How does a microwave turn electricity into heat? Like this!
In a conventional oven, heat has to pass from electric heating elements (or gas burners) positioned in the bottom and sides of the cooker into the food, which cooks mostly by conduction from the outside in—from the outer layers to the inner ones. That's why a cake cooked in a conventional oven can be burned on the edges and not cooked at all in the middle. People sometimes say microwave ovens cook food from the "inside out," which is a bit of a gloss and isn't quite correct. When people say this, what they really mean is that the microwaves are simultaneously exciting molecules right through the food, so it's generally cooking more quickly and evenly than it would otherwise.
Artwork: Microwaves (orange) cook food mainly by making the water molecules (red and blue) "vibrate" inside it.
Exactly how the food cooks in a microwave depends mostly on what it's made from. Microwaves excite the liquids in foods more strongly, so something like a fruit pie (with a higher liquid content in the center) will indeed cook from the inside out, because the inside has the highest water content. You have to be very careful eating a microwaved apple pie because the inside may be boiling hot, while the outside crust is barely even warm. With other foods, where the water content is more evenly dispersed, you'll probably find they cook from the outside in, just like in a conventional oven. Since they work by energizing water molecules, microwaves also tend to dry food out more than conventional ovens.
Another important factor is the size and shape of what you're cooking. Microwaves can't penetrate more than a centimeter or two (perhaps an inch or so) into food. Like swimmers diving into water, they're losing energy from the moment they enter the food, and after that first centimeter or so they don't have enough energy left to penetrate any deeper. If you're cooking anything big (say a joint of meat in a large microwave oven), only the outer "skin" layer will be cooked by the waves themselves; the interior will be cooked from the outside in by conduction. Fortunately, most of the things people cook in small microwave ovens aren't much more than a couple of centimeters across (think about a microwaveable meat or fruit pie). Unfortunately, since the inside and outside of the food are cooking in different ways, at different speeds, it's easy to end up with something that's cooked on the outside and uncooked in the middle, or overdone on the outside and cooked just right in the middle. Like every other cooking method, microwaving has its drawbacks and takes some getting used to.
You'll notice that microwaveable dinners specify a "cooking time" of so many minutes, followed by a "standing time" that's often just as long (where you leave the cooked food alone before eating it). During this period, the food effectively keeps on cooking: the hotter parts of the food will pass heat by conduction to the cooler parts, hopefully giving uniform cooking throughout.
The way microwave ovens distribute their microwaves can also cook things in unusual ways, as Evil Mad Scientist Laboratories found out when they tried cooking Indian snack food in a selection of different microwave ovens.
Like many great inventions, microwave ovens were an accidental discovery. Back in the 1950s, American electrical engineer Percy Spencer (1894–1970) was carrying out some experiments with a magnetron at the Raytheon Manufacturing Company where he worked. At that time, the main use for magnetrons was in radar: a way of using radio waves to help airplanes and ships find their way around in poor weather or darkness.
One day, Percy Spencer had a chocolate bar in his pocket when he switched on the magnetron. To his surprise, the bar quickly melted because of the heat the magnetron generated. This gave him the idea that a magnetron might be used to cook food. After successfully cooking some popcorn, he realized he could develop a microwave oven for cooking all types of food. He was granted a series of patents for this idea in the early 1950s, including one for a microwave coffee brewer (US patent 2,601,067, granted June 17, 1952) and the one I've illustrated here (US patent 2,495,429 "Method of Treating Foodstuffs" on January 24, 1950), which shows the basic operation of a microwave oven. In this patent, you can find Spencer's own pithy summary of how his invention works:
"...by employing wavelengths falling in the microwave region of the electromagnetic spectrum... By so doing, the wavelength of the energy becomes comparable to the average dimension of the foodstuff to be cooked, and as a result, the heat generated in the foodstuff becomes intense, the energy expended becomes a minimum, and the entire process becomes efficient and commercially feasible."
Spencer's early equipment was relatively crude compared to modern wipe-clean microwaves—his first oven was around 1.5 meters (5 ft) high! Since then, microwave ovens have become much more compact and millions of them have been sold throughout the world.
It's easy to put Spencer's invention down as a "mere" happy accident, but there was more to it than that: it takes the right kind of inventive mind to seize on a discovery and make something of it. As Reader's Digest magazine later reported, Spencer "demonstrated that nothing is beyond the grasp of a man who wants to know what is going on, and who feels a sense of responsibility for doing something about it." The 130 patents he was granted in his lifetime attest to that, and to his inventive ability to put scientific ideas into practical action.
