ozone? 

message: Again the story of Percy Spencer and his melting chocolate bar. This sounds like a tall story to me (though I’ve seen it repeated several times); I believe the cooking effect of microwaves was known in the 1930’s. 
I have a children’s book (Boys Annual 1938 ) which mentions an experiment which involved cooking a sausage between two radio emitters; amazingly, the sausage cooked from the inside!  
Of course, the book didn’t call them microwaves; the article was called “Death Rays Across The Ether”, and featured a futuristic picture of blokes in space suits using a ray-gun to shoot down a plane resembling a DC-3. The account also said the effect was noticed when it was found the mice at Alexandra Palace were getting fried by the radio transmitters there (this may also be a tall story; it was written for 12 year olds)
Also, I can think of a number of reasons why a chocolate bar in someone’s pocket might melt, but not a way for microwaves to do this without also cooking any adjacent anatomy.[[62.6.139.11 15:54, 12 October 2006 (UTC)
replies: text: Uhh... high power microwave emitters did not exist in the 1930s.  You're confusing microwave heating with simple RF heating which was known since the late 1890s.  High-power microwave dates from the klystron and magnetron which were a military secret invention from the early 1940s.  A few seconds of research shows that Percy Spencer was an uneducated tech at Raytheon whose problem-solving skills won Raytheon the WWII contract to build the first military radar tubes, got Spencer the job as department head, and later led to Spencer's inventing of the microwave oven.  The candy bar story and the popcorn story are "repeated several times" since they are a well-known part of that history, see [raytheon site http://www.raytheon.com/about/history/leadership/index.html].   Also: microwave radar leakage that gently warms a candy bar will gently warm the adjacent person. --Wjbeaty 20:33, 9 November 2006 (UTC)
replies: text:  Thanks...you're probably right about the confusion, I don't know a huge amount about RF heating; so you could get the same cooking effect that way? I know the history, and I wasn't trying to dispute Spencer's invention of the oven, I was just querying whether he was the first to notice the effect. And I know the story is a well-known part of the history, I'm just wondering if it's actually true (...and that children is where microwaves came from ) Richard 62.6.139.11 16:19, 15 November 2006 (UTC)
replies: text:  RF heating is a similar principle (radio waves induce losses in the object-to-be-heated), but I think the exact principle varies somewhat. Whereas microwaves tend to heat most food via a pretty specific interaction between the electrical field and water molecules, RF heating in general tends to heat by inducing electrical currents into the object leading to Joule heating. Where some confusion arises is that microwaves (being a specific class of radio waves) also do this (so not all microwave heating is caused by water-molecule friction). RF heating is widely used in industrial processes. At the other end of the EM spectrum is yet another related process called induction cooking, but here the energy is transfered mostly as a relatively-low-frequency magnetic field. Atlant 16:40, 15 November 2006 (UTC)
replies: text:  See . RF heating via ion-drag (fluid conductivity) from radio frequency capacitors, is actually how some diathermy machines work to heat tissue, if I'm not mistaken (this is not the surgical diathermy which uses a high frequency A.C. arc to cut tissue, but rather an external device used to warm tissue in physical therapy). These things have been known since the 30's, I think. Such things don't need to touch the patient, so they certainly work via EM fields and not contact conduction. But unlike microwaves, they don't work on pure water, or non-conductive stuff like the fat in a chocolate bar. Microwaves do heat fat (not as well as water, but they do heat them) because fat molecules have an electric dipole moment (you can demo this with a little cooking oil in your own oven). Also, chocolate bars aren't completely dry and water free. I can well imagine that a microwave beam from a magnetron in 1945 might have melted a candy bar in Percy's pocket (hard to do with RF or diathermy) while still not depositing enough energy in the underlying tissue to feel warm, simply due to blood circulation (a type of forced convective cooling) carrying away such heat (a luxury not available to the candybar). BTW, the same "more heating in the thing with no circulation" thing happens in the lenses of the eyes of a person exposed to microwaves at moderate power, which gave cataracts to some of the early people who alligned working waveguides by sight). SBHarris 20:32, 26 November 2006 (UTC)
thread_title:  History: Percy Spencer