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Basic Microwave Oven Information, Safety Issues and Principles of Operation

© 1997-2013 by J. Carlton Gallawa All Rights Reserved Worldwide 
Last updated 3/14/2012

Microwave ovens use microwave energy to heat or cook food in a fraction of the time needed to cook with conventional ovens. Unlike conventional ovens, a microwave oven heats food without applying external heat.   

A magnetron tube is used to produce short electromagnetic waves known as microwaves, or R. F. energy. Microwave energy is directed into the cooking chamber where the food is placed to be heated.  

The microwaves readily pass through many materials, such as glass, most plastics, paper and china, with little or no effect. Generally, these materials make excellent utensils for cooking in a microwave oven.  

Some other materials, such as metal and foil, tend to reflect microwave energy. Except for certain recommended procedures that involve the use of metal or foil as outlined in the use and care manual, use of metal utensils in microwave ovens should be avoided. Why? For the following reasons:  

  • Metal utensils do not allow complete penetration of the food by the microwaves, so cooking efficiency is greatly reduced. 
  • If the cooking load is not large enough to absorb the microwave energy, the oven could be damaged by an arc between the metal utensils and the cavity interior or door assembly.
  • The life of the magnetron tube can be shortened by extended periods of back-feeding R. F. energy, which raises the magnetron tube's filament temperature.
Because metal reflects microwave energy, the metal walls of the cooking cavity are not affected by microwaves and do not get hot.  

Materials with high moisture content, like most foods, absorb microwave energy. As the electromagnetic waves at a frequency of 2450 million cycles per second enter the food, the molecules tend to align themselves with the cycling energy (animated illustration). Since the microwaves are changing polarity with every half cycle, the food molecules follow these alternations and thus move rapidly back and forth. In effect, the food molecules are changing direction every half cycle, so they are oscillating back and forth 4,900,000,000 times each second. This high-speed oscillation causes friction between the molecules, thereby converting the microwave energy to heat. 

    Microwave Facts and Safety Information

Microwaves are a form of radiant energy. Other common forms are radio waves, visible light, infrared heat and electricity. All forms of radiant energy are a part of the electromagnetic spectrum. To distinguish between the forms, they are classified according to the wave length which may vary from miles to thousandths of an inch.   

Microwaves are located in the non-ionizing portion of the energy spectrum between radio waves and visible light. The first application of microwaves was in radar during world war II (See History of the Microwave Oven). Today microwaves are widely used in communication systems, radar and many other commercial and industrial applications. 

Significantly large segments of the population are exposed to infrared rays, visible light waves and microwaves every day. One characteristic of microwaves is their ability to bounce or deflect off metal surfaces, a characteristic basic to its use in radar. Another is its thermal or heating effect utilized in microwave cooking. 

The difference between microwave energy and other forms of ionizing radiation, such as X-rays, Alpha, Beta and Gamma rays, is that microwave energy is non-ionizing. In other words, it does not alter the molecular structure of the item being heated. The effects of microwave energy are strictly thermal and do not cause cellular change as with ionizing radiation. 

Microwave Oven Questions & Answers 

QUESTION: Is it possible for heated liquids to suddenly and mysteriously erupt when being removed from a microwave oven, possibly causing severe burns?
ANSWER: Yes. Heated liquids can erupt, boil over and cause severe burns. Hot liquids that appear dormant while being removed from the oven can suddenly--without warning--erupt like an exploding volcano. This is a consequence of a phenomenon known as SUPERHEATING. Superheated liquids are in fact at or above the boiling point, but to all appearances they show no signs of boiling--that is, until they are disturbed. The simple motion of removing the container from the microwave may provide the vibration needed for the seething liquid to erupt in a scalding blast. Superheating can be prevented by stirring the liquids before heating them, or by pouring in another ingredient, which mixes air with the liquid, thus preventing the fulminate phenomenon from occurring from the book, The Complete Microwave Oven Handbook, pgs. 386-387. (Available on CD ROM. Click HERE for more)
QUESTION: Is it true that microwaves can cause bodily harm before  the body  is able to feel the heat?
ANSWER: No. Since microwaves have a thermal effect, the body's sensory nerves can feel the heat created by the microwaves at levels far below the levels necessary to cause injury even during short exposures. See How Dangerous are Microwaves?
QUESTION: Have microwave ovens been known to cause cataracts or sterility?
ANSWER: To our knowledge, there is no known case or responsible claim of cataracts or any other effect such as sterility recorded from using a microwave oven.
QUESTION: Can microwave ovens affect cardiac pacemakers?
ANSWER: As with other types of electronic products, microwave energy could cause  undesirable currents to flow in the electric leads of a pacemaker. Implanted pacemaker dysfunction has been observed near electro-cautery and diathermy apparatus,  radar and communication systems,  walkie-talkie radios,  auto ignition systems and even electric razors.  
This is because devices such as these can generate electronic interference that can be intercepted by the tiny  electronic circuits and wires of a pacemaker.  If the pacemaker is exposed to sufficiently intense levels of microwave energy, unwanted currents can flow in its wires and circuits that may cause the pacemaker to dysfunction. While such problems are unlikely, it makes sense to warn pacemaker users whenever a microwave oven is in use. All patients with pacemakers should consult with their physicians for the final word on this matter.
QUESTION: Is eating food cooked in a microwave oven harmful?
ANSWER: No. Microwaves are simply a source of heat energy just like gas and electricity. All three produce cooking in the food itself. Food cooked in a microwave oven is not radioactive or contaminated and is not harmful to eat.
QUESTION: What prevents microwave energy from escaping through the exhaust vents or through the perforations in the door where the light shines through?
ANSWER: Microwaves do not escape through the small perforations in the door, nor do they get out through the exhaust vents, because of the physical characteristics microwave energy. In RF energy applications there is a direct relationship between the frequency (cycles per second or Hz) and the wavelength (physical size of the wave): The higher the frequency, the proportionately shorter the wavelength; and, the lower the frequency, the proportionately longer the wavelength. The inverse is also true. 

