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September 19, 2006 > How does a microwave oven work?

How does a microwave oven work?

The convenience of microwave ovens has led to very broad usage in a relatively short period of time.  Many chefs do not consider them an essential, or perhaps even desirable, kitchen appliance; but for most of us they are indispensable.  Indeed, it has been estimated that approximately 95% of all U.S. households have a microwave oven.

Defense contractor Raytheon patented the microwave cooking process in 1947, though a practical and affordable device was not developed until the 1960s.  However, many consumers considered it a potentially dangerous gimmick until the 1980s, when the microwave oven attained widespread acceptance as a standard kitchen fixture.  This explosion of popularity in the 1970s and 1980s was due in part to design improvements, including less expensive magnetrons and efficient solid-state electronics.

Before discussing the workings of a microwave oven, let's briefly define some basic terms related to electricity.  The power provided by a standard electrical outlet is alternating current or AC.  While direct current (DC), such as that provided by a battery, pushes electrons (an electrical charge) in one direction continuously, AC rapidly reverses the direction of flow back and forth.  Voltage can be loosely defined as a measure of the difference in charge between two points, or the "force" pushing the electrons through a circuit.  The standard voltage in the U.S., as we all know, is 120 volts (120V).

As this standard power enters the microwave oven, it is fed into a transformer, which steps up and "transforms" the voltage from 120V AC to, typically, 4kV (4000 Volts) DC.  This high voltage, direct current power is required to drive the "engine" of the microwave oven, the magnetron, which produces the microwaves.

The magnetron converts the electrical input to an electromagnetic output.  Essentially, it creates radio waves, similar to those that carry music to the radio antenna on a car, data across a wireless network, or a voice transmission to a cell phone.  Radio waves vary in wavelength from a few inches to a few miles.  The wavelength of a microwave in an oven is about five inches.

Finally, the microwave signal is transmitted to the cooking cavity via a wave guide.  Most microwave ovens also use a mode stirrer, yet another electronic component, whose function is to spread the microwaves evenly throughout the cooking area.  The fact that most ovens produce "hot spots" is testament to the less-than-perfect nature of these devices.  To compensate for the uneven distribution of the microwave radiation, most ovens now include a rotating cooking surface.

In summary, a microwave oven uses a transformer and magnetron to convert standard electrical energy to a very concentrated microwave signal.  While this is a form of electromagnetic radiation, it should not be confused with nuclear radiation, which is "ionizing" radiation.  This much shorter wavelength radiation has the ability to change the atomic structure of living tissue through which it passes.  Microwaves do not have this power, though they can be dangerous in other ways, as we shall see.

So how does a microwave oven cook food?  It heats food by agitating water molecules.  Again, let's take a short detour through some basic concepts of atomic physics.  We all know a water molecule is composed of two hydrogen atoms and one oxygen atom.  Since the hydrogen atoms share their negatively charged electrons with the oxygen atom, the molecules become "electric dipoles," meaning they are negatively charged on one side and positively charged on the other.  They are, in effect, tiny magnets.

When the microwaves flood through the food, their wavelengths oscillate with a frequency (typically 2.45 billion cycles per second) that causes the water molecules to whirl and spin at the same rate, as they try to align themselves with the constantly changing electrical field.  When millions of water molecules all begin this frenzied dance, they rub and bounce against each other.  This friction causes heat, which quickly diffuses through the food, and dinner is ready.

This explains why microwave ovens do not do a particularly good job of heating frozen food.  There are plenty of water molecules, but they are tied up in the form of solid ice, and unable to dance.  Most ovens provide a defrost cycle, which instead of applying continuous microwaves, radiates the food for a few seconds, then turns off for a few seconds.  This allows the ice to gradually melt and the resulting heat to be transferred slowly throughout the food, with the intent of heating the food fairly uniformly.

By the way, the common belief that microwave ovens heat from the inside out is incorrect.  The microwaves penetrate most foods to a depth of a couple inches or more, so the food begins heating up throughout, all at once.  This can give the appearance of heating from the inside, opposed to a standard convection oven, which creates hot air to heat only the outer surface of the food, relying on that heat to eventually warm the entire mass.  A microwave oven does not heat the air, where there are relatively few water molecules, but acts directly on the food, making it not only quicker but more efficient as well.

But as every cook knows, the microwave oven cannot produce a nice, crispy pie crust or effectively brown meat, since the surface material is not heated any more than the interior.  To address this deficiency, a "convection microwave" oven has been marketed in recent years, combining the two types of heating.

Now let's discuss the hazards of microwaves.  Again, since microwave energy is not ionizing radiation, it does not present the same threats to health.  Nonetheless, direct exposure to the concentrated energy inside a microwave oven would be very dangerous.  Modern ovens are designed to prevent any significant amount of radiation to escape as long as they are in undamaged condition and used as directed.

They do this by employing a charged enclosure (called a Faraday cage) that prevents the radiation from escaping.  The material pierced with small holes seen on the glass doors of many microwave ovens is part of this enclosure.  Recall that the wavelength of a microwave is about five inches.  The diameter of the holes is much smaller than this, blocking the microwaves from getting out while allowing us to see in. 

Never operate an oven with a damaged door, and keep the areas where the door seals against the oven body clean.  Do not place any metals in the oven, except those intended for the purpose.  Many metal objects can become electrically charged by the microwave beam, and may discharge this energy into the air.  In addition, foods such as eggs, potatoes, and closed containers can build up enough internal pressure to explode.  Always allow such foods a way to vent excess heat and steam.

Finally, there has been considerable material circulating on the Internet about the dangers of superheated liquids and microwave ovens.  Superheating is a condition in which a liquid is heated beyond its boiling point, but yet does not boil.  This condition is caused by rapid heating combined with a lack of "seed bubbles" or dissolved gases and impurities to aid the formation of bubbles needed to initiate boiling.

Because the microwave oven heats so quickly, it is possible that a liquid can be superheated, and much hotter than it appears.  Distilled water or a liquid that has been sitting on the kitchen counter overnight are more likely to experience superheating than a cup of water freshly drawn from the tap.  It is prudent to be cautious with any food heated in a microwave, not just liquids.

 
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