Science Fair Project Encyclopedia
The idea for using microwaves to cook food was discovered by Percy Spencer who was working for Raytheon and was building magnetrons for radar sets. One day he was working on an active radar set when he had noticed a sudden and strange sensation, and saw that a chocolate bar he had in his pocket had melted. The holder of 120 patents, Spencer was no stranger to discovery and experiment, and realized what was happening. The first food to be deliberately cooked with microwaves was popcorn, and the second was an egg (which exploded in the face of one of the experimenters).
In 1946 Raytheon patented the microwave cooking process and in 1947, they built the first commercial microwave oven, the Raydarange. It was almost 6 feet (1.8 m) tall and weighed 750 pounds (340 kg). It was water-cooled and produced 3000 watts, about three times the amount of radiation produced by microwave ovens today. This first step was so successful that Raytheon eventually purchased Amana to round-out a complete home appliance product suite.
A number of other companies joined in the market, and for a time most systems were built by defense contractors, who were the most familiar with the magnetron. Litton was particularly well known in the restaurant business. By the late 1970s the technology had improved to the point where prices were falling rapidly. Formerly found only in large industrial applications, microwaves were increasingly becoming a standard fixture of most (western) kitchens. The rapidly falling price of microprocessors also helped by adding electronic controls to make the ovens easier to use. By the late 1980s they were almost universal and currently it is estimated that nearly 95% of American households have a microwave.
A microwave oven consists of:
- a magnetron,
- a magnetron control circuit (usually with a microcontroller),
- a waveguide, and
- a cooking chamber
A microwave oven works by passing microwave radiation, usually at a frequency of 2450 MHz (a wavelength of 12.24 cm), through the food. Water, fat, and sugar molecules in the food absorb energy from the microwave beam in a process called dielectric heating. Most molecules are electric dipoles, meaning that they have a positive charge at one end and a negative charge at the other, and is therefore twisted to and fro as it tries to align itself with the alternating electric field induced by the microwave beam. This molecular movement creates heat. Microwave heating is most efficient on liquid water, and much less so on fats, sugars, and frozen water. Microwave heating is sometimes incorrectly explained as resonance of water molecules, but this occurs only at much higher frequencies, in the tens of gigahertz.
The cooking chamber itself is a Faraday cage enclosure to prevent the microwaves escaping into the surroundings. The oven door is usually a glass panel, but has a layer of conductive mesh to maintain the shielding. Since the mesh width is much less than the wavelength of 12 cm, the microwave radiation can not pass through the door, while visible light (with a much shorter wavelength) can.
With wireless computer networks gaining in popularity, microwave interference has become a concern among those with wireless networks. Microwave ovens are capable of disrupting wireless network transmissions due to the fact that the microwave creates radio waves at about 2450 MHz. This is about the same frequency that wireless networks use, so microwave ovens in use can interfere with network signals and cause connection issues.
A microwave oven does not convert all electrical energy into microwaves. A typical consumer microwave oven could consume 1100 W, and deliver 700 W of microwave power. The remaining 400 W