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]

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?

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]

I'm wondering now if I misunderstood my prof.

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

 

[Gokul43201]

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.

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 won't give a damn harm to you and she's very sure of it :) She says if microwave is going to 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]

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.

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.

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.

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]

Unless you personally dick around with the interlocks, there's no way you can get a bad dose.

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 don't 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 don't know, either way, Gokul, don't say 'dicking around' that just makes me laugh uncontrollably and spilling the hot coffee all over my laptop

 

[Gokul43201]

I don't know, either way, Gokul, don't say 'dicking around' that just makes me laugh uncontrollably and spilling the hot coffee all over my laptop

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

(Quoted text removed: no need to give anyone dangerous ideas)

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

 

[Alpha2005]

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.

This is what I was talking about for:

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?

1)simplify it: Does it matter If I stand in front of the microwave for 10sec and 10 hrs?(what i mean by accumulative). I mean it does right cause this is part of wave properties, unlike X-ray or high frequency waves that shows particle properties.Please clarify :)

2)Er and also anyway I don't think what I have said is the source of causing harm to us:)

 

[Grogs]

This is what I was talking about for:
1)simplify it: Does it matter If I stand in front of the microwave for 10sec and 10 hrs?(what i mean by accumulative). I mean it does right cause this is part of wave properties, unlike X-ray or high frequency waves that shows particle properties.Please clarify :)

OK. Simply put, microwaves heat water. Other molecules are affected as well, so long as they have some sort of electric moment, but overall the effects should be the same: energy, in the form of heat is added to the body. At the same time, the body has processes for removing heat. So you've got energy in (the microwaves) and energy out (body's heat removal mechanisms.) For a long term exposure, if the microwaves are depositing energy faster than the body can remove it, the temperature in your body will rise. If it rises high enough, cells will begin to die. The critical quantity you're looking for here is the dose rate of the microwaves, measured in something like grays (Gy)/s = Joules / (kilogram * second). You've got 2 cases:

a) The dose rate is lower than this critical dose rate. Your body can remove the energy faster than it is taking it in, so no damage is done. Whether you're exposed for a second, an hour, or a week, no harm should be done.

b) The dose rate is higher than the critical rate. In this case, the longer you're exposed, the greater the damage to your body. If you're exposed for long enough, you'll die.

I suspect that, for a microwave oven, the amount of microwaves leaking out is far below the critical dose rate, meaning long term exposure carries no additional risks over a brief one.

 

[Locrian]

TIf 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.

This is of course correct, but it seemed to me to be so obvious as to not need mentioning, seeing as how microwaves are energetic enough to damage human tissues and obviously above threshold. Once that threshold is passed varying power does vary the damage caused. If you would like to test that we can stuff you into our 30kW chamber (running at a mere 915mhz) and see! The microwaves from a 1kw microwave oven are significantly more dangerous to humans than the microwaves from a cell phone. It would require very little exposure to a microwave oven to do more damage than cell phones ever do.

 

[Gokul43201]

Not sure what the debate is about here. It is known that microwaves have the right frequency to excite vibrational modes in several organic molecules as well as certain modes in water. We are not talking about photoionization or photoelectron emission...because we don't have to. Since the frequency requirement is met, the intensity and exposure time are relevant to the extent of damage done. The analogy with the PE Effect works too - the number of photoelectrons emitted is proportional to the number of incident photons.

If you can stick your hand into your oven while it's on, you will get severely burned in about the time it takes to boil a small cup of water.

 

[Grogs]

I didn't mean to set off a debate. For most direct effects from radiation, there's a cutoff energy/frequency associated with it. For long term radiation damage, the effects that concern us the most are the formation of free radicals (from the disassociation of water molecules) and DNA strand breaking. These effects have a definite cutoff energy associated with them and the energy of microwaves falls far below the cutoff. Obviously, there are other effects that happen at these lower frequencies, namely the vibration of water and other types of molecules.

The point is, you can't just integrate to find the total energy deposited in the body and say 'this is the effect.' 30 kJ of gammas will have a different effect than 30 kJ of visible light or 30 kJ of microwaves.
One big difference is that with microwaves you're looking at a cutoff intensity (dose rate) before any effects start to accumulate. With gammas and x-rays, this cutoff intensity simply doesn't exist because a single photon can cause direct and measurable damage.

 

[Alpha2005]

Okie got it. Thankz a lot for the info. Yeah, I agree with you guys that we sort of go offtopic haa.
So it's just simple :
1)Microwaves makes the water molecules vibrate thus produces heat.
2)The frequency(or energy) of microwaves is far too low to break DNA strand or cause mutation, even with a negotiable dose rate of exposure to it.

 

[microwaveguru]

The concept of keeping a glass of water inside a microwave oven goes back over 30 years and was a recommendation by oven manufacturers in order to protect the magnetron - the early magnetrons were glass envelope tubes (I have one of these ovens in my lab) that could not withstand much reflected microwave energy, However, in the late 70's and early 80's ceramic dome magnetrons were used and these can withstand lots of reflected energy. Also, the wall and floor or turntable are designed to absorb lots of energy if it is used without much in it. That's why the turntable can get bloody hot! So, there is absolutely no need to keep water inside the oven.

 

[microwaveguru]

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?

That is a not necessary and, in fact, will negatively affect the heating rate. This concept arises because early microwave ovens, pre-mid 1970's had glass envelope magnetrons (the microwave generating tube I have such an oven in my lab - it's huge) that could not take much reflected microwave power and could be damaged if the oven was run empty, with very little in it, or even frozen food (look on another thread for my answer regarding microwaving ice). So, you were told by manufacturers and in microwave cookbooks to always keep a cup of water in the oven in order to protect the magnetron. Today's ovens use ceramic dome magnetrons that can withstand large amounts of reflected energy without damage. So, there is o longer a need for the water.