The small holes in the door allow light waves to pass through, thus making it possible to see the food cooking inside, because the frequency of light waves is extremely high, which means that their wavelength very small--physically small enough to pass through small holes. On the other hand, microwaves are lower in frequency, thus they have a longer wavelength (at 2450 MHz, about 4 3/4 inches, in fact). Therefore, microwaves are simply too large to pass through the small holes in the door and in the exhaust vent openings. To the microwaves, these small perforations actually represent a solid metal wall that effectively blocks or reflects the microwaves back in the opposite direction.

QUESTION: Is there such a thing as a microwave oven with a left-handed door?

While there are many models with doors that open downward, to our knowledge, a basic microwave oven with a door that opens from the left simply does not exist. Evidently, this is because most people are right handed and there has not been enough demand for manufacturers to give attention to this market.

By the way, if anyone finds a left-handed microwave oven, please let us know and we will post it on this page.

QUESTION: How can the burnt smell be removed from a microwave oven?
ANSWER: First, clean away all remnants of the burned product using a manufacturer-recommended cleaner. Then add fresh slices of lemon to a microwave-safe container of water and place the container in the microwave. Next bring the water to a slow boil for about 10 minutes. Be careful to make sure the container does not boil dry. Allow the water cool sufficiently to safely remove the container. Finally, place freshly used coffee grounds on a paper towel and leave them in the oven overnight (do not operate the oven) to absorb any residual odor.
QUESTION: Is it safe to operate a microwave oven when it is empty?

Generally speaking, microwave ovens should not be operated when empty. Operating the oven empty can result in damage to the magnetron, excessive RF leakage, and can even start a fire.

Microwave ovens heat by showering the food or liquid in the oven with many hundreds (even thousands in commercial models) of watts of microwave energy. The microwaves penetrate and are absorbed by the food or liquid, causing it to heat up. If there is nothing to absorb the microwaves, they bounce around the cooking cavity endlessly, building up to excessively high levels of electromagnetic energy within a matter of seconds.

Eventually some of the energy will find its way back to the magnetron. The magnetron does not want this energy back, and it tends to dissipate these returning waves in the form of heat. This excess heat reduces the efficiency of the magnetron and shortens its life. Long periods of unchecked backfeeding will cause permanent magnetron failure.

This intense build up of energy can also result in excessive microwave leakage around and through the door and through the air and light vent holes. It can also produce varying degrees of electrical breakdown of metal surfaces in the cooking cavity, such as the antenna or stirrer blade, or along the walls, ceiling or floor, resulting in arcing and burning.

Some models have cooking trays (shelves) that are designed to absorb certain amounts of microwave energy. These types of trays can also be damaged by prolonged no-load operation.

More Frequently Asked Questions About Microwave Ovens

 More questions and answers coming soon... 

Copyright Information: Unless otherwise noted, all materials at this cite (including without limitation all text, html markup, graphics, and graphic elements) are copyrighted ©, 1989-2012 by J. Carlton Gallawa. The material available through this site may be freely used for attributed noncommercial educational purposes only. We ask that due credit and notification be given the author.  

All materials appearing on this Web site may not be reproduced, stored in any retrieval system, or used in any way for commercial purposes without the express prior written permission of the copyright holder.  

Disclaimer: The author assumes no liability for any incidental, consequential or other liability from the use of this information. All risks and damages, incidental or otherwise, arising from the use or misuse of the information contained herein are entirely the responsibility of the user. Although careful precaution has been taken in the preparation of this material, we assume no responsibility for omissions or errors.  

Send correspondence to: Microtech, P.O. Box 940, Gonzalez, Florida 32560 

Copyright © 1997-2013 by J. Carlton Gallawa All Rights Reserved Worldwide