Artwork: One of Percy Spencer's original patent drawings for the microwave oven. I've colored it in here so you can see it more clearly and recognize how very similar it is to the microwave I've described up above. On the left (red), we have the incoming electrical power. That makes a pair of magnetrons (blue) generate microwaves, which are channeled down transmission lines (yellow) and a wave guide (orange) to the cooking compartment (green). Artwork courtesy of US Patent and Trademark Office.
You might expect a microwave to be much more efficient than other forms of cooking: in other words, you'd expect more of the energy going in from the power cable to be converted into heat in your food and less to be wasted in other ways. Broadly speaking, that's correct: cooking in a microwave is cheaper and quicker than cooking with a conventional oven because you don't have to heat up the oven itself before you can cook.
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But that's not the whole story. If you want to heat up only a small quantity of food (or a cup of hot water), a microwave oven is not necessarily the best thing to use. When you microwave something, apart from putting energy into the food, you're also powering an electric motor that spins a relatively heavy glass turntable. Although you don't have to heat up the food compartment for the oven to cook, a microwave oven does, in fact, get fairly warm after it's been on for a while, so there are some heat losses. A magnetron is not perfectly efficient at converting electricity into microwaves: it will get hot. And you also have to power an electronic circuit, a timer display, and probably a cooling fan. Taken together, all these things make a microwave less efficient than it might be.
How much less efficient? Physicist Tom Murphy recently compared the energy efficiency of different methods of boiling water and found (perhaps surprisingly) that it was only about 40 percent efficient, which is about half as efficient as using an electric kettle.
Do you worry about standing too near your microwave as it hums and whirrs and blasts that frozen block into a steaming, tasty dinner? Don't! The cooking cavities in microwave ovens are sealed metal containers: use a microwave normally and the waves can't leak out. If you look closely at the inside of the glass door, you'll find it has a grid of metal stuck to the back; those holes you can see in it are too small to let microwaves through. Another safety feature (called an interlock) keeps you safe and sound: if you try to open the door, the magnetron stops buzzing immediately; most microwaves actually have two independent interlocks in case one fails. Of course, it still pays to take precautions. You don't want microwaves leaking out of your oven, so if the door doesn't close properly (perhaps because it's gummed with spilled food), if the grid on the back of the glass has started rusting and peeling away, if the interlocks don't work, or the machine gives you any reason to think it might be leaking, get it repaired or replaced straight away.
Photo: A microwave oven has a protective metal grid on the inside of its door. You can see into the oven when the door's shut because light can get through the holes in the gauze. Microwaves, however, are much bigger than light waves, so they're too big to get through the holes and remain safely "locked" inside.
Even if your microwave is "leaking," it's unlikely to do you any harm. Although microwave ovens can produce very high power inside (up to 1000 watts in a typical large oven), the power drops off very quickly the further away you go. Outside the cooking cavity and some distance away, even a leaky microwave would produce only tiny amounts of electromagnetic radiation—less than you'd pick up from a cellphone. According to the US Food and Drug Administration, at a distance of about 5cm (2 inches), the amount of power a microwave can leak is about 5 milliwatts per square centimeter, which is "far below the level known to harm people," while at a distance of about 50cm (20in), it's about 1 percent as much again. Even standing right up close to a leaky microwave, you'd need to be exposed to much higher levels of radiation for much longer for there to be any real risk to your health. The World Health Organization is reassuring on this point: "thermal damage would only occur from long exposures to very high power levels, well in excess of those measured around microwave ovens." In other words, there's simply too little power to heat your body tissue up enough to do damage.
And if you've ever wondered why you can't microwave your dinner with a cellphone (which, remember, uses similar-sized waves), the explanation is exactly the same: there isn't enough power. Even if you stood your cellphone right on top of a frozen dinner, it wouldn't release enough power to generate the heat required for cooking, no matter how long you left it there.
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The Food and Drug Administration (FDA) has regulated the manufacture of microwave ovens since 1971. Microwave oven manufacturers are required to certify their products and meet safety performance standards created and enforced by the FDA to protect the public health. Based on current knowledge about microwave radiation, the Agency believes that ovens that meet the FDA standard and are used according to the manufacturer's instructions are safe for use.
Microwave ovens are a convenient means to heat food and are generally safe when used correctly. Microwave ovens heat food using microwaves, a form of electromagnetic radiation similar to radio waves. Microwaves have three characteristics that allow them to be used in cooking: they are reflected by metal; they pass through glass, paper, plastic, and similar materials; and they are absorbed by foods.
A device called a magnetron inside the oven produces microwaves. The microwaves reflect off the metal interior of the oven and cause the water molecules in food to vibrate. This vibration results in friction between molecules, which produces heat that cooks the food.
