If a microwave oven door were to open while it was still on, what would happen? Could it hurt you? - JP
The microwaves would flow out of the oven's cooking chamber like light streaming out of a brightly illuminated mirrored box. If you were nearby, some of those microwaves would pass through you and your body would absorb some of them during their passage. This absorption would heat your tissue so that you would feel the warmth. In parts of your body that have rapid blood circulation, that heat would be distributed quickly to the rest of your body and you probably wouldn't suffer any rapid injuries. But in parts of your body that don't have good blood flow, such as the corneas of your eyes, tissue could heat quickly enough to be permanently damaged. In any case, you'd probably feel the warmth and realize that something was wrong before you suffered any substantial permanent injuries.
Are microwaves attenuated in air?
Not significantly. Air doesn't absorb them well, which is why the air in a microwave oven doesn't get hot and why satellite and cellular communication systems work so well. The molecules in air are poor antennas for this long-wavelength electromagnetic radiation. They mostly just ignore it.
There is a story circulating by email about a 26 year old man who heated a cup of water in a microwave oven and had it "explode in his face" when he took it out. He suffered serious burns as a result. Is this possible and, if so, how did it happen? -- JJ, Kirksville, Missouri
Yes, this sort of accident can happen. The water superheated and then boiled violently when disturbed. Here's how it works:
Water can always evaporate into dry air, but it normally only does so at its surface. When water molecules leave the surface faster than they return, the quantity of liquid water gradually diminishes. That's ordinary evaporation. However, when water is heated to its boiling temperature, it can begin to evaporate not only from its surface, but also from within. If a steam bubble forms inside the hot water, water molecules can evaporate into that steam bubble and make it grow larger and larger. The high temperature is necessary because the pressure inside the bubble depends on the temperature. At low temperature, the bubble pressure is too low and the surrounding atmospheric pressure smashes it. That's why boiling only occurs at or above water's boiling temperature. Since pressure is involved, boiling temperature depends on air pressure. At high altitude, boiling occurs at lower temperature than at sea level.
But pay attention to the phrase "If a steam bubble forms" in the previous paragraph. That's easier said than done. Forming the initial steam bubble into which water molecules can evaporate is a process known as "nucleation." It requires a good number of water molecules to spontaneously and simultaneously break apart from one another to form a gas. That's a rare event. Even in a cup of water at several degrees above the boiling temperature, you might have to wait minutes before such a rare event occurred. In reality, it usually occurs at a defect in the cup or an impurity in the water--anything that can help those first few water molecules form the seed bubble. When you heat water on the stove, the hot spots at the bottom of the pot or defects in the pot bottom usually assist nucleation so that boiling occurs soon after the boiling temperature is reached. But when you heat pure water in a smooth cup using a microwave oven, there is virtually nothing present to help nucleation occur. The water can heat right past its boiling temperature without boiling. The water then superheats--its temperature rising above its boiling temperature. When you shake the cup or sprinkle something like sugar or salt into it, you initiate nucleation and the water then boils violently.
Fortunately, serious microwave superheating accidents are unusual--this is the first injury I've ever heard about. You could minimize the chance of this sort of problem by deliberately nucleating boiling before removing the cup from the microwave. Inserting a metal spoon or almost any food into the water should trigger boiling in superheated water. A pinch of sugar will do the trick, something I've often noticed when I heat tea in the microwave.
If a microwave oven with painted inside walls has some of the paint removed due to a very small fire caused by arcing, is it still safe to use?
Yes. The paint is simply decoration on the metal walls. The cooking chamber of the microwave has metal walls so that the microwaves will reflect around inside the chamber. Thick metal surfaces are mirrors for microwaves and they work perfectly well with or without thin, non-conducting coatings of paint.
My science book said that a microwave oven uses a laser resonating at the natural frequency of water. Does such a laser exist or was that a major typo?
It's a common misconception that the microwaves in a microwave oven excite a natural resonance in water. The frequency of a microwave oven is well below any natural resonance in an isolated water molecule, and in liquid water those resonances are so smeared out that they're barely noticeable anyway. It's kind of like playing a violin under water--the strings won't emit well-defined tones in water because the water impedes their vibrations. Similarly, water molecules don't emit (or absorb) well-defined tones in liquid water because their clinging neighbors impede their vibrations.
Instead of trying to interact through a natural resonance in water, a microwave oven just exposes the water molecules to the intense electromagnetic fields in strong, non-resonant microwaves. The frequency used in microwave ovens (2,450,000,000 cycles per second or 2.45 GHz) is a sensible but not unique choice. Waves of that frequency penetrate well into foods of reasonable size so that the heating is relatively uniform throughout the foods. Since leakage from these ovens makes the radio spectrum near 2.45 GHz unusable for communications, the frequency was chosen in part because it would not interfere with existing communication systems.
As for there being a laser in a microwave oven, there isn't. Lasers are not the answer to all problems and so the source for microwaves in a microwave oven is a magnetron. This high powered vacuum tube emits a beam of coherent microwaves while a laser emits a beam of coherent light waves. While microwaves and light waves are both electromagnetic waves, they have quite different frequencies. A laser produces much higher frequency waves than the magnetron. And the techniques these devices use to create their electromagnetic waves are entirely different. Both are wonderful inventions, but they work in very different ways.
The fact that this misleading information appears in a science book, presumably used in schools, is a bit discouraging. It just goes to show you that you shouldn't believe everything read in books or on the web (even this web site, because I make mistakes, too).
Would it be possible to put a thermometer inside a microwave oven? Would the microwaves have an effect on an electronic thermometer? Would they have an effect on a mercury thermometer? -- R
This is an interesting question because it brings up the tricky issue of what is the temperature in a microwave oven. In fact, there is no specific temperature in the oven because the microwaves that do the cooking are not thermal. Rather than emerging from a hot object with a well-defined temperature, these microwaves are produced in a coherent fashion by a vacuum tube. Like the light emerging from a laser, these microwaves can heat objects they encounter as hot as you like, or at least until heat begins to escape from those objects as fast as it's being added.
So instead of measuring the "temperature of the microwave oven," people normally put thermometers in the food to measure the food's temperature. This works well as long as the thermometers don't interact with the microwaves in ways that make them either hotter or inaccurate. Electronic thermometers are common in high-end microwaves. There is nothing special about these electronic thermometers except that they are carefully shielded so that the microwaves don't heat them or affect their readings. By "shielded," I mean that each of these thermometers has a continuous metallic sheath that reflects the microwaves. This sheath extends from the wall of the oven's cooking chamber all the way to the thermometer probe's tip so that the microwaves themselves can't enter the measurement electronics. Since the sheath reflects microwaves, the thermometer isn't heated by the microwaves and only measures the temperature of the food it contacts.
On the other hand, putting a mercury thermometer in a microwave oven isn't a good idea. While mercury is a metal and will reflect most of the microwaves that strike it, the microwaves will push a great many electric charges up and down the narrow column of mercury. This current flow will cause heating of the mercury because the column too thin to tolerate the substantial current without becoming warm. The mercury can easily overheat, turn to gas, and explode the thermometer. (A reader of this web site reported having blown up a mercury thermometer just this way as a child.) Moreover, as charges slosh up and down the mercury column, they will periodically accumulate at the upper end. Since there is only a thin vapor of mercury gas above this upper surface, the accumulated charges will probably ionize this vapor and create a luminous mercury discharge. The thermometer would then turn into a mercury lamp, emitting ultraviolet light. I used microwave-powered mercury lamps similar to this in my thesis research fifteen years ago and they work very nicely.
You said that microwaves heat food by twisting water molecules back and forth and having those water molecules rub against one another to experience a molecular form of "friction." Since vibrating molecules are the fundamental manifestation of heat, why is the friction necessary at all? -- GS, Kanata, Canada
While it's true that microwaves twist water molecules back and forth, this twisting alone doesn't make the water molecules hot. To understand why, consider the water molecules in gaseous steam: microwaves twist those water molecules back and forth but they don't get hot. That's because the water molecules beginning twisting back and forth as the microwaves arrive and then stop twisting back and forth as the microwaves leave. In effect, the microwaves are only absorbed temporarily and are reemitted without doing anything permanent to the water molecules. Only by having the water molecules rub against something while they're twisting, as occurs in liquid water, can they be prevented from remitting the microwaves. That way the microwaves are absorbed and never remitted--the microwave energy becomes thermal energy and remains behind in the water.
Visualize a boat riding on a passing wave--the boat begins bobbing up and down as the wave arrives but it stops bobbing as the wave departs. Overall, the boat doesn't absorbed any energy from the wave. However, if the boat rubs against a dock as it bobs up and down, it will converts some of the wave's energy into thermal energy and the wave will have permanently transferred some of its energy to the boat and dock.
You said that some rooms in the physics building are made with metal to specifically keep electromagnetic waves out. How does that work?
Some experiments are so sensitive to electromagnetic waves that they must be performed inside "Faraday cages". A Faraday cage is a metal or metal screen box. Its walls conduct electricity and act as mirrors for electromagnetic waves. As long as a wave has a wavelength significantly longer than the largest hole in the walls, that wave will be reflected and will not enter the box. This reflection occurs because the wave's electric field pushes charges inside the metal walls and causes those charges to accelerate. These accelerating charges redirect (absorb and reemit) the wave in a new direction--a mirror reflection. Just as a box made of metal mirrors will keep light out, a box made with metal walls will keep electromagnetic waves out.
Can microwave ovens leak microwaves? Is my mother's warning not to stand in front of the microwave while it's on valid?
