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Microwave Ovens
Page 11 of 13 (127 Questions and Answers)

1487. I saw the story on Primetime tonight (Superheated Water Produced in Microwave Ovens on ABC Primetime 3/15/2001), and at weird timing. Just yesterday, a co-worker and I were standing around the kitchen area talking, while she warmed up some coffee. All of a sudden, there was a loud POP, which startled both of us. Not knowing exactly what had happened, we stopped the microwave and opened the door, only to find the contents of the mug (coffee) everywhere on the inside of the cooking chamber, less a few drops at the bottom of the cup.

The story provided SOME insight into what exactly had happened, however, it was reported that the surface of the super-heated liquid had to be broken by something for an explosion to be triggered. In the explosion with the coffee, there were no other objects in the microwave other than the mug and the coffee it held. What then, caused the explosion if nothing was present to break the surface? - MM, Denver, CO

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Superheated water doesn't always wait until triggered before undergoing sudden boiling. All that's needed to start an explosion is for something to introduce an initial "seed" bubble into the liquid. Sometimes the container already has everything necessary to form a seed bubble and it's just a matter of getting the water hot enough to start that process. Many seed bubbles begin as trapped air in tiny crevices. As the water gets hotter, the size of any trapped air pocket grows and eventually it may be able to break free as a real seed bubble. When water is sufficiently superheated, just a single seed bubble is enough to start an explosion and empty the container completely. In your case, the coffee flash boiled spontaneously after something inside it nucleated the first bubble.

This sort of accident happens fairly often and we rarely think much about it as we sponge up the spilled liquid inside the microwave oven. But had your friend been unlucky enough to stop heating the coffee a second or two before that POP, she might have been injured while taking the coffee out of the oven. The moral of this story is to avoid overcooking any liquid in the microwave oven. If you must drink your coffee boiling hot, pay attention to it as it heats up so that it doesn't cook too long and then let it sit for a minute after the oven turns off. If you don't like your coffee boiling hot, then don't heat it to boiling at all.


1490. I thought microwave ovens were sealed shut to keep the waves inside. Why then can you smell the food as it is being cooked? - E
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The cooking chamber of a microwave oven has mesh-covered holes to permit air to enter and exit. The holes in the metal mesh are small enough that the microwaves themselves cannot pass through and are instead reflected back into the cooking chamber. However, those holes are large enough that air (or light in the case of the viewing window) can pass through easily. Sending air through the cooking chamber keeps the cooking chamber from turning into a conventional hot oven and it carries food smells out into the kitchen.

1491. I am planning to do an experiment with a microwave oven and want to videotape it. I want to operate the microwave oven with the door open. Will I be safe if I'm 15 feet away? Will opening the door nullify the "chamber" effect that the oven normally has? - E
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Don't operate the oven open. You're just asking for trouble. The oven will emit between 500 and 1100 watts of microwaves, depending on its rating, and you don't need to be exposed to such intense microwaves. The chamber effect is important; without the sealed chamber, the microwaves pass through the food only about once before heading off into the kitchen and you. The food won't cook well and you'll be bathed in the glow from a kilowatt source of invisible "light."

Imagine standing in front of a 10-kilowatt light bulb (which emits about 1 kilowatt of visible light and the rest is other forms of heat) and then imagine that you can't see light at all and can only feel it when it is causing potential damage. Would you feel safe? Your video camera won't enjoy the microwave exposure, either.

If you want to videotape your experiments without having to view them through the metal mesh on the door, you can consider drilling a small hole in the side of the cooking chamber. If you keep the hole's diameter to a few millimeters, the microwaves will not leak out. Then put one of the tiny inexpensive video cameras that widely available a centimeter or so away from that hole. You should get a nice unobstructed view of the cooking process without risking life and limb.


1493. In regards to your discussion of superheating water in a microwave oven, I've found that it occurs most often when (1) I reheat water that has been heated before and (2) I heat water that has sat in the cup overnight. Why does that seem to reduce the number of seed bubbles? - JS
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Both processes allow dissolved gases to escape from the water so that they can't serve as seed bubbles for boiling. When you heat water and then let it cool, the gases that came out of solution as small bubbles on the walls of the container escape into the air and are not available when you reheat the water. When you let the water sit out overnight, those same dissolved gases have time to escape into the air and this also reduces the number and size of the gas bubbles that form when you finally heat the water. Without those dissolved gases and the bubbles they form during heating it's much harder for the steam bubbles to form when the water reaches boiling. The water can then superheat more easily.