Microwaves are non-ionizing radiation, so they do not have the same risks as x-rays or other types of ionizing radiation. But, microwave radiation can heat body tissues the same way it heats food. Exposure to high levels of microwaves can cause skin burns or cataracts. Less is known about what happens to people exposed to low levels of microwaves.
To ensure that microwave ovens are safe, manufacturers are required to certify that their microwave oven products meet the strict radiation safety standard created and enforced by the FDA.
Microwave energy will not leak from a microwave in good condition. A damaged microwave oven may present a risk of microwave energy leaks. Contact your microwave's manufacturer for assistance if your microwave oven has damage to its door hinges, latches, or seals, or if the door does not open or close properly.
Microwaves are a form of "electromagnetic" radiation; that is, they are waves of electrical and magnetic energy moving together through space. Electromagnetic radiation spans a broad spectrum from very long radio waves to very short gamma rays. The human eye can only detect a small portion of this spectrum called visible light. A radio detects a different portion of the spectrum, and an X-ray machine uses yet another portion.
Visible light, microwaves, and radio frequency (RF) radiation are forms of non-ionizing radiation. Non-ionizing radiation does not have enough energy to knock electrons out of atoms. X-rays are a form of ionizing radiation. Exposure to ionizing radiation can alter atoms and molecules and cause damage to cells in organic matter.
Microwaves are used to detect speeding cars and to send telephone and television communications. Industry uses microwaves to dry and cure plywood, to cure rubber and resins, to raise bread and doughnuts, and to cook potato chips. But the most common consumer use of microwave energy is in microwave ovens. Microwaves have three characteristics that allow them to be used in cooking: they are reflected by metal; they pass through glass, paper, plastic, and similar materials; and they are absorbed by foods.
Microwaves are produced inside the oven by an electron tube called a magnetron. The microwaves are reflected within the metal interior of the oven where they are absorbed by food. Microwaves cause water molecules in food to vibrate, producing heat that cooks the food. That's why foods that are high in water content, like fresh vegetables, can be cooked more quickly than other foods. The microwave energy is changed to heat as it is absorbed by food, and does not make food “radioactive” or "contaminated."
Although heat is produced directly in the food, microwave ovens do not cook food from the "inside out." When thick foods are cooked, the outer layers are heated and cooked primarily by microwaves while the inside is cooked mainly by the conduction of heat from the hot outer layers.
Microwave cooking can be more energy efficient than conventional cooking because foods cook faster and the energy heats only the food, not the whole oven compartment. Microwave cooking does not reduce the nutritional value of foods any more than conventional cooking. In fact, foods cooked in a microwave oven may keep more of their vitamins and minerals, because microwave ovens can cook more quickly and without adding water.
Glass, paper, ceramic, or plastic containers are used in microwave cooking because microwaves pass through these materials. Although such containers cannot be heated by microwaves, they can become hot from the heat of the food cooking inside. Some plastic containers should not be used in a microwave oven because they can be melted by the heat of the food inside. Generally, metal pans or aluminum foil should also not be used in a microwave oven, as the microwaves are reflected off these materials causing the food to cook unevenly and possibly damaging the oven. The instructions that come with each microwave oven indicate the kinds of containers to use. They also cover how to test containers to see whether or not they can be used in microwave ovens.
The FDA received reports in the past of serious skin burns or scalding injuries around people's hands and faces as a result of hot water erupting out of a cup after it had been overheated in a microwave oven. Super-heated water (water heated past its boiling temperature) does not appear to be boiling and occurs when water is heated by itself in a clean cup. If super-heating has occurred, a slight disturbance or movement such as picking up the cup, or pouring in a spoon full of instant coffee, may result in a violent eruption with the boiling water exploding out of the cup. Adding substances such as instant coffee or sugar before heating greatly reduces this risk.
Users should closely follow the precautions and recommendations provided in the microwave oven instruction manuals, specifically regarding heating times. Users should make sure that they do not exceed the recommended heating times when determining the best time settings to heat water to the desired temperature.
Through its Center for Devices and Radiological Health (CDRH), the FDA sets and enforces standards of performance for electronic products to assure that radiation emissions do not pose a hazard to public health.
A Federal standard (21 CFR 1030.10) limits the amount of microwaves that can leak from an oven throughout its lifetime to 5 milliwatts (mW) of microwave radiation per square centimeter at approximately 2 inches from the oven surface. This limit is far below the level known to harm people. Microwave energy also decreases dramatically as you move away from the source of radiation. A measurement made 20 inches from an oven would be approximately 1/100th of the value measured at 2 inches from the oven.