A properly built and maintained microwave oven leaks so little microwave power that you needn't worry about it. There are also inexpensive leakage testers available that you can use at home for a basic check, or for a more reliable and accurate check--as recommended by both the International Microwave Power Institute (IMPI) and the FDA--you can take your microwave oven to a service shop and have it checked with an FDA certified meter. It's only if you have dropped the oven or injured its door in some way that you might have cause to worry about standing near it. If it were to leak microwaves, their main effect would be to heat your tissue, so you would feel the leakage.
The frequency at which microwave ovens operate is about 2.45 GHz, which is about the resonant frequency of the free water molecule. Can you calculate this resonant frequency or was it determined experimentally? -- GW
While most microwave ovens operate at 2.45 GHz, that frequency is not a resonant frequency for the water molecule. In fact, using a frequency that water molecules responded to strongly (as in a resonance) would be a serious mistake--the microwaves would all be absorbed by water molecules at the surface of the food and the center of the food would remain raw. Instead, the 2.45 GHz frequency was chosen because it is absorbed weakly enough in liquid water (not free water molecules) that the waves maintain good strength even deep inside a typical piece of food. Higher frequencies would penetrate less well and cook less evenly. Lower frequencies would penetrate better, but would be absorbed so weakly that they wouldn't cook well. The 2.45 GHz frequency is a reasonable compromise between the two extremes.
When I warm more than one cup of coffee or milk together in a microwave oven, some of them warm more than others. Why does this happen? Is there something wrong with our microwave oven? -- ON, Istanbul, Turkey
When the microwaves bounce around inside the oven's cooking chamber, they experience an effect called interference. Interference occurs when similar waves, or portions of the same wave, follow different paths to the same region in space. As they pass through that region, their crests and troughs ride up on top of one another and they interfere. Sometimes the crests of one wave ride on the crests of the other wave, creating enormous crests--an effect called constructive interference. However, it is also possible for the crests of one wave to ride on the troughs of the other wave, so that they cancel one another out--an effect called destructive interference.
These interference effects are quite visible in wave waves, but they also make themselves apparent in microwaves. In your oven, they lead to regions of the cooking chamber that heat quickly (regions where the microwaves experience constructive interference) and regions that don't heat well at all (regions where they experience destructive interference). Because these fast and slow cooking regions can't be avoided, many microwave ovens incorporate turn tables to keep the food moving through the various regions inside the oven. Some ovens use rotating metal paddles to stir that microwaves around inside the cooking chamber, so that the fast and slow cooking regions move about.
Your experience with uneven heating of coffee or milk is an example of this interference problem. The solution is to move the cups occasionally while they are being heated.
You claim that the metal walls of the cooking chamber in a microwave oven protect us from the microwaves. How can they protect us from microwaves when they aren't even able to keep sound contained? You can hear popcorn popping through the walls. -- RB, Beltsville, MD
The fact that sound waves can pass through the cooking chamber's metal walls doesn't mean that microwaves can. These two types of waves are very different and the chamber's walls handle them very differently.
Any type of wave will partially reflect from a surface if passing through that surface causes the wave's speed to change or, more generally, introduces a change in the "impedance" the wave experiences. Impedance is a quantity that relates various parts of a wave to one another--it relates pressure to velocity in sound and it relates the electric field to the magnetic field in a microwave. Since both sound waves and microwaves change speeds and impedances when they encounter the cooking chamber's metal walls, they both partially reflect. The sound that you hear when popcorn pops inside the oven is slightly muffled because the sound is having some trouble escaping from the cooking chamber. However, the impedance change for the microwaves is so enormous that the reflection is complete. No microwaves at all escape from the cooking chamber! The same effect occurs when you hold a large mirror up in front of your face. You can hear what's happening on the other side of the mirror because some sound can pass through the mirror. But light is completely reflected and you can't see through the mirror at all.
Could microwave heating be used to treat sewage to wipe out disease organisms in it? -- KO
While microwave heating could be used to sterilize sewage, it's not the most energy efficient or inexpensive technique. Microwave heating is really only worthwhile in cases where you can't reach the inside of an object directly--as is the case in most solid foods. Since sewage is essentially liquid, it can be heated quickly and efficiently by passing it close to a hot surface. Just about anything can be used to heat that surface--electricity, natural gas, coal, you name it.
But to be even more energy efficient, the sewage that was just sterilized a minute ago and is still hot can be used to heat the sewage that is about to be treated! A well designed thermal treatment facility could employ "counter-current exchange"--that is it could pass the hot, treated material through a heat exchanger to allow it to transfer most of its excess heat to the cooler, untreated material that is about to be sterilized. By recycling the heat in this manner, the facility could avoid having to burn so much fuel. The only drawback with this technique is that the heat exchanger must be leak-proof--it must keep the sterilized material from touching and being contaminated by the unsterilized material.
What about the effects of microwaves on the cellular structure of the item in the oven? I've heard that cells are ruptured violently by microwave radiation and that the ingestion of such materials affects the immune system. - AB
Just about any cooking damages the cells of the food being cooked, so microwave cooking is nothing unusual. Since our digestive systems destroy cells in the food we eat, cellular damage in cooking is inconsequential. As for the rumors about the unhealthiness of food cooked in a microwave oven, these are simply myths promulgated by people who don't understand what microwaves are and fear them irrationally. The world was awash in microwaves from natural sources long before the developments of electricity and microwave ovens.
Is there a standard time that one should wait before eating food that has been heated in a microwave oven? - M
Apart from the usual precautions with hot food, there is nothing unsafe about food cooked in a microwave oven. You can eat it the instant the microwave oven turns off. The microwaves in the oven are absorbed so quickly that they vanish almost immediately after the oven stops producing them. By the time you get the oven door open, there is nothing hazardous left inside the cooking chamber or in the food. However, a microwave oven tends to heat foods unevenly, particularly if they were initially frozen. Thus you should be careful to stir the food or test its temperature at various places so that you don't burn yourself. You should be particularly wary of solid foods, such as raisin biscuits, that are generally dry but have moist, microwave-absorbing objects inside them. Those moist objects can become dangerously hot and have been known to cause life-threatening burns in people who tried to swallow them without letting them cool off.
That said, a reader notes that the uneven cooking in a microwave oven can lead to bacterial safety problems--if parts of the food aren't heated sufficiently to kill dangerous bacteria, then you could be exposing yourself to those bacteria. He suggests using the microwave oven for reheating only. He also notes that the lack of surface heating leaves the food relatively tasteless, as compared to more conventional cooking.
Does microwave cooking affect the nutritional value of food?
No more so than conventional heating does. Overheating some nutrients can damage them, so that microwave cooking does affect food's nutritional value. But microwave cooking is far less likely to cause serious molecular damage to food than flame broiling or frying.
I recently place a green tomato in the microwave oven. I forgot to turn on the microwave and in the morning the tomato was ripe. Can you explain this? -- KH
No. When a microwave oven is off, the cooking chamber contains nothing special at all--just some trapped air and perhaps a little light that enters through the window. Even when it is operating, a microwave oven never produces any ionizing (high energy) radiation so there are no long term effects such as radioactivity present in the cooking chamber when the oven is off. The tomato was simply sitting in a sealed metal box overnight. Since some fruits ripen faster in sealed environments, perhaps that accounts for your observation.
When two identical items are cooked, one with a microwave oven and the other on the stove, which will cool faster? -- CR
If the distributions of temperatures inside the items were the same after cooking, they would cool at the same rate. However, a microwave oven tends to cook relatively evenly throughout the food while the stove tends to cook from the outside of the food inward. That means that food cooked in a microwave oven tends to have more thermal energy near its center than food cooked on a stove, even when those foods contain the same total amount of thermal energy. Since foods lose heat through their surfaces, the extra thermal energy in the food cook by microwave will take longer to flow out to the surface of the food and from there to its surroundings. All else being equal, I would expect the food cooked in the microwave oven to cool slightly slower than the food cooked on the stove top.
I know that microwaves only heat polar molecules but what about aluminum foil and graphitic carbon, which are both heated by microwaves even though they have no dipole moments? -- EB
Aluminum foil and graphitic carbon are both conductors of electricity. When they're exposed to microwaves, the electric fields in those microwaves causes currents to flow through them. If the aluminum were thick enough, it would be able to handle the currents without trouble. But aluminum is very thin and the current that flows through it may be more than it can tolerate, particularly if it's only a narrow strip. It then becomes very hot. The effect is the same as would happen if you plugged the aluminum foil into an electric outlet and sent current through it that way. The same heating occurs in the carbon--the current that flows in it heats it up. In short, relatively poor conductors of electricity become hot in a microwave because they permit currents to flow in response to the microwave electric fields but then can't tolerate those currents without becoming hot.
Assuming microwave ovens cook on the principle of "moist" heat cookery, what are the general effects of microwave cooking on various foods, including effects on chemical structure? -- EJ, Sydney, Australia
Microwave ovens cook by depositing thermal energy in the water molecules, which isn't the same as cooking food in moist hot air. Microwave cooking tends to heat food uniformly throughout where as more conventional "moist" heat cooking still heats food from the outside in. Nonetheless, the chemical effects on food are very similar for both types of cooking. Virtually all of these effects are caused by elevating the temperatures of the food. I'm not an expert on the chemistry of cooking, but elevated temperatures certainly denature proteins and caramelize sugars.
