# Talk:Microwave oven/Archive 2

## Food heating

Can anyone explain why some microwaved foods heat mostly on the outside (and inner parts stay cold), while other foods get heated evenly? Thanks, --Abdull 10:37, 7 April 2006 (UTC)

From this web page:

Food is partially transparent to the radio waves, so the energy is able to shine through it. But at the same time the waves are partly absorbed by the food. Usually most of the heat is produced in an outer layer about an inch thick. So, large pieces of meat will be quickly cooked to a depth of about an inch, while the inside portions are cooked by heat conduction, just like in a conventional oven. This effect is different for different foods of course. If a food is mostly water, then only the outside inch cooks at all. If a food contains both air and water (like bread, cake, etc.,) then the radio energy penetrates all the way through, and the food gets heated everywhere, even deep inside.

- Bloodshedder 17:11, 7 April 2006 (UTC)

## Heating mechanisms

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 (talkcontribs) 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 (talkcontribs) 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)
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)
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)

One of the claims of the people who question microwave oven dangers is that there have been no government studies to prove its safety.

They also claim that microwave ovens were forbidden in Russia in 1976 due to its health risks, and that there was a woman killed by a simple blood transfusion when a nurse "warmed the blood for the transfusion in a microwave oven". This wouldn't make sense unless microwave oven did something else than heating. And then there are the testimonies of people like the swiss scientist Hans Hertel.

As with any new procedure or invention, the burden of the proof should be on those who propose its use, not on those who question its safety. In other words, we should not expect studies proving microwave oven dangers, but just the opposite: convincing scientific proof of its safety before even considering its use.

If it is true what they say, that there have not been a large scale study to prove microwave oven safety, I think we should be very careful about its use. --200.63.240.128 22:06, 23 April 2006 (UTC) Nelson Cevallos Estarellas

It's impossible to prove that something is safe, it's only possible to prove that something is unsafe. I've not been able to find the original source of the Urban Legend about the nurse heating blood in a microwave oven. If it did happen, the explanation is fairly obvious. As explained in the article, microwave ovens generate heat spots. It would be very hard to heat blood without causing solids to appear. Injecting solids in to the bloodstream is not the smartest thing to do.
There's plenty of people about on the Internet making bizarre claims regarding microwave ovens. They don't understand Science and as a result fear anything they don't understand. In the past people used to burn people as witches, now they just create websites.
Microwaves are non ionising electromagnetic waves. People realise that some forms of electromagnetic radiation cause cancer and they apply their limited knowledge and deduce that microwave ovens must be dangerous. However to this date, no peer-reviewed study can validate these fears.
If people believe that Microwave Ovens are dangerous, then they have the choice not to use them. However I'd question how these people manage to live their lives. As mentioned above, there is no way to prove that anything is safe. Science isn't perfect, but it does give people a logic to decision making rather than random assumptions based on gut instinct. 82.209.178.74 03:35, 19 August 2006 (UTC)
I beg to differ. Microwave Ovens are not be required to "live" ones life. Ha. Thats just silly;)
Perhaps someone more interested in types of technology and how it operates might instead, ask, WHY there hasn't been more research into what microwaving does to our food, and in turn, to ourselves. I don't think its unfounded to desire more awareness on this topic.
FYI, safety and what that constitutes, is a POV, at least when it comes to something MILLIONS have and use EVERYDAY in their homes. What we are referencing is the health effects and what those might be, no matter what increment of effect.
—The preceding unsigned comment was added by 71.246.208.226 (talkcontribs) .
There's been relatively little research because there are essentially no known mechanisms by which microwave ovens can do anything much different to food than ordinary thermal ovens already do, and in all the years that we've been using them, there's esentially no evidence that anything is going wrong in microwave cooking (other than the known risks of undercooking foods). Atlant 00:06, 18 September 2006 (UTC)
Perhaps we should just revert back to fire...Safe, safe fire. Oh, wait, what's the score?
Microwave oven: 1 (due to botched transfusion)
Cooking with fire, ovens, stoves: <1000 deaths per year (due to residential fires)!
I think its obvious where "more awareness" is needed.
Mikeeg555 13:36, 2 April 2007 (UTC)