can’t see the forest

How do microwave ovens work?

Posted in Uncategorized by Curtis on 12/20/08

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Don’t make fun of me. It’s fascinating!!

It is!! See, she thinks so.


After the kingly refrigerator, I can think of no kitchen gadget which has more extensively transformed the way people buy, prepare, and eat food (and not necessarily for the better) than the illustrious microwave oven. The modern kitchen isn’t itself without one, and even people—like me—who tend to eschew convenience cooking for more traditional practices find the microwave a handy friend to have around for certain things.

The microwave is truly a child of the space age, as its development came about accidentally from work on radar equipment. Things got cooking in the 1940s when Percy Spencer, an engineer working on a magnetron for a Raytheon radar, noticed that a candy bar in his pocket was being melted by the device. Raytheon filed a patent for the microwave oven in 1945, and after extensive testing, the first production model was built two years later. Early microwaves were used almost exclusively in restaurants, as their cost ran into the thousands; it wasn’t until the mid 1960s that affordable home models hit the market, the first of which was Amana’s Radarange (pictured above). Microwave ovens became more commonplace as their prices fell through the 1970s, and by the late 1980s, with the addition of microprocessors to make the devices easier to use, about 25% of homes in the U.S. were nuke-equipped. Today, the figure is higher than 90%.

Microwaves are a form of electromagnetic radiation lying in the spectrum between infrared (thermal heat) and radio frequencies. The powerful magnetron in a microwave oven produces an electromagnetic field whose waves oscillate typically at 2.45 GhZ (2.45 billion cycles per second); as the field oscillates between a positive and negative charge, the water molecules in food are heated dielectrically.

polarity-of-waterWater is a polar molecule, and this is one of the substance’s most valuable characteristics. One end of the water molecule is positively charged, and the other end is negatively charged. As the field inside the microwave oven very quickly oscillates, the polar water molecules in food rotate in tandem in a futile attempt to align with the charge of the field. Molecular motion means heat, of course, and the water molecules bump against the other molecules in the food to achieve a heating that is more or less even through a process very different from infrared radiation and thermal convection, as in a traditional oven.

Modern microwaves feature various power settings, but these do not typically change the intensity of the microwave field, which is static. Instead, they control the frequency with which the magnetron is turned on and off during heating; high power means the magnetron is in operation more of the time than at lower power. The average microwave oven operates with an electrical efficiency of about 64%, using around 1100 W to produce 700 W of microwave energy, for example.

Despite the common fear of radiation in the atomic age, microwave ovens pose very little, if any, health risks to humans. Microwave radiation is non-ionizing, and thus incapable of inducing radiation sickness or cancer. The body of the microwave essentially consists of a Faraday Cage, a self-contained box which safely holds in all the microwave radiation. You may have noticed that the doors of microwave ovens are covered with a fine mesh. This mesh is designed so that the spaces are smaller than the wavelength of the microwaves themselves, preventing their escape from the chamber in a way that just the glass would not.

However, in cases where a microwave oven might be unwisely rigged to operate with the door open, the escaping waves are capable of cooking human tissue and causing insidious damage to internal organs, even across a small distance.

microwave-consoleBecause microwave heating works by inducing molecular motion rather than conducting heat through air or directly applying infrared radiation (as does a traditional oven element), microwaves cannot cause flavor-enhancing reactions such as caramelization. For this reason, microwaved food has the reputation of tasting more bland than traditionally prepared fare. Also, the evenness and thoroughness of cooking achievable with a microwave oven can vary considerably depending on the substances being cooked. For example, ice does not respond as well to dielectric heating as liquid water, so frozen foods can cook more unevenly—this can be a problem, since heating frozen foods is a common application. And very dry foods, such as pasta or rice, may not cook at all because of the low presence of water. In fact, it is dangerous to operate a microwave for a substantial length of time without moisture in the cooking chamber, because, without water to vibrate in sympathy, the radiation can feedback and destroy the oven’s magnetron. The same effect is responsible for the phenomenon of scarring that occurs when a CD or DVD is heated in a microwave.

It is not technically true that microwaves cook “from the inside out.” Rather, microwave radiation penetrates the non-conductive surfaces of certain foods more deeply than infrared radiation from a regular oven, so the inside of foods can become cooked more quickly than one might expect, although the radiation is being applied more or less evenly throughout the food.