I know that an electromagnetic wave cannot pass through the holes in a metal cage (a Faraday cage) if those holes are significantly smaller than the wavelength of the wave. But what if it is just a constant electric field? What determines the hole size now? -- KBH, Logan, Utah
If the electric field isn't changing with time, then it can't enter a metal cage no matter how large the cage's holes are. In effect, the constant electric field has an infinite wavelength and can't propagate through holes of any finite size. However, the holes don't stop an electromagnetic wave instantly--the wave does penetrate a short distance into the cage before it dwindles to insignificance. The distance over which the wave diminishes by a factor of about 3 is roughly the size of the hole through which it is trying to pass. So if your Faraday cage has holes that are 1 centimeters in diameter, the constant electric field will take several centimeters to diminish to nearly zero. If the holes are much larger than that, the electric field will penetrate far into the cage and the cage will only be an effective shield if it is extremely large. To avoid having to use a very large cage, it's better to use small holes.
How do microwave ovens affect people fitted with pacemakers? -- W
If a microwave oven doesn't leak microwaves, then it won't affect such people at all. However, if microwaves do leak from a particular microwave oven, they will cause undesirable currents to flow in the electric leads of the pacemaker. That's because a microwave consists of electric and magnetic fields, and an electric field exerts forces on charged particles. The mobile charged particles in the pacemaker's electric wiring will experience these forces as the microwave encounters them and they will move back and forth with the microwave's fluctuating electric field. The pacemaker's wiring isn't meant to carry these unexpected current flows, and the pacemaker and/or the person attached to it may experience unpleasant effects. While such problems are very unlikely, it makes sense to warn pacemaker users whenever a microwave oven is in use.
Do hand carried microwave heaters exist or must the microwaves always be enclosed, as they are in a microwave oven? -- AL, Umea, Sweden
My understanding is that there are microwave heating systems that are not enclosed and that are used in medical therapies to provide deep warming to injured tissues in medical patients. But apart from such devices, I've never heard of unenclosed microwave heaters. That's because such heaters would be dangerous, since a user would be exposed to the heating effects of the microwaves. To keep the microwave heating under control, microwave ovens always carefully enclose the microwaves in a metal cooking chamber from which they can't escape.
Why does food become soggy after heating in the microwave oven; particularly pastry?
A normal oven heats foods by exposing them to hot air and thermal radiation. It cooks the foods from the outside in. As a result, a normal oven tends to make the surfaces of food dry and crispy because it heats those outer surfaces first and drives the water out of them. A microwave oven heats the food by heating the water in that food. It cooks foods from the inside out. As a result, a microwave oven tends to drive water out of the middle of the food and into the outer layers of that food. The outer layers are essentially "steamed" and steaming makes everything soggy.
Why can you put a can of frozen concentrate juice in the microwave? The metal doesn't spark or burn.
The microwaves in a microwave oven consist of electric and magnetic fields. Since electric fields push on electric charges, microwaves cause electric currents to flow through any metal objects they encounter. These movements of current don't necessarily cause any problems in a microwave oven. In fact, metal objects only cause trouble in the microwave oven when they are so thin or narrow that they can't tolerate the electric currents that flow through them or when they have such sharp ends that electric charges leap off them as sparks. A thin object like a twist-tie can't tolerate the currents and becomes very hot. Its sharp ends also allow charges to leap out into the air as sparks. But the thick, rounded end of a juice concentrate can easily tolerates the currents sent through it by the microwaves and doesn't have the sharp ends needed to send charges into the air as sparks. It doesn't present any problem for the microwave oven.
If you stand near a microwave oven, looking at your food, is it dangerous--tissue damage or make you blind?
Properly built and undamaged microwave ovens leak so few microwaves that they aren't dangerous at all. Even if they did leak enough to be in violation of the safety limits, those safety limits are very conservative. While there is no reason to court disaster by holding your face right up to the microwave for hours and hours, it shouldn't hurt you at all.
Is it possible isolate a room or part of it totally from microwaves? -- DMJ
Because conducting surfaces reflect electromagnetic waves, you can shield a room from electromagnetic waves by enclosing it in conducting surfaces. For example, a room surrounded by metal mirrors will be completely black inside because light won't be able to enter it. Furthermore, if the electromagnetic waves that you're trying to exclude have reasonably long wavelengths, you can put holes in the conducting surfaces because electromagnetic waves can't pass through holes in a conducting surface if those holes are substantially smaller than their wavelengths. So, to shield your room from microwaves, I'd suggest enclosing it in copper screening with holes that are no more than a few millimeters in diameter. Many scientific experiments are performed in such screen rooms, which are generally called Faraday cages.
Have you made RF leakage measurements on a sample of microwave ovens? I understand that the FDA requires that if measured 5 cm away from any of the oven's surfaces, the RF leakage must be less than 1 mW/cm2 for new ovens and less than 5 mW/cm2 over the oven's life time. I'm just curious what actual measurements reveal about a "typically used" oven. -- S
I've measured several ovens and have only found one that leaks a measurable amount of microwave power. That leaker is an oven that I've used in countless demonstrations and have taken apart several times (it appears on page 514 of my book). Considering the abuse that poor oven has had, it's doing pretty well. At a talk I gave yesterday, I couldn't get it to leak more than about 1 mW/cm2 even though I was measuring microwave power directly on the edge of the oven door--the most vulnerable point in the oven. Given that this oven's door sags several millimeters as the result of its rough treatment, that's not bad. In short, I doubt that there are many leaky microwave ovens around that haven't been dropped, crushed in shipping, or otherwise suffered serious mechanical injury.
Is it true that microwaves cause cancer?
I think that it's very unlikely that microwaves cause cancer. Microwaves are not ionizing radiation--they don't directly damage chemical bonds. Instead, they heat materials, particularly those containing water. As a result, they may cause damage to proteins in the same way that cooking damages proteins (and hardens egg protein, for example). But while such protein damage can easily cause cell death, I wouldn't expect it to cause the genetic damage associated with cancer.
Is it harmful for children to sit too close to microwave ovens? Is it possible to get "burned" opening the microwave oven during a cycle or too soon after a cycle? I realize the oven shuts off, but is there residual radiation? -- C
As long as the microwave oven hasn't been damaged and doesn't leak excessive microwaves, there should be no harm in having children sit near it. I wouldn't hold my face right up against the door edges because that would be asking for trouble with leakage, but it's extremely unlikely that even doing that once in a while would cause injury.
As for being injured by microwave radiation after the cycle has stopped, that's essentially impossible. As soon as the high voltage disappears from the magnetron tube and it stops emitting microwaves, the microwaves in the cooking chamber begin to diminish. Even if they bounce 1000 times off the metal walls of the chamber before they're absorbed by those walls or the food in the microwave, that will only take about 2 millionths of a second. You can't open the door fast enough to let them out before they're already gone.
What metals and other substances are used in microwave ovens? Specifically, what is the substance on the inside of the microwave that absorbs all the microwaves? -- AD, San Anselmo, CA
The walls of a microwave oven's cooking chamber are made of highly conductive metals so that they reflect the microwaves almost completely. Only a very small fraction of the microwaves inside the oven are absorbed by these metal walls and virtually none of the microwaves escape into the room. However, there is a substance inside the cooking chamber that absorbs the microwaves: water in the food! If you don't put water-containing food inside the microwave oven, there will be nothing to absorb the microwaves and they will reflect back to the magnetron and may damage it. The absence of an absorber in the cooking chamber will also increase any minor leakage of microwaves from the oven because the microwave intensity inside the cooking chamber will be much higher than normal.
Why does microwave radiation affect plant seeds differently? If you microwave sunflower seeds 30 seconds, they germinate faster than if you did not microwave them at all, and yet if you microwave them for 60 seconds, the seeds do not germinate at all. If you do this same experiment with carrot seeds, the non-radiated seeds, the 30 second and 60 second seeds all germinate within 14 days. Why? Is it because the sunflower seeds are larger and absorb more radiation than the smaller carrot seeds? -- ST, Mobile, AL
When you expose the seeds to microwave radiation, you are selectively heating portions of the insides of the seeds. Fats and oils don't absorb microwaves well but water does, so the parts of the seeds that become hottest are those that contain the most water molecules. Evidently, heating the water-containing portion of a sunflower seeds slightly cause that seed to germinate faster, but heating that same portion too much sterilizes the seed. That observation indicates that a moderate temperature rise causes the chemical reactions of germination to occur more rapidly while a more severe temperature rise denatures some of the critical biological molecules and kills the seed. The absence of any effect in carrot seeds may indicate that they don't have enough water in them to absorb the microwaves. It may also indicate that they can tolerate higher temperatures without undergoing the chemical reactions of germination and without experiencing damage to their critical molecules.
I heard recently of someone with a pacemaker who went near a microwave oven and his pacemaker faulted, with him needing urgent medical attention. How did this happen? I also know of someone currently undergoing chemotherapy, who was told by his doctor not to eat food from a microwave oven. Why?
A pacemaker contains electronic circuits and wires that can act as antennas for microwaves. If a pacemaker is exposed to sufficiently intense microwaves, currents will begin to flow in those wires and circuits, and these currents may cause computational errors to occur or they may cause the circuitry to overheat. But while a pacemaker is far more sensitive to microwave radiation than say your hand is, I'm still surprised that enough microwave radiation leaked out of the oven to cause trouble. I'd suspect a real problem with that oven.
As for the chemotherapy question, I can't think of any reason why the doctor would suggest avoiding cooking food in a microwave oven. Unless I hear otherwise, I would suspect ignorance on the part of the doctor. The doctor may not understand the difference between "microwave radiation" and "gamma radiation".