1497. Many of the new cordless phones operate at 2.4GHz like a microwave oven. Are we microwaving our ears when we use them, or is the wattage so small it doesn't affect us? - R
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As far as anyone has been able to determine so far, the wattage is so small that this microwave radiation doesn't affect us. Not all radiations are the same, and radio or microwave radiation is particularly nondestructive at low intensities. It can't do direct chemical damage and at low wattage can't cause significant RF (radio frequency) heating. At present, there is thus no plausible physical mechanism by which these phones can cause injury. I don't think that one will ever be found, so you're probably just fine.

1499. When you are defrosting and the magnetron is turning on and off, when it is off, are the microwaves still bouncing around or is the food just sitting there warming itself up? - LEA, PA
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During the defrost cycle, the microwave oven periodically turns off its magnetron so that heat can diffuse through the food naturally, from hot spots to cold spots. These quiet periods allow frozen parts of the food to melt the same way an ice cube would melt if you threw it into hot water. While the magnetron is off, it isn't emitting any microwaves and the food is just sitting there spreading its thermal energy around.

1504. Is it possible to heat up the surface of a stealth aircraft by exposing it to strong microwaves? Also, I heard that local forces in the recent Balkans conflict used cellular phone technology to down the U.S. stealth aircraft. Is that possible? - JG
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Stealth aircraft are designed to absorb most of the microwave radiation that hits them and to reflect whatever they don't absorb away from the microwave source. That way, any radar system that tries to see the aircraft by way of its microwave reflection is unlikely to detect anything returning from the aircraft. In effect, the stealth aircraft is "black" to microwaves and to the extent that it has any glossiness to its surfaces, those surfaces are tipped at angles that don't let radar units see that glossiness. Since most radar units emit bright bursts of microwaves and look for reflections, stealth aircraft are hard to detect with conventional radar. Just as you can't see a black bat against the night sky by shining a flashlight at it, you can't see a stealth aircraft against the night sky by shining microwaves at it.

Like any black object, the stealth aircraft will heat up when exposed to intense electromagnetic waves. But trying to cook a stealth aircraft with microwaves isn't worth the trouble. If someone can figure out where it is enough to focus intense microwaves on it, they can surely find something better with which to damage it.

As for detecting the stealth aircraft with the help of cell phones, that brings up the issue of what is invisibility. Like a black bat against the night sky, it's hard to see a stealth aircraft simply by shining microwaves at it. Those microwaves don't come back to you so you see no difference between the dark sky and the dark plane. But if you put the stealth aircraft against the equivalent of a white background, it will become painfully easy to see. Cell phones provide the microwave equivalent of a white background. If you look for microwave emission near the ground from high in the sky, you'll see microwaves coming at you from every cell phone and telephone tower. If you now fly a microwave absorbing aircraft across that microwave-rich background, you'll see the dark image as it blocks out all these microwave sources. Whether or not this effect was used in the Balkans, I can't say. But it does point out that invisibility is never perfect and that excellent camouflage in one situation may be terrible in another.


1508. I don't want to sound like I know everything in the world or even like I know quite a lot. But you had a question regarding "If a microwave oven door were to open while it was still on, what would happen? Could it hurt you?- JP"

Well ..Having the thought process that I have, kinda how should I put it? ...Stupid? or inventive or even in-between. Well, my microwave door did happen to come off. Magic Chef 900-watt microwave. Well, I did my best to try to fix it but the hinge on one side did not attach properly, therefore having a gap between the door and the appliance. Being me (stupid) I wondered if it would burn fast or would it gradually warm up. I slid my finger between...You probably dying to hear what happened... But it didn't gradually warm up at all. It was instant heat! It didn't scar me or anything like that, but sure scared the H*** out of me to find out it got so hot so quick. I didn't get any blisters either. But it just burned like touching something hot on the tip of my finger being that is the only thing I put in. Well you know the old adage, "You learn from your mistakes", stands true. lol - Anonymous

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What a remarkable story! As much as I like to think I can predict what should happen in many cases, there is just nothing like a good experiment to bring some reality to the situation. Your microwave evidently sent a significant fraction of its 900 watts of microwave radiation through that crack between cooking chamber and door and roasted your finger instantly. This is a good cautionary tale for those who are careless or curious with potentially dangerous household gadgets. While I continue to think that serious injuries are unlikely even in a leaky microwave oven, you have shown that there are cases of real danger. Fortunately, you had time to snap you finger away. It's like Class 3 lasers, which are now common in the form of laser pointers and supermarket checkout systems: they can damage your vision if you stare into them, but your blink reflex is fast enough to keep you from suffering injury. Thanks for the anecdote and I'm glad your finger recovered.