The reason users are cautioned not to place metal objects inside an operating microwave oven is because a metallic object in a microwave field will essentially act as an antenna, multiplying any radiation. In the case of metal objects with exposed points, such as the tines of a fork, the metal can resonate with the electromagnetic field and cause air at the tips to become superheated plasma, inducing arcs. This effect is potentially destructive to a microwave oven and may cause a fire inside the cooking chamber, although microwave ovens by their nature can contain fires rather well.

Is it safe to microwave plastics and styrofoam? There’s no simple answer, but, according to a Harvard Medical School publication, it’s generally safer than you might think. While certain chemicals in plastics can migrate into foods—especially fatty foods—in trace amounts, there is a lack of any direct evidence showing specific harmful effects on humans. Just to be safe, it’s a good idea to stay away from microwaving soft plastics such as water bottles and certain take-out containers. Styrofoam is actually safe, according to the article. The best course of action is to follow your common sense and carefully check labels on plastic containers, microwave dinners, etc.

Also, one should be careful handling liquids in a microwave. When in a smooth-surfaced container, certain liquids can be dielectrically heated beyond their boiling points without actually boiling. The boiling can start explosively once the liquid is physically disturbed, e.g., removed by the user from the cooking chamber. This is why it is a safe bet to allow heated liquids to stand for a moment or two before removing them.

Microwaves can’t be said to have encouraged meaningful mealtimes in our society—but used in moderation with a bit of common sense, they aren’t half-bad.

Wikipedia – microwave oven

Physics (Suite101) – How do microwave ovens work?



7 Responses

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  1. Bluebear2 said, on 12/20/08 at 12:50 pm

    Ah the old Radar Range. My first experiences with a microwave was one of those.

    I was temporarily staying with some friends who had one. One morning I saw my friend take some toast out of the microwave, so later I tossed in some bread and started the box. Nothing seemed to be happening other than after a while the bread started to shrink a little. It definitely wasn’t toasting. After a while I gave up setting more time and took out the bread, finding it had been turned into a sheet of foam rubber or some similar object.

    Giving up on the toast I tossed in some raw pork sausages – I had seen her take out some nicely browned sausages as well.

    Over time the sausages shrank, but also did not brown. Again I finally gave up, having cycled the sausages several times only to find that the sausages had been fossilized.

    Later I asked her what I had done wrong only to find that the toast had been pre-toasted in a toaster and the sausages had been pre-cooked in a frying pan. She was only reheating them in the microwave.

    Once I put some food on a plate and covered it with plastic wrap prior to putting it in the microwave. It didn’t occur to me that the plate had a silver metal trim around the edges. I returned prior to the end of the cook time to find the edges of the plate sparkling with little plasma discharges racing around the perimeter. The plastic wrap became fused to the plate.

    • Curtis said, on 12/20/08 at 1:09 pm

      Those are good microwave anecdotes. I keep trying to get grapes in the microwave to arc, but haven’t had any success so far.

      I was born in 1980, but my parents had a thing for old appliances, so we had an early 70s model Tappan (to go with our early 70s model Kenmore under-the-counter-trash-smasher-thing) until I was in my teens.

      I recently saw where someone had converted an old microwave into a computer casemod.

  2. Bluebear2 said, on 12/21/08 at 12:20 pm

    I recently saw where someone had converted an old microwave into a computer casemod.

    Don’t be giving me Ideas now….

    Toaster Oven laptop…

    Nesco Roaster gamer…

    Clothing Iron mouse…

  3. Monte said, on 12/21/08 at 9:03 pm

    Tea bags – those are the culprits at our house for disturbing water above the boiling point. Wow.

  4. Bluebear2 said, on 12/22/08 at 2:50 pm

    Yup, anything to provide nucleating sites for the water to change state at.

    A good example of the phenomenon is the Menthos & Coke trick. In that example it is the carbon dioxide rather than water vapor that causes the eruption.

  5. Curtis said, on 12/22/08 at 4:40 pm

    I would especially like to see the Iron Mouse. If you hurry, you can beat Apple to the punch on their new multi-touch.

  6. Chris said, on 4/6/09 at 4:33 am

    Nobody I knew had a microwave oven until the mid-1980s! The first time I saw one was around 1981 when a local cafe had one installed. We were alarmed, thinking that the radiation may leak and kill us!

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