Is there an inexpensive device for detecting leaks from a microwave oven?
Yes. You can get one from a hardware or appliance store for about $5 to $10. MCM Electronics sells one by mail order at (800) 543-4330, part number 72-4080, price $6.99. They also sell a device called "Magic Light", part number 72-6110, price $7.99, that lights up to show you where the microwaves are strongest--it's great fun to watch. While I have tended to downplay the leakage issue in the past, I bought one of the MCM testers and found that the microwave oven in my laboratory actually leaked significantly. I had used it in many class demonstrations, so it had been abused and the door wasn't properly aligned any more. I retired it. Incidentally, the tester contains only two components: a fast diode and a current meter. It detects microwave in the same way that a crystal radio detects an AM radio broadcast. However, I should note that both the International Microwave Power Institute (IMPI) and the FDA caution against trusting those simple and not particularly accurate meters, and recommend that you take your microwave oven to a service shop for inspection with an FDA certified meter.
What are the key components of a microwave oven?
In addition to the digital controller that runs the microwave, it contains (1) a power relay that allows the controller to turn on and off the microwave source, (2) a power transformer that produces the high voltage electricity needed by the magnetron, (3) a power rectifier that converts the alternating current from the transformer into the direct current needed by the magnetron, (4) a capacitor that smoothes out ripples in the direct current leaving the rectifier, (5) a magnetron that uses the high voltage direct current to produce an intense beam of microwaves, (6) a wave guide that transports the microwaves from the magnetron to the cooking chamber, and (7) a cooking chamber in which the food absorbs the microwaves and becomes hotter.
Do microwaves have no effect on gas?
While water vapor can absorb microwaves at certain frequencies, the absorption mechanism is very different from the one that causes liquid water to become hotter. Steam isn't affected very much by a microwave oven.
Can one's health be adversely affected by the use of certain wraps, films, or containers, when heating food in the microwave?
When various plastics become hot, their molecules become more mobile. The most obvious such case is when a plastic actually melts. But even before it melts, a plastic can begin to lose molecules to objects that are touching it. However, the plastics used in cooking are pretty non-toxic, so that even eating pieces of those plastic won't cause you any significant trouble. On the other hand, I would be careful with plastics that weren't intended for cooking. Some non-food related plastics are mixed with additives called "plasticizers" that keep them softer than they would be if they were pure. These plasticizers have a tendency to migrate out of the plastics, giving such things as "vinyl" their characteristic odors. Heating a plastic containing a plasticizer can drive this plasticizer out of the plastic and into something else. I don't think that it's a good idea to eat plasticizers so I would suggest not cooking with plastics that weren't intended for use with food. Still, not all plasticizers are bad--water is an excellent plasticizer for such common plastics as hair and cotton.
In your explanation of why microwaves don't penetrate the oven door, you said it is because the holes in the screen are smaller than the wavelength of a microwave. Wouldn't it be the amplitude of the wave and not its wavelength? - P
When a microwave tries to pass through the holes in the metal screen, electric charges in that screen begin to move. The microwave's electric field fluctuates back and forth rapidly and the charges reverse directions rapidly as a result. If the electric current made up of these charges has enough time to travel all the way around each hole before it reverses directions, it will be as though the screen were made of solid metal and the screen will be able to completely reflect the microwave.
Like any electromagnetic wave, a microwave has a wavelength (the spatial distance between adjacent wave crests) and a period (the temporal spacing between adjacent wave crests). The electric current that a microwave propels through a metal travels about one microwave wavelength during one microwave period. Therefore, the current can work its way around a hole in the metal only if the hole is significantly smaller than the microwave wavelength. The amplitude of the microwave doesn't matter--increasing the amplitude of the microwave just makes more current flow.
In cooking, what are some examples of absorbing microwaves, transferring microwaves, and reflecting microwaves? - K
In a microwave oven, water-containing foods absorbs microwaves. The microwaves disappear as they pass through the food and the food becomes hotter. Microwaves are transferred from the small antenna near the magnetron to the cooking chamber by sending those microwaves through a metal pipe. This rectangular pipe is typically a few inches wide and an inch or so tall, and is called a "wave guide." Finally, the walls of the cooking chamber reflect the microwaves. When a microwave encounters a metal surface, it pushes electric charges back and forth in the metal and this moving charge causes the microwave to reflect.
Where is the best place to put a microwave oven? Is it dangerous to place it on the refrigerator? - PTW
You can put a microwave oven anywhere that it's stable and where it has adequate ventilation. A microwave oven has a fan and vents through which it gets rid of its excess heat. You mustn't block the vents or the oven will overheat.
Is it possible to eat a microwave while you eat food that was cooked in the microwave oven? - PTW
Not one that came from the microwave oven. Microwaves are all around us and are completely innocuous. Your body emits weak microwaves all the time, as part of its thermal radiation! Like light, microwaves don't remain still in objects so you can't eat one that was put in the food by the oven.
If the condenser in a microwave is bad, what is the most likely reaction the microwave generator will exhibit? -- IF, Bakersfield, CA
Once again, I'm not really an expert on microwave oven repair. However, I'd expect the broken capacitor to act as a short circuit, in which case the microwave will probably blow a fuse or trip a circuit breaker.
How can we clean the microwave oven? - PTW
Since the cooking chamber of a microwave oven doesn't get hot, there is no way to make a "self-cleaning" microwave oven. Instead, you have to clean it by hand with a sponge and perhaps a little soapy water. As long as you get the soap or any other cleaning agents out, you can clean the cooking chamber just as you'd clean the top of a stove.
Don't microwaves penetrate metal at all? -- DR, Tampa, FL
If the metal is a good conductor, then the microwaves don't penetrate more than a fraction of a millimeter. That's because the microwave electric fields push on the metal's mobile electrons and those electrons immediately rearrange in such a way that they cancel the microwave fields inside the metal. Only the skin of the metal responds to the fields and it shields the rest of the metal from the microwaves.
How can I check the magnetron in a home microwave oven? I have checked the HV (high voltage) transformer, the rectifier, and capacitor and all are OK. Does the magnetron output decrease with age? The oven has a hum which is much louder than normal. -- AA, Ontario, CA
While I have only a little experience repairing microwave ovens, I can make reasonable guesses. The loud hum you hear is probably an indication that something is overloading the power transformer. That suggests that the diode, capacitor, or magnetron are bad. If you have checked the first two carefully, at full operating voltage, and found no problems, then I would suspect the magnetron. I think that magnetrons fail suddenly when their filaments burn through. When that happens, the pieces may short against the insides of the tube and then draw excessive currents from the high voltage transformer. That has probably happened in your case. Still, free advice like mine is only worth what you've paid for it. I'd suggest you consult a local repairperson, who has test equipment that can pinpoint the problem in seconds.
Is heating milk by microwave advisable? - I
Microwave cooking leaves no permanent mark on the food. It causes virtually no chemical damage and absolutely no radioactivity. The only drawback with heating milk by microwave is that the heating may be uneven and may denature some protein molecules in regions of the milk that become excessively hot. Since most protein molecules are disassembled by your digestion anyway, this treatment probably has no effects worth worrying about. Even with infant formula, my only concern would be the hot spots. If you carefully shake the milk after heating, so that its temperature is uniform, it should be just fine. I suspect that companies warn you not to heat milk in a microwave because they are worried that you will either not shake the milk to distribute its temperature evenly or that you will overcook it until it boils and the bottle explodes.
Why do metal objects spark/arc in the microwave? Why don't the metal walls of the microwave spark? - JR
Like all electromagnetic waves, microwaves are composed of electric and magnetic fields. Since an electric field exerts forces on charged particles, a microwave pushes electrons back and forth through any metals it encounters. It is this motion of electrons back and forth through the metal walls of the microwave oven that allow that metal to reflect the microwaves and keep them inside the oven. If you leave a spoon in you cup of coffee as you heat it in the microwave, electrons will move back and forth through the spoon. This motion of charge will cause no problems so long as (1) the spoon can tolerate this flow of charge without overheating and (2) the spoon doesn't allow the charges at its ends to leap into the air as a spark. To keep the spoon from overheating, it must be a good conductor of electricity. Since most spoons are pretty thick, the modest currents flowing through them in the microwave will leave little energy inside them and they won't overheat. But a thin twist-tie or small bit of aluminum foil may well overheat and begin to burn. To keep the spoon from sparking, it should have smooth ends. Electrons are more likely to leave the end of a metal surface at a sharp point, so avoiding points is important. Most spoons are smooth enough that no sparks will occur. But a fork, a sharp piece of foil, or a twist-tie may well begin to emit electrons into the air as those electrons pile up at one end of the wire while the microwave oven is on. Like a spoon, the walls of the oven are good conductors of electricity and they have no sharp points. While electrons move back and forth in these walls, they simply reflect the microwaves without becoming very hot and without emitting any sparks. You'll note that the light bulb for the microwave is always outside the cooking chamber because it contains small bits of metal that would have trouble inside a microwave oven.
Why is it bad to put metal in a microwave oven? - OR
It isn't necessarily bad to put metal in a microwave oven, but it can cause cooking problems or other trouble. Microwaves causes currents to flow in metals. In a thick piece of metal, these currents won't cause problems for the metal. However, in thin pieces of metal, the currents may heat the metal hot enough to cause a fire. Metallic decorations on fine porcelain tend to become hot enough to damage the porcelain. But even thick pieces of metal can cause problems because they tend to reflect the microwaves. That may cause cooking problems for the food nearby. For example, a potato wrapped in aluminum foil won't cook at all in a microwave oven because the foil will reflect the microwaves. The currents flowing in the metal can also produce sparks, particularly at sharp points, and these sparks can cause fires. In general, smooth and thick metallic objects such as spoons aren't a problem, but sharp or thin metallic objects such as pins or metal twist-ties are.