The standard also requires all ovens to have two independent interlock systems that stop the production of microwaves the moment the latch is released or the door is opened. In addition, a monitoring system stops oven operation in case one or both of the interlock systems fail.
All ovens must have a label stating that they meet the safety standard. In addition, the FDA requires that all ovens have a label explaining precautions for use. This requirement may be dropped if the manufacturer has proven that the oven will not exceed the allowable leakage limit even if used under the conditions cautioned against on the label.
To make sure the standard is met, FDA tests microwave ovens in its own laboratory. The FDA also evaluates manufacturers' radiation testing and quality control programs at their factories.
Microwave radiation can heat body tissue the same way it heats food. Exposure to high levels of microwaves can cause a painful burn. Two areas of the body, the eyes and the testes, are particularly vulnerable to RF heating because there is relatively little blood flow in them to carry away excess heat. Additionally, the lens of the eye is particularly sensitive to intense heat, and exposure to high levels of microwaves can cause cataracts. But these types of injuries – burns and cataracts – can only be caused by exposure to large amounts of microwave radiation.
Consumers should take common sense precautions regarding handling of hot foods and beverages. For more safety recommendations see the section of this page titled: Tips on Safe Microwave Oven Operation.
Most injuries related to microwave ovens are the result of heat-related burns from hot containers, overheated foods, or exploding liquids. Most injuries are not radiation-related. That said, there have been very rare instances of radiation injury due to unusual circumstances or improper servicing. In general, microwave oven radiation injuries are caused by exposure to large amounts of microwave radiation leaking through openings such as gaps in the microwave oven seals. However, the FDA regulations require that microwave ovens are designed to prevent these high-level radiation leaks.
At one time there was concern that radiation leakage from microwave ovens could interfere with certain electronic cardiac pacemakers. Similar concerns were raised about pacemaker interference from electric shavers, auto ignition systems, and other electronic products. While the FDA does not specifically require microwave ovens to carry warnings for people with pacemakers, this problem has largely been resolved as today’s pacemakers are designed to shield against such electrical interference. However, patients with pacemakers are encouraged to consult their physicians if they have concerns.
There is little cause for concern about excess microwaves leaking from ovens unless the door hinges, latch, or seals are damaged. The FDA recommends looking at your oven carefully, and not using an oven if the door doesn’t close firmly or is bent, warped, or otherwise damaged.
The FDA also monitors appliances for radiation safety issues and has received reports of microwave ovens that appear to stay on – and operate – while the door is open. When operating as intended, microwave ovens have safety features to prevent them from continuing to generate microwaves if the door is open. However, if an oven does continue to operate with the door open, consumers cannot be 100 percent sure that microwave radiation is not being emitted. Thus, if this occurs, the FDA recommends immediately discontinuing use of the oven.
If you suspect a radiation safety problem with your microwave oven, you may contact the microwave oven manufacturer. Manufacturers who discover that any microwave ovens produced, assembled, or imported by them have a defect or fail to comply with an applicable Federal standard are required to immediately notify FDA. In addition, manufacturers/importers are required to report all accidental radiation occurrences to the FDA, unless the incident is associated with a defect or noncompliance that has previously been reported (21 CFR 1002.20).
You may also report any suspected radiation-related problems or injuries to the FDA by completing and mailing the Accidental Radiation Occurrence Report form.
For more consumer information on microwave oven radiation, contact the Staff of the Division of Industry and Consumer Education (DICE) by email at DICE@fda.hhs.gov.
Manufacturers of electronic radiation emitting products sold in the United States are responsible for compliance with the Federal Food, Drug and Cosmetic (FD&C) Act, Chapter V, Subchapter C - Electronic Product Radiation Control.
Manufacturers of microwave ovens are responsible for compliance with all applicable requirements of Title 21 Code of Federal Regulations (Subchapter J, Radiological Health) Parts 1000 through 1005:
1000 - General
1002 - Records and Reports
Microwave oven manufacturers are exempt from product reports (§ 1002.10) except the first product report. Please also see 21 CFR 1002.1, Table 1 Record and Reporting Requirements by Product, Footnote 8
1003 - Notification of defects or failure to comply
1004 - Repurchase, repairs, or replacement of electronic products
1005 - Importation of electronic products
In addition, microwave ovens must comply with radiation safety performance standards in Title 21 Code of Federal Regulations (Subchapter J, Radiological Health) Parts 1010 and 1030.10:
1010 - Performance standards for electronic products: general
1030.10 - Microwave Ovens