water behind the microwave oven

Math Is Hard

My chemistry teacher recommends (for safety) putting a glass of water behind a microwave oven. She says this directs the microwaves (gives them somewhere to go) and helps to keep us from absorbing them and any harmful effects. Does it really do any good?

Math Is Hard

I never thought of microwaves as particularly harmful since they aren't very energetic and there wouldn't be a large amount coming from a microwave oven, but I know that pregnant women are told not to stand in front of them when they are in use. I didn't think they would pose any significant danger to me.
Any thoughts?

gravenewworld

Hmm well a microwave works by spinning the water molecules in food which heats it up. Some of the microwaves would probably be absorbed by the bowl of water behind the oven. Pregnant women probably have a lot of fluid/water in their uterus which the microwaves could interact with.

Grogs

MIH: I'm sure the glass of water will absorb any microwaves that happen to run into it, but attract microwaves? I've never heard of anything like that. Microwaves are uncharged electromagnetic radiation, just like visible light, radio waves, or gamma rays. They travel in a straight line unless they run into something that deflects them. Antennas don't attract radio waves and glasses of water don't attract microwaves.

As for the danger, I doubt there is much unless the case of your microwave is somehow damaged. Microwaves are quite low energy actually, even lower than light. It lacks the energy to break water molecules or break DNA strands like X-Rays and Gamma rays. The reason microwaves are able to cook food is because the water (and some other) molecules have an electric moment that interracts with the radiation, causing the molecule to vibrate and heat up the food. This radiation *can* cook you, just like your food, but it would take something rather extreme (like removing the door of the microwave) to get that sort of dose. Also, since the microwaves aren't breaking DNA strands or water molecules, we don't expect there would be any long term effects from exposure.

Math Is Hard

Thanks, Grogs. Everything you have mentioned here about microwave radiation are things that I have learned in other physical science classes and have always accepted. So the comments from my chemistry teacher were very surprising. I just had to ask and see if there was something I was misunderstanding about this. I'm wondering now if I misunderstood my prof.

Gokul43201

If you didn't, she needs to be fired !

Gokul43201

The FDA requires that all microwave ovens (in the US) be equipped with at least two safety switches that turn off the magnetron when the door opens. Unless you personally dick around with the interlocks, there's no way you can get a bad dose.

Moonbear

MIH, you should ask your prof why your grapes don't glow if they're sitting behind the microwave.

Math Is Hard

well, Gokul, you know I never monkey with my microwave.. well, except for the occasional light bulbs, tin foil, and AOL CDs I put in there. Thanks for the info!

Moonbear, I am so glad you mentioned grapes, because one thing I was thinking about is that I always keep a dish of fruit on top of my microwave. Sometimes it sits there for days before I get to it - yet I have never peeled open a "cooked" banana!

Gokul43201

MIH : Is this at UCLA ? If it is, I wasn't kidding with my earlier post.

What your prof is proposing is blatantly unscientific crackpottery. No one with such a gross misunderstanding of basic science should have a PhD, let alone teach science at the college level to hundreds or thousands of students.

Alpha2005

I have heard my teacher say microwave wont give a damn harm to you and she's very sure of it :) She says if microwave is gonna harm you, then you got to stop using your mobile phone instead.

Just a thought: If microwave is such a weak EM wave(weaker than visible light), why does it bring harm? Is it because of the accumulative intensity of it?

Locrian

Your teacher is stupid. Microwaves can absolutely harm you. The microwaves produced by your home cooking device are many many orders of magnitude more powerful than the ones produced by a phone. Continued exposure will leave you blind and sterile.

As for why electromagnetic waves can harm you, I'm not sure what you are asking. Different frequencies of light have different effects on matter, and the higher the total power the stronger those effects. The 2.45ghz range your microwave uses is perfect for strong heating of soft organic materials - and we need most of our soft organic materials.

Gokul43201

I'm quite sure Alpha's teacher meant that the leakage level from a commercial microwave oven is so low that it won't harm you in any way.

Grogs

That's what I'm referring to, of course. Is there some sort of an interlock with the glass/screen on the front? I suppose it would be possible to break that out and run it. In either case, you pretty much have to *try* and get yourself cooked.

Locrian, this isn't quite correct. If you shine a beam of light onto a metal plate, the photons will knock an electron out of the metal. The number of electrons freed varies with the intensity of the light. It was found, however, that there was a certain cutoff wavelength of light. If the wavelength of the light was greater than that cutoff, no electrons would be freed, no matter how intense the beam of light was. Early 20th Century physicists were unable to explain this cutoff wavelength. They thought as you that so long as the total Energy was the same the effect should be the same. Einstein postulated that photons of light are quantized, and for a given wavelength this energy is determined by E=h*nu = h * lambda / c. If the quantized energy is less than the binding energy of the last electron in the metal, the electron cannot absorb it, and thus no electrons will be released. This is called the photoelectric effect and it's what Einstein won his nobel prize for.

When you're talking about gamma rays, they've got all sorts of energy, enough to break water molecules down, break strands of DNA, and just generally produce havoc with the human body. The damage done by these processes, especially DNA strand breaks, is what we believe causes a long term increase in the cancer rate of people receiving relatively large doses of radiation. Because the energy of microwaves is too low to cause these effects, we don't believe the long-term effects people generally mean when they're talking about radiation exposure.

Because of their fairly long wavelengths though, microwaves *can* interact with the entire water molecule. The water molecule absorbs some of the energy from the MW's and begins to vibrate more quickly, and increased kinetic energy is increased temperature. With a strong enough dose, this will kill cells, or in layman's terms it will cook you. There is some concern in the scientific community that you may get enough microwaves from your cell phone to cook a few brain cells, which especially in teenagers could add up to quite a few dead brain cells over an extended period of time. This seems unlikely to me, but scientists admit they don't completely understand the process, so they are conducting studies to determine if there may be long term risks to heavy cell-phone usage.

Monique

At work I would be walking around with a walkman on, it would zzoom if I came close to a working microwave oven. I just avoided standing close to it (also worrying that it might erase my tapes or something ), not sure whether the radiation was normal.

cronxeh

gee thanks I just coughed my coffee all over the keyboard and monitor
To answer the original question:
the microwave oven waves are 2450 MHz, and water has a dipole moment (negative on oxygen, slightly positive on hydrogen side), and when exposed to this electric field the water molecule tries to move to that field, but bumps into another water molecule, thus creating heat. This is not the resonant frequency of water, and the peak absorption of waves decreases as the temperature goes up because of the dielectric properties of water. ( Refer to http://www.lsbu.ac.uk/water/explan4.html )
I dont know why the bowl of water behind the microwave would make a difference, perhaps your professor thought of the lightning being attracted to the pointiest metal rod and drew similarities between that and the electric field and its preferrential collateral attack?
I dont know, either way, Gokul, dont say 'dicking around' that just makes me laugh uncontrollably and spilling the hot coffee all over my laptop

Gokul43201

I won't. I guess I was just a little mad at this professor at the time.


This will make more than just a practical joke. Do not attempt it !