What is the black holey stuff on the doors of microwave ovens? Is it for looks, protection, or what? - K
The black holey stuff on the window of a microwave oven is a metal shield that keeps the microwaves inside the cooking chamber. Because the holes in this metal sheeting are so much smaller than the wavelengths of the microwaves (about 12 cm), the microwaves respond to the sheeting as though it were solid metal and they reflect almost perfectly. By keeping the microwaves inside the oven, this sheeting speeds cooking and protects you from the microwaves.
Why don't microwaves get stuck in the food we put in the microwave oven?
Microwaves are like light--both are electromagnetic waves and both move extremely quickly. While it is possible to trap a light wave briefly between two mirrors, that wave will eventually be absorbed or released. The same is true of a microwave. It's almost impossible to trap a microwave for more than 1 second, even in very exotic enclosures, so you needn't worry about them becoming trapped in food. The food simply absorbs them and turns their energy into thermal energy.
There is an experiment involving grapes and microwaves that we found on the internet. If a grape is cut in half--with a piece of skin attached between the two halves--and it is then microwaves, sparks are produced. What is happening? -- GB, Antioch, CA
This experiment is described in Fun with Grapes - A Case Study. While I haven't tried it yet myself, I believe I know why it works. Grape juice is somewhat able to conduct electricity and the two halves of the grape are connected by a weak conducting path: the skin bridge. When the microwave oven is turned on, the microwaves not only heat the water in the grapes, they also push a few mobile electric charges back and forth through the skin bridge from one side of the grape to the other. This current releases energy as it passes through the narrow bridge and it heats the bridge extremely hot. The bridge soon catches fire and the electric current driven by the microwaves begins to pass through the flame. When current passes through a gas, it tends to ionize that gas (remove electrons from the gas atoms) so that the gas itself begins to conduct electricity. When current flows through atmospheric pressure air, it forms an brilliant arc. In this case, the arc that you see is powered by the microwaves as they push electric charges back and forth from one side of the grape to the other. An excellent set of movies showing this and other microwave oven experiments appears at http://www.physics.ohio-state.edu/~maarten/microwave/microwave.html.
Don't microwaves change the molecular structure and composition of food, by ejecting some electrons from atoms and forming cancer-causing free radicals? If I should stand away from a microwave to avoid possible leakage, why would I eat microwaved food?
Microwaves don't affect the molecular structure of the food, except through the thermal effects we associate with normal cooking (e.g., denaturing of proteins with heat and caramelizing of sugars). That's because, like all electromagnetic waves, microwaves are emitted and absorbed as particles called "photons." The energy in a microwave photon is so tiny that it can't cause any chemical rearrangement in a molecule. Instead, it can only add a tiny amount of heat to a water molecule. During the microwave cooking process, microwave photons stream into the food and heat it up. But millions of them would have to work together in order to cause non-thermal chemical changes in the food molecules and they don't normally do that. The photons can only work together if there is a conducting material, such as a metal wire, inside the oven. In that case, the photons can accelerate mobile electric charges along the conducting paths and create sparks. Such sparks can cause chemical damage, but nothing worse than the chemical damage caused by scorching food with a flame or broiler. Even if your microwave is full of sparks for some reason, I doubt that the food will be any worse for you than it would be if you cooked it over an open flame or barbecue.
How does radar absorbent materials work. How effective is stealth technology? -- DP, Scottsbluff, NE
I believe that most radar absorbing materials are partially conducting plastic composites. As a microwave from the radar transmitter penetrates these composites, the electric field in that wave drives charges back and forth through the composites. Since the composites don't conductor electricity well, they turn the wave's energy into thermal energy and thereby absorb it. A similar effect occurs for light waves when you shine them on a pile of powdered charcoal. (According to David Ingham, some radar absorbing materials include lossy magnetic materials--materials such as ferrite and carbonyl iron that respond to the magnetic field in a microwave.) Because there is always some reflection whenever an electromagnetic wave enters a material that slows the wave down, stealth aircraft are also careful to deflect the reflected wave away from the radar transmitter so that its receiver won't detect the return wave. In fact, these materials can be corrugated so that any microwaves hitting them reflect into the corrugations and have many opportunities to be absorbed. As I understand it, the microwaves that return to the radar receiver from a stealth plane are remarkably weak. I wouldn't be surprised if a whole stealth plane reflected less microwaves back at the radar unit than would reflect from a foil chewing gum wrapper.
In a microwave oven, does food cook from the inside out or outside in? -- KS, Essex, England
If the piece of food isn't too large, it all cooks at once. The microwaves that heat the food pass deep into it and they deposit energy in every part of the food simultaneously. Only if the piece of food is so large that an appreciable amount of microwaves are absorbed before they reach the center will the center cook more slowly than the outside. I doubt that this shielding of the center is a problem with foods small enough to fit inside a normal microwave oven. However, the microwaves in a microwave oven aren't perfectly uniform, so that some parts of a meal will cook a bit faster than others. That's why it's important to move the food about during cooking to achieve uniform heating throughout.
I recently acquired a microwave that "doesn't cook as fast as it used to." Does this sound right? What type of service might need to be performed? - W
It is possible for a microwave to lose cooking speed. If the microwave source isn't able to produce as intense microwaves as before or if it doesn't turn on reliably and steadily, it won't cook as fast. For the source to produce less intense microwaves, the high voltage power supply would probably have to be weak. Its storage capacitor could have failed or one or more of its high voltage diodes could have burned out. I suppose that the magnetron itself could be about to die, but I'll bet that most of them go suddenly and without warning. For the source to not turn on reliably, it would probably have to have a bad connection to the power line. One good possibility is that the relay that turns on power to the high voltage power supply is not making good contact.
Listen to the microwave as it operates on a medium setting. It should cycle on and off every five or ten seconds. You should hear it hum softly during the on half of the cycle and then stop humming during the off half of the cycle. Different power levels simply vary the fractions of on time and off time. If you don't hear the hum or the hum is intermittent, then something is probably wrong with the power relay or with something else in the high voltage power supply. If the relay is flaky, a little cleaning of its contacts may cure the problem. Be careful of the high voltage capacitor, which can store a lethal charge even when the unit is unplugged.
If microwave cookers are so energy-efficient, why can't similar machines be used as hot water heaters or in central heating systems? - GB
Microwave ovens transfer about 50% of the electric energy they receive from the electric company to the food. Conventional ovens transfer only something like 10%. Cooking just isn't a very energy efficient process because you're trying to get heat into an object from outside that object. In contrast, an electric space heater transfers 100% of the electric power it receives to the room around it. Home heating is much more energy efficient because you're getting heat into an object from inside that object. In effect, your microwave oven is also 100% efficient at heating your room--every bit of electric energy it consumes eventually enters your room as heat. But it's an expensive sort of "space heater" and you do better just to use conventional heating systems.
What does the inside of a microwave oven look like? Please show illustrations. -- Dade County, FL
A microwave oven contains (1) a magnetron that produces the microwaves, (2) a high voltage direct current power supply (a high voltage transformer, a set of rectifiers, and a capacitor) that provides power to the magnetron, and (3) a computerized control system that turns the power supply and magnetron on and off. A metal pipe connects the magnetron to the cooking chamber of the oven. While there are photographs and drawings of the insides of a microwave oven in my book, I can't reproduce them here because of copyright issues.
How does a magnetron work? -- MM, Czech Republic
A magnetron has a ring of resonant electromagnetic cavities around a hot central filament. Each resonant cavity acts like an electromagnetic "tuning fork"--electric charges and electromagnetic waves swing back and forth inside a resonant cavity at a particular frequency; the cavity's resonant frequency. As electrons are "boiled" off the hot filament, a high voltage attracts them toward the walls of the resonant cavities. The resonant cavities tend to have at least small amounts of electric charge "sloshing" back and forth in them at their resonant frequencies and the electrons from the filament are attracted more strongly to the cavities' positively charged walls than to their negatively charged walls.
However, there is also a magnetic field present in the magnetron and this field deflects the streams of electrons so that they hit the wrong walls of the resonant cavities. Instead of canceling the charge sloshing in the walls of the resonant cavities, the newly arrived electrons add to it. As electrons flow to the resonant cavities, more and more charge sloshes in the resonant cavities and these cavities accumulate huge amounts of energy. Some of this energy is tapped by a small wire loop and a microwave antenna. This antenna radiates some of the energy from the cavities into a metal channel that leads away from the magnetron. In a microwave oven, this channel leads to the cooking chamber so that energy from the resonant cavities is delivered to the food in the oven. Energy is extracted from the magnetron slowly enough that the filament and high voltage power supply can replace it and the operation continues indefinitely.
I have heard that microwaving can destroy certain nutrient molecules in food, such as vitamins. Is this true? -- D, Boulder, CO
A microwave oven heats the food it cooks; nothing more. If it damages nutrients, then it's by overheating those nutrients. Such overheating could happen in a microwave oven if you don't move the food about during cooking. That's because the microwaves aren't uniformly distributed in the cooking chamber and some parts of the food heat faster than others. Some parts of the food could become hotter than you intend and this overheating could damage sensitive molecules. However, I think that microwave cooking is probably less injurious to the food than conventional cooking. It's pretty hard to burn food in a microwave!