1516. While shopping for a new microwave I was asking the salesperson at a local store some questions regarding microwaves. He proceeded to tell me how dangerous they were and that they used to sell some sort of testers to see if the new microwaves they were selling "leaked radiation". He told me that they all did and that microwaves give off "harmful" radiation. He said that it affects the food that we cook in it and can cause cancer. He said "Think about it, when you get an x-ray the tech covers himself with a lead shield and here we are putting our food into this and there is no lead shield. Needless to say I did not purchase a microwave yesterday, and was wondering if you could please give me some insight on this and tell me is what this salesperson told me is true. Are microwave ovens really harmful? Do they cause cancer? What about the food, does it become toxic. A friend of mine is totally into all organic food and she "unplugged" her microwave years ago and never used it since. She swears it is harmful. Please help. Heating food in a pot is so inconvenient!! — KO
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The salesperson you spoke to was simply wrong. If you'll allow me to stand on my soapbox for a minute, I'll tell you that this is a perfect example of how important it is for everyone to truly learn basic science while they're in school and not to simply suffer through the classes as a way to obtain a degree. The salesperson is apparently oblivious to the differences between types of "radiation," to the short- and long-term effects of those radiations, and to the importance of intensity in radiation.

Let's start with the differences in types of radiation. Basically, anything that moves is radiation, from visible light, to ultraviolet, to X-rays, to microwaves, to alpha particles, to neutrons, and even to flying pigeons. These different radiations do different things when they hit you, particularly the pigeons. While "ionizing radiations" such as X-rays, ultraviolet, alpha particles, and neutrons usually have enough localized energy to do chemical damage to the molecules they hit, "non-ionizing radiation" such as microwaves and pigeons do not damage molecules. When you and your organic friend worry about toxic changes in food or precancerous changes in your tissue, what really worry you are molecular changes. Microwaves and pigeons don't cause those sorts of changes. Microwaves effectively heat food or tissue thermally, while pigeons bruise food or tissue on impact.

Wearing a lead apron while working around ionizing radiation makes sense, although a simple layer of fabric or sunscreen is enough to protect you from most ultraviolet. To protect yourself against pigeons, wear a helmet. And to protect yourself against microwaves, use metal. The cooking chamber of the microwave oven is a metal box (including the screened front window). So little microwave "radiation" escapes from this metal box that it's usually hard to detect, let alone cause a safety problem. There just isn't much microwave intensity coming from the oven and intensity matters. A little microwaves do nothing at all to you; in fact you emit them yourself!

If you want to detect some serious microwaves, put that microwave detector near your cellphone! The cellphone's job is to emit microwaves, right next to your ear! Before you give up on microwave ovens, you should probably give up on cellphones. That said, I think the worst danger about cellphones is driving into a pedestrian or a tree while you're under the influence of the conversation. Basically, non-ionizing radiation such as microwaves is only dangerous if it cooks you. At the intensities emitted by a cellphone next to your ear, it's possible that some minor cooking is taking place. However, the cancer risk is almost certainly nil.

Despite all this physics reality, salespeople and con artists are still more than happy to sell you protection against the dangers of modern life. I chuckle at the shields people sell to install on your cellphones to reduce their emissions of harmful radiation. The whole point of the cellphone is to emit microwave signals to the receiving tower, so if you shield it you spoil its operation! It would be like wrapping an X-ray machine in a lead box to protect the patient. Sure, the patient would be safe but the X-ray machine would barely work any more.

Returning to the microwave cooking issue, once the food comes out of the microwave oven, there are no lingering effects of its having been cooked with microwaves. There is no convincing evidence of any chemical changes in the food and certain no residual cooking microwaves around in the food. If you're worried about toxic changes to your food, avoid broiling or grilling. Those high-surface-temperature cooking techniques definitely do chemical damage to the food, making it both tasty and potentially a tiny bit toxic. One of the reasons why food cooked in the microwave oven is so bland is because those chemical changes don't happen. As a result, microwave ovens are better for reheating than for cooking.


1517. I recently bought a used microwave oven. The enamel coating under the glass turntable tray is rusted in a ring around the track that the turntable rotates on. Should I repair this or is it ok to just use it as is? — AA, Kettering, Ohio
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As long as the oven's metal bottom is sound underneath the rust, there isn't a problem. The cooking chamber walls are so thick and highly conducting that they reflect the microwaves extremely well even when they have a little rust on them. However, if the metal is so rusted that it loses most of its conductivity in the rust sites, you'll get local heating across the rusty patches and eventually leakage of microwaves. If you're really concerned that there may be trouble, run the microwave oven empty for about 20 seconds and then (carefully!) touch the rusty spots. If they aren't hot, then the metal underneath is doing its job just fine.

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