|MLA Citation:||Bloomfield, Louis A. "Microwave Ovens" How Everything Works 19 Apr 2018. Page 10 of 13. 19 Apr 2018 <http://www.howeverythingworks.org/prints.php?topic=microwave_ovens&page=10>.|
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 absorb 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.
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 is 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.
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).
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 an extraordinarily rare event. Even in a cup of water many degrees above the boiling temperature, it might never happen. In reality, nucleation 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 may be 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 fairly unusual. However, they occur regularly and some of the worst victims require hospital treatment. I have heard of extreme cases in which people received serious eye injuries and third degree burns that required skin grafts and plastic surgery.
You can minimize the chance of this sort of problem by not overcooking water or any other liquid in the microwave oven, by waiting about 1 minute per cup for that liquid to cool before removing it from the microwave if there is any possibility that you have superheated it, and by being cautious when you first introduce utensils, powders, teabags, or otherwise disturb very hot liquid that has been cooked in a microwave oven. Keep the water away from your face and body until you're sure it's safe and don't ever hover over the top of the container. Finally, it's better to have the liquid boil violently while it's inside the microwave oven than when it's outside on your counter and can splatter all over you. Once you're pretty certain that the water is no longer superheated, you can ensure that it's safe 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. However, don't mess around with large quantities of superheated water. If you have more than 1 cup of potentially superheated water, don't try to nucleate boiling until you've waited quite a while for it to cool down. I've been scalded by the stuff several times even when I was prepared for an explosion. It's really dangerous.
As far as the food is concerned, the presence of the spoon redirected the microwaves somewhat, but probably without causing any noticeable changes in how the food cooked. There is certainly no residual radiation of any sort and the food is no more likely to cause cancer after being cooked with metal around than had there been no spoon with it. In general, leaving a spoon in a cup of coffee or bowl of oatmeal isn't going to cause any trouble at all. I do it all the time. In fact, having a metal spoon in the liquid may reduce the likelihood of superheating the liquid, a dangerous phenomenon that occurs frequently in microwave cooking. Superheated liquids boil violently when you disturb them and can cause serious injuries as a result.
Nucleation usually occurs at hot spots during stovetop cooking or at defects in the surfaces of cooking vessels. Glass containers have few or no such defects. When you cook water in a smooth glass container, using a microwave oven, it is quite possible that there will be no nucleation on the walls of the container and the water will superheat. This situation becomes even worse if the top surface of the water is "sealed" by a thin layer of oil or fat so that evaporation can't occur, either. Superheated water is extremely dangerous and people have been severely injured by such water. All it takes is some trigger to create the first bubble-a fork or spoon opening up the inner surface of the water or striking the bottom of the container-and an explosion follows. I recently filmed such explosions in my own microwave (low-quality movie (749KB), medium-quality movie (5.5MB)), or high-quality movie (16.2MB)). As you'll hear in my flustered remarks after "Experiment 13," I was a bit shaken up by the ferocity of the explosion I had triggered, despite every expectation that it would occur. After that surprise, you'll notice that I became much more concerned about yanking my hand out of the oven before the fork reached the water. I recommend against trying this dangerous experiment, but if you must, be extremely careful and don't superheat more than a few ounces of water. You can easily get burned or worse. For a reader's story about a burn he received from superheated water in a microwave, touch here.
Here is a sequence of images from the movie of my experiment, taken 1/30th of a second apart:
Glass containers are clearly the most likely to superheat water because their surfaces are essentially perfect. Glasses have the characteristics of frozen liquids and a glass surface is as smooth as... well, glass. When you overheat water in a clean glass measuring cup, your chances of superheating it at least mildly are surprisingly high. The spontaneous bubbling that occurs when you add sugar, coffee powder, or a teabag to microwave-heated water is the result of such mild superheating. Fortunately, severe superheating is much less common because defects, dirt, or other impurities usually help the water boil before it becomes truly dangerous. That's why most of us avoid serious injuries.
However, even non-transparent microwaveable containers often have glass surfaces. Ceramics are "glazed," which means that they are coated with glass for both sealing and decoration. Many heavy mixing bowls are glass or glass-ceramics. As you can see, it's hard to get away from trouble. I simply don't know how plastic microwaveable containers behave when heating water; they may be safe or they may be dangerous.
If you're looking for a way out of this hazard, here are my suggestions. First, learn to know how long a given amount of liquid must be heated in your microwave in order to reach boiling and don't cook it that long. If you really need to boil water, be very careful with it after microwaving or boil it on a stovetop instead. My microwave oven has a "beverage" setting that senses how hot the water is getting. If the water isn't hot enough when that setting finishes, I add another 30 seconds and then test again. I never cook the water longer than I need to. Cooking water too long on a stovetop means that some of it boils away, but doing the same in a microwave oven may mean that it becomes dangerously superheated. Your children can still "cook" soup in the microwave if they use the right amount of time. Children don't like boiling hot soup anyway, so if you figure out how long it takes to heat their soup to eating temperature and have them cook their soup only that long, they'll never encounter superheating. As for dad's coffee water, same advice. If dad wants his coffee boiling hot, then he should probably make it himself. Boiling water is a hazard for children even without superheating.
Second, handle liquids that have been heated in a microwave oven with respect. Don't remove a liquid the instant the oven stops and then hover over it with your face exposed. If the water was bubbling spasmodically or not at all despite heavy heating, it may be superheated and deserves particular respect. But even if you see no indications of superheating, it takes no real effort to be careful. If you cooked the water long enough for it to reach boiling temperature, let it rest for a minute per cup before removing it from the microwave. Never put your face or body over the container and keep the container at a safe distance when you add things to it for the first time: powdered coffee, sugar, a teabag, or a spoon.
Finally, it would be great if some entrepreneurs came up with ways to avoid superheating altogether. The makers of glass containers don't seem to recognize the dangers of superheating in microwave ovens, despite the mounting evidence for the problem. Absent any efforts on their parts to make the containers intrinsically safer, it would be nice to have some items to help the water boil: reusable or disposable inserts that you could leave in the water as it cooked or an edible powder that you could add to the water before cooking. Chemists have used boiling chips to prevent superheating for decades and making sanitary, nontoxic boiling sticks for microwaves shouldn't be difficult. Similarly, it should be easy to find edible particles that would help the water boil. Activated carbon is one possibility.
Last night's report wasn't meant to scare you away from using your microwave oven or keep you from heating water in it. It was intended to show you that there is a potential hazard that you can avoid if you're informed about it. Microwave ovens are wonderful devices and they prepare food safely and efficiently as long as you use them properly. "Using them properly" means not heating liquids too long in smooth-walled containers.
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