Can plastic melt in a microwave oven? How does this process work? Can plastic burn in a microwave oven? - HD
Most plastics are unaffected by microwaves and do nothing at all in a microwave oven. For them to absorb energy from the microwaves, the plastics must either conduct electricity or their molecules must undergo the twisting motions that water molecules experience in the microwave oven. There are a few conducting plastics and these may melt or burn in a microwave as the microwave electric fields propel electric currents through them. There are also some plastics that trap water molecules and these may also melt or burn as the water molecules gather energy from the microwaves. I suppose that there are also a few plastics that have polar molecules in them that respond to the microwaves the way water does. However, most plastics do none of these and only melt or burn if they accidentally come in contact with very hot food or pieces of metal that happen to be in the microwave oven.
I've heard from many people that you should not stand directly in front of and no closer than 30 feet while a microwave oven is on? Why? If this is a myth how did it get started?
This idea is just a myth. There should be virtually no microwaves leaking from the oven so it shouldn't matter where you stand. If you're concerned about microwaves, you can buy a microwave oven tester from a local appliance store or from MCM Electronics at (800) 543-4330, part number 72-4080, for $6.99 (or for a more accurate and reliable measurement, take your microwave to a service shop for inspection with an FDA certified meter). I have no idea how such a myth got started, but it's clear that microwave ovens scare people because they don't understand them. Given how easy it is to burn yourself on a conventional oven, I'd guess that there are fewer health risks with microwave cooking than with conventional cooking.
Does cooking in a microwave oven destroy the nutritional value of foods? Are microwaves radioactive? Does radiation "leak" from the oven? - DL
Microwaves are essentially high frequency radio waves. They heat food by twisting its water molecules back and forth so that those water molecules rub against one another. Like all electromagnetic waves, microwaves are absorbed and emitted as particles or "photons," but the photons of microwaves have so little energy that they are unable to cause chemical changes in the molecules they encounter. They simply heat food; they don't "irradiate" it. The only way a microwave oven damages the nutritional value of foods is if it overheats. Microwaves are not radioactive--radioactivity is the spontaneous fragmentation of the nuclei of atoms and is usually associated with the emission of high energy particles; particles that can induce chemical changes in the molecules they encounter. Finally, if a microwave oven was properly constructed and hasn't been damaged, virtually no microwaves leak from it. A small amount of microwaves won't hurt you anyway--they are present all around us already because of satellite transmissions, cellular telephones, and even the thermal radiation from our surroundings.
Why does a microwave oven heat organic material and not inorganic material? -- JM, Columbus, OH
A microwave oven heats anything that contains liquid water. Since many organic materials contain water, they will become hot in a microwave oven. But some organic materials such as pure salad oil don't contain water and won't become hot in a microwave oven. There are also some inorganic materials such as damp unglazed pottery that contain water and that will become hot.
Is 2.45 gigahertz the best frequency for a microwave oven? Is that frequency at or near a water molecule resonant frequency? Do water molecules have a resonant frequency?
The frequency of the microwaves used in most microwave ovens, 2.45 gigahertz or 2,450,000,000 cycles per second, isn't related to any resonance of the water molecules themselves. While the isolated water molecules in steam or moist air have clear resonances associated with various vibrational and rotational modes of oscillation, these resonances are smeared out in liquid water. The water molecules in liquid water touch one another and their resonances are disturbed in much the same way that the resonances of a bell are disturbed when you touch it.
Rather than interacting with the water molecules via a resonance, the microwaves in an oven heat the water by twisting its molecules rapidly back and forth so that they rub against one another. The molecules are heated by the molecular equivalent of sliding or dynamic friction. The choice of 2.45 gigahertz gives the water molecules about the right amount of time to twist in each direction. The precise frequency isn't important, but microwave ovens are required to operate at exactly 2.45 gigahertz so that they don't interfere with communication systems using nearby frequencies. I believe that there are 2 other frequencies allocated to microwave ovens, but only a few ovens make use of those frequencies.
My mother owns a microwave oven that is about 20 years old. It looks like new and has always been well taken care of. However, I was wondering whether it is still safe to use. Should I have it tested for leakage? -- KE, Milwaukee, WI
As long as it still cooks, it's probably fine. Leakage of microwaves can only occur if the cooking chamber has holes in its metal walls. These walls include the metal grid over the front window and the seals around the door. If the metal grid is intact and the door still appears to close properly, the oven shouldn't leak any more microwaves now than it did 20 years ago. However, to set your mind at ease, you can have it tested or test it yourself. MCM Electronics at (800) 543-4330 sells a simple microwave leak tester, part number 72-4080, for $6.99. You can probably find similar devices at local appliance stores or, for a more accurate and reliable test, take your microwave oven to a service shop for inspection with an FDA certified meter. [Note added 1/10/97: I have finally found one microwave oven that leaks enough that the MCM detector identifies it as dangerous--it's the microwave oven in my laboratory and I've moved it around frequently and taken it apart several times for my classes. Evidently, I damaged its door hinges during my experiments because the door now sags a bit and doesn't seal properly. The tester worked nicely in finding the leaks.]
If you microwaved bean plant seeds over a period of weeks while they were growing, would they grow faster or longer, and if they would, would that be due to the heat or some effect of the microwave radiation? - DS
Microwaving the bean plant seeds would be no different from heating them, except that the distribution of temperatures in the seeds and soil might be a little different from what you would get if you simply used a space heater. The particles or photons of ultraviolet light, X-rays, or gamma rays have enough energy to cause chemical changes in organic molecules and can induce mutations in living organisms. However, the photons of microwaves have so little energy that all they can do is heat living things. The most likely result of microwaving the bean plant seeds will be that the seeds will overheat and won't grow at all. You'll have bean stew.
Were microwaves invented for the microwave oven?
While microwaves were known long before anyone know how to produce them efficiently, they became important during World War II as the basis for radar. The ability to detect and locate enemy aircraft at long distances and at night was crucial to the defense of Allied cities during the war. The 1945 discovery that microwaves also cooked food was an accidental offshoot of radar development.
What is the purpose of the grid on the glass door of the microwave oven?
The metal grid reflects microwaves and keeps them inside the oven. Electromagnetic waves are unable to pass through holes in conducting materials if those holes are significantly smaller than their wavelengths. The wavelengths of visible light are very short, so light has no trouble passing through the holes in this grid. But the microwaves used in the oven have wavelengths of about 12.4 cm and are unable to propagate through the grid. Thus you can see the food cook while the microwaves are trapped inside the oven.
Can we add a section to a microwave oven that gets the food or drinks cold? - MH
Not without adding a full-blown refrigerator. While it's relatively easy to add thermal energy to food or drink, it's much harder to remove that thermal energy. Since energy is conserved, the thermal energy that you remove from the food must be transferred elsewhere. Since heat (moving thermal energy) normally flows from a hotter object to a colder object, you must make something colder than the food before the heat will leave the food. While it's possible to cool an object to a temperature lower than its surroundings, this cooling process requires a heat pump, a device that actively pumps heat from a cold object to a hot object (against its natural direction of flow). A refrigerator is such a heat pump.
Does the volume in the cooking chamber of a microwave oven affect the rate at which it cooks the food? In other words, which cooks faster, a small microwave oven or a large one? - RP
The size of a microwave oven's cooking chamber should have little or no effect on how quickly it cooks food. The oven's magnetron tube delivers a certain amount of microwave power to the cooking chamber and virtually all of that power will eventually be absorbed in the food. It may take a few moments longer for a large cooking chamber to fill with microwaves when you first start the oven, but soon the food inside it will be exposed to the same intensity of microwaves as food cooking inside a smaller microwave oven with a similar magnetron power.
On the other hand, the magnetron's power does affect cooking speed so that an oven with a more powerful magnetron will cook food faster than one with a less powerful magnetron. The speed of cooking in a microwave oven also depends on how much food it contains because the food shares the microwave power. In general, doubling the amount of food in the microwave doubles the cooking time.
What is a microwave and what does it do? -- AH, Rochester, MN
A microwave is an electromagnetic wave with a frequency and a wavelength that are intermediate between those of a radio wave and those of light. An electromagnetic wave consists of both an electric field and a magnetic field. These two fields travel together in space and perpetually recreate one another as the wave moves forward at the speed of light. An electric field is a phenomenon that exerts forces on electric charges, while a magnetic field is a phenomenon that exerts forces on magnetic poles. Electric and magnetic fields are intimately connected, so that whenever an electric field changes, it creates a magnetic field and whenever a magnetic field changes, it creates an electric field. By combining a changing electric field and a changing magnetic field, the electromagnetic wave uses their abilities to create one another to form a self-perpetuating entity--the wave's changing electric field creates its changing magnetic field and its changing magnetic field creates its changing electric field.
If you were to freeze an electromagnetic wave at one instant and look at its structure in space, you would find that its electric and magnetic fields had a periodic spatial structure. Just as a water wave has crests and troughs, an electromagnetic wave has spatial fluctuations in its two fields. The distance between adjacent "crests" in either one of these fields is that wave's wavelength. Different types of electromagnetic waves have different wavelengths. Radio waves have long wavelengths that range from about 1 meter to hundreds or even thousands of meters and visible light has short wavelengths that range from about 400 billionths of a meter (400 nanometers) to about 750 billionths of a meter (750 nanometers). Microwaves are those electromagnetic waves with wavelengths between 1 millimeter and 1 meter. The microwaves used in microwave cooking have wavelengths of 12.2 centimeters.
Microwaves are often used to carry information in satellite communication and telephone microwave links. Whenever you see a dish antenna (a satellite dish or a communication link dish on a building or tower), you are looking at a microwave system. Astronomers use radio telescopes to look at microwave emissions from celestial objects. Radar bounces microwaves from objects to determine where they are or how fast they're moving. And microwave ovens use microwaves to add thermal energy to water molecules in order to heat food.
How can we polarize a molecule? -- AD, Manaus City, Amazonia, Brazil
Some molecules, including water, are naturally polarized. This means that they have a positively charged end and a negatively charged end. But even normally non-polar molecules such as carbon dioxide can be polarized by exposing them to strong electric fields. Electric fields exert forces on electric charges and cause the electric charges in a molecule to rearrange--the positive charges in the molecule shift in one direction and the negative charges in that molecule shift in the other. As a result of this applied electric field, the molecule acquires a polar character--a negatively charged end and positively charge end. However, this polar character disappears as soon as the electric field is removed.
Who invented the microwave oven and how did he think of it?
In 1945, American engineer Percy Le Baron Spencer was working with radar equipment at Raytheon and noticed that some candy he had in his pocket had melted. Radar equipment detects objects by bouncing microwaves from them and Spencer realized that it was these microwaves that had heated the candy (as well as his body...oops!). Raytheon soon realized the potential of Spencer's discovery and began to produce the first microwave ovens: Radaranges. These early devices were large and expensive and it wasn't until 1967, when Amana, a subsidiary of Raytheon, produced the first household microwave oven, that microwave ovens became widely available.
If microwaves "bounce" or reflect inside the cooking chamber, is it important for all of the surfaces (walls) of the oven to be flat? What would happen if the cooking chamber were cylindrical or circular? Would the microwaves bounce off the walls and then cancel each other out?
A microwave oven with a cylindrical or spherical cooking chamber would have a problem with non-uniform cooking. But before I look at why, I should note that even a microwave oven with a box-like cooking chamber exhibits non-uniform cooking. That's because the microwaves that are bouncing around inside the cooking chamber are all coherent--they are parts of a single, giant wave--and they can interfere strongly with one another. That means that several reflected microwaves can cancel or enhance one another as they cross, leading to regions inside the cooking chamber that cook quickly and other regions that cook slowly. That's why it's important to move the food around the cooking chamber during cooking--so that the food cooks evenly.
If the oven's cooking chamber weren't box-like, there would be a new problem to contend with: a tendency for the microwaves to be concentrated or focused in a particular region. Just as a cylindrical or spherical mirror bends the light rays it reflects, so the curved walls of a non-boxlike cooking chamber would bend the microwaves it reflects. It would tend to focus those microwaves in particular regions (such as the center of the cylinder or sphere) so that there would certain regions inside the chamber where the microwaves would be particularly intense and cooking would proceed very quickly.
Why are you required to have an item in the microwave oven while it is operating?
When a microwave oven is cooking food, electrons move rhythmically back and forth inside the magnetron tube and create the microwaves. These microwaves flow through a metal pipe and into the cooking chamber, where they are absorbed by the water in the food and thus heat the food (the twisting back and forth of the water molecules, described elsewhere on this page, not only heats the food--it also absorbs the microwaves). If there is no food in the cooking chamber, the microwaves build up in the cooking chamber until they are so intense that large numbers of them flow backward through the pipe to the magnetron. These microwaves reenter the magnetron and disrupt the motion of electrons inside it. The magnetron begins to misbehave and can be damaged as a result. To avoid such damage, you want to be sure that there is something in the cooking chamber to absorb the microwaves before they return to the magnetron and cause trouble. In short, don't run the microwave empty for any long periods of time.
How does a microwave oven heat food?
A microwave oven uses a vacuum tube called a magnetron to create intense microwaves inside the cooking chamber. These microwaves are electromagnetic waves with a frequency of 2.45 gigahertz or 2,450,000,000 cycles per second. They are similar to normal radio waves, except that they have a higher frequency. Because of these microwaves, the electric field at any point inside the cooking chamber fluctuates back and forth 2.45 billion times each second. That means that an electrically charged particle at any point in the cooking chamber will be pulled first one way and then the other, back and forth 2.45 billion times each second. While water molecules aren't electrically charged overall, they do have electrically charged ends--one end is positively charge and the other is negatively charged. In the presence of the microwave radiation, these water molecules find themselves twisted back and forth very rapidly. As they twist, they rub against one another and friction heats them up. The water becomes hot and this hot water, in turn, cooks the food. Food that doesn't contain water (like salt or oil) won't get hot. Neither will food in which the water molecules can't turn (like ice or frozen food). That's why it's hard to defrost frozen food in a microwave.
Does microwave cooking break molecular chains? Does any recombination of ions take place in the food and, if so, is there a possibility of eating some type of toxin formed during cooking?
The answers to all of these questions are no. Microwave cooking merely heats the water molecules, which in turn heat the food. The only molecular rearrangements that occur are those that are caused by warming the food toward the boiling temperature of water. In fact, there is less chemistry done during microwave cooking than is done in a normal oven. For example, one of the problems with microwave cooking is that food doesn't brown because the high temperatures needed to chemically modify the food molecules (and cause browning) aren't reached in microwave cooking. So you shouldn't have any fear of food cooked in a microwave oven. The microwaves don't damage it any more than heating it in boiling water would.
You said an ice cube will not get hot in the microwave because the molecules won't "flip". If this is so, then why do frozen foods cook in the microwave?
As noted previously, the water molecules in frozen foods are not all bound up perfectly inside ice crystals. As long as there are a few relatively mobile water molecules, even frozen food will eventually absorb enough energy to melt. Once that happens, the food can cook easily. Of course, the melting process is frequently very non-uniform so that food comes out with hot and cold regions. In general, frozen food cooked in a microwave is not very satisfying.
Why do some microwave ovens not seem to have a metal surface in the cooking area?
The cooking chamber of a microwave oven is always metallic. Even the glass door has a metal grid across it to keep the microwaves inside. This metal chamber may be coated with paint or plastic but it is there nonetheless. Without it, the microwaves would leak out and the oven would be hazardous and inefficient. It would cook objects throughout the kitchen.
Why do microwave ovens cook so rapidly?
When you put solid food (a potato, not soup) into a conventional oven, the heat flows slowly into the center of that food. This heat must work its way into the food via thermal conduction, in which adjacent atoms and molecules transfer their motional energies in a long bucket-brigade process. The last part of a potato to become hot is its center. However, in a microwave oven, the microwaves travel well into the solid food and deposit their energy everywhere. The potato cooks throughout at a relatively even rate. The actual amount of heat and energy involved in conventional and microwave cooking is about the same. However, the microwaves can heat the food throughout without having to wait for the slow process of conduction to carry it inward from the food's surface.
What happens if you start the microwave oven with nothing inside?
The magnetron creates microwaves that travel into the cooking chamber and should be absorbed there. If there is no food (or rather no water-containing food), those microwaves will not be absorbed and will eventually find their way back to the magnetron. Eventually the magnetron will absorb as many microwaves as it emits. This situation is hard on the magnetron, which works best when it has very little radiation returning to it. That's why you should never run a microwave empty for more than a second or two.
What exactly goes on when you're cooking a potato in the microwave and it explodes?
A microwave oven heats food by depositing energy in its water. If you cook the food long enough, that water can begin to boil. If the food has a hard outer shell (e.g. a potato or a corn kernel), the boiling water can create enough pressure in the food to make it explode. That is what pops the corn in microwave pop-corn and why the potato explodes if you don't pierce it so that steam can escape.
What containers are not safe to use in a microwave? I am particularly concerned about Styrofoam containers as I use them to make TV dinners for my family. Is it OK to heat directly in these containers?
The two critical issues with containers in a microwave are (1) that they do not absorb or reflect microwaves and (2) that they tolerate high temperatures. Concerning the first issue, a container that absorbs microwaves will become extremely hot and may be damaged or destroyed. Most plastics (including Styrofoam) don't absorb microwaves and are fine. Glazed water-free ceramics and glasses are usually also fine, as long as they don't have any metallic trim. Metal dishes are a poor choice because they reflect microwaves and lead to uneven heating. Unglazed ceramics absorb water and will overheat.
Concerning the second issue, many plastics melt or soften below the temperature of boiling water. Polystyrene, the plastic from which Styrofoam is made, has a glass transition temperature of almost exactly 212° Fahrenheit (100° Celsius). That means that it will begin to soften at just about the temperature of boiling water. While pure water will boil without much problem in Styrofoam, water containing dissolved solids such as sugar or salt will boil at a higher temperature and may melt the Styrofoam. You'll know when this happens...it's not really a health issue, just a potential for a messy oven. I've only encountered the problem once myself, when a Polystyrene gravy separator melted in the microwave and let the gravy spill.
On the subject of defrosting frozen food in a microwave oven, you must refer to the old BTU formula which states "It takes one BTU to raise the temperature of 1 pound of water 1° (Fahrenheit), but when water is changing state from a solid (ice) to a liquid (water), it must absorb 144 BTUs (per pound)." - George R.
This observation accounts for much of difficulty with defrosting food in general and defrosting food in a microwave oven in particular. It often takes more heat to melt ice in the food than it does to actually cook the food once the ice has melted. Since ice doesn't absorb microwaves well, heating frozen foods in a microwave oven is a tricky business. Any region of food that melts early will absorb microwaves strongly and overheat while any region of food that remains frozen won't absorb microwaves well and won't receive the enormous amounts of heat it needs just to melt. The result is typically a food item with some frozen parts and some boiling hot parts. To avoid this problem, microwave oven defrost cycles let the food sit in between bursts of microwave heating. That way, there is time for heat to flow through the food and keep the internal temperatures relatively uniform. Parts of the food that heat well have time to transfer heat to parts that don't heat well and the whole item thaws and heats together.
Inside the microwave oven, what is it that heats the food? How does the heat come out; where did it come from?
The food is heated by the microwaves themselves and these microwaves are piped into the cooking chamber from the magnetron. The magnetron has electric charge sloshing back and forth in its tines. A small antenna uses that sloshing charge to emit microwave radiation. The water molecules in the food absorb this microwave radiation and turn its energy into heat. The usual rules of heat transfer don't apply in the heating process--the energy arrives at the food as microwaves, not heat.
In microwaves - you heat up food really fast. Is it true that microwaved food will cool down faster than oven heated food? Someone told me "if it heats fast, it will then cool fast."
No. Microwaves cook the food in a very different manner than normal thermal heating, but microwaved food has the same thermal energy that it would have if it had been warmed by more traditional methods. Microwaves heat food by exerting torques on the individual water molecules in the food. These molecules jiggle back and forth and sliding friction between them heats the food. This peculiar route to energy addition explains why frozen portions of the food don't heat well: the water molecules are rigidly oriented and can't jiggle back and forth in order to become hot. But despite the fancy heating scheme, the food retains no memory of how it was heated. Once it is uniformly hot, it cools at a rate that depends only on how heat is transported out of it. Microwaved food cools just as slowly as normally cooked food.
If a radio station operated at 2.45 gigahertz, could you pick it up when your microwave was turned on and attached speakers?
If some radio station were to operate at 2.45 gigahertz, the main effect would be very poor reception of that channel on your radio. The oven isn't a transmitter for microwaves; it just makes them like crazy. Most of the microwaves never leave the cooking chamber and there are strict regulations on any leakage. But it would only take a few thousandths of a watt of leaking microwave power to cause trouble in your reception of the radio station. Your radio wouldn't be able to distinguish that station's transmission from microwaves leaking out of your oven. The radio would struggle to pick up the signal and you would probably hear lots of noise in the background.
If a microwave does not melt ice, how does the "Defrost" setting on the microwave work?
I've already noted the issues of warming frozen food. However, the "defrost" setting is an interesting issue. If you've ever watched a microwave trying to defrost food, you've probably noticed that it heats the food briefly and then waits. It repeats this process many times. What it is doing is depositing energy (via the microwaves) into whatever water molecules are able to absorb microwaves. It then waits for this energy to flow as heat into the nearby food. Once the heat has been distributed rather evenly, the oven adds some more energy by turning the magnetron back on. This cycle of heating and waiting allows the food to defrost fairly evenly. Still, microwaves are likely to create hot and cold regions in the food so that some parts of the food will cook rather than defrost while some parts remain frozen.
I'd heard that if I cook in the microwave oven, there will be a possible formation of free radicals. Is it true? If yes, how? -- Angela I.
It's doubtful that microwave cooking forms free radicals in food. The microwaves in a microwave oven cook by exerting torques on the water molecules and gradually increasing the water molecules' thermal energies through friction-like effects. There is never enough energy present in a single molecule at one time to shatter that molecule and form a free radical. While ultraviolet light, such as that found in sunlight, carries enough energy per photon (particle of light) to split a molecule and form a free radical, microwave radiation carries very little energy per photon. That's why microwave photons can't do chemical damage the way ultraviolet photons can. However, even if microwave radiation could form free radicals in food, that wouldn't necessarily cause you trouble when you eat that food. So much happens to the food before it enters your blood stream that a free radical probably won't survive. The more harmful free radicals are ones that are actually created inside your body, where they can immediately attack important molecules in your cells.
I have a friend who refuses to stand in front of the microwave oven in his kitchen, because he feels the "nuclear waves" leak and will cause his sperm to deform (and he doesn't want ugly kids). Is this true? What about car phones? He heard they were bad, too!
Both microwave ovens and car phones emit electromagnetic radiation. But that radiation has relatively long wavelengths (about 12 cm in the case of microwave ovens and about 40 cm in the case of car phones) and is not at all like the electromagnetic waves emitted by nuclear processes. Nuclear electromagnetic radiation, usually called gamma rays, has extremely short wavelengths (less than 0.001 nanometer or about a millionth of the wavelength of visible light). All electromagnetic waves are emitted and absorbed as particles called photons. The energy in a photon is inversely proportional to its wavelength (in vacuum). Gamma rays, with their short wavelengths, have very energetic photons that can do lots of chemical damage to your tissue. But the longer wavelength radiation from microwave ovens and car phones comes as very low energy photons. These photons can't do chemical damage. The only thing those waves can do is heat things. Microwave ovens are carefully shielded so that they keep most of the microwaves inside. If those waves did emerge, they would simply warm your tissue up. This warming won't cause genetic damage but it could cook your tissue. There has been recent concern about low frequency electromagnetic fields causing subtle damage to tissue, but these have not be substantiated by scientific research and no physically reasonable scenarios for how such damage could occur have been offered.
How does the resonant cavity in the magnetron work?
When it's active, the magnetron's cavity has electric charge sloshing back and forth along its tines. The charge moves at a frequency determined by the shape and size of the cavity and these are carefully controlled so that the cavity's natural resonance frequency is 2.45 gigahertz. To keep the charge sloshing, the magnetron adds negative charge from a hot filament wire located in the center of the cavity. Electrons flowing off of this wire are steered toward the negative tines by a magnetic field. As a result, the charges continue to slosh back and forth indefinitely. A small wire connected inside the magnetron extracts some of the energy in the magnetron and converts it into microwaves outside the magnetron. This wire acts as an antenna. The antenna is located in the pipe that carries the microwaves to the cooking chamber.
How does a microwave oven defrost foods? Doesn't it only work with water, not ice?
In any frozen food, there are some water molecules that are relatively free to turn about. These molecules may be at the surfaces of ice crystals or sitting on the surface of food particles. These water molecules can absorb microwaves and heat. However, the heating is very uneven because as soon as any water crystal absorbs enough heat to melt, the resulting liquid water will begin to absorb microwaves much more strongly. That is why defrosting must be done slowly. Then the microwave deposited heat will have time to flow through the food and melt it uniformly. Otherwise, you can end up with boiling hot spots mixed together with frozen icy spots.
How do metal rods short out the microwaves?
If you arrange a metal rod so that it's parallel to a microwave's electric field, the microwave will push electric charges up and down that rod. This moving charge will waste some of the microwave's energy by creating heat in the rod. But the main effect will be that the rod will reflect or scatter the microwave. The moving charge will emit its own microwave and this new microwave will interfere with the original one.
How can microwaves heat something? Radio waves don't warm things very much.
The electric field of a microwave flips back and forth at just about the right frequency to have the largest effect on water molecules. The water molecules try to follow the reversing electric field and, in doing so, become hotter and hotter. Radio waves flip too slowly to have very much effect on water. Furthermore, the microwaves in an oven are far more intense than the radio waves that we're used to have around us so that common radio waves just don't do very much cooking.
Can microwaves be emitted to travel in one direction?
Yes. Like all electromagnetic waves, microwaves can be focused and concentrated in a particular direction. That is exactly what microwave dish antennas (e.g., satellite dishes) do. At the transmitter, they focus the microwaves emitted by a smaller antenna so that those microwaves travel as a parallel beam. At the receiver, they focus the parallel beam of microwaves onto a smaller antenna. You can think of the microwaves as very long wavelength light waves, so that anything you can do with light (e.g., focus it, form images with it, or bend it with optical devices), you can also do with microwaves. The only problem is that the optical elements you use for microwaves must be larger, because the microwaves have longer wavelengths.
Are microwaves harmful to you? Is eating microwaved food harmful?
Microwaves can heat your body by adding thermal energy to the water molecules in you. This heating can be damaging if it's not controlled. Most of your body is protected from slow heating because your blood carries heat away from any local hot spots so that you warm evenly. However there are a few places that aren't cooled by your circulation and can heat up locally enough to denature the protein molecules and cause biological injury. The cornea of your eye is a good example. It can be heated and damaged because it's not cooled well. That's why you must be careful not to look into a strong beam of microwaves. As for microwaved food, the only effect of cooking with microwaves is hot food. There is no "radiation damage" or "radioactivity," as there might be with x-ray or gamma radiation. Some foods should not be cooked in a microwave only because the uneven heating may allow certain parts to become too hot. Those parts may burn you when you eat them or they may suffer thermal damage that diminishes their nutritional value.
Are microwaves distributed unevenly in the oven? Why do manufacturers claim that microwaves with turntables are more effective than microwaves without turntables?
As the microwaves bounce around the inside of the cooking chamber, they tend to interfere with one another. There are usually regions in which the waves that follow various paths almost cancel one another and regions in which the waves reinforce one another. These regions don't cook food equally well. If the microwaves are canceled in one region, cooking will be slow there. If the microwaves reinforce one another in another region, cooking will be fast there. If you simply leave food in one place and try to cook it in the microwaves, the cooking will be uneven. However, if the food is rotated continuously, these good and bad cooking regions will be blurred away so that the food will all cook at about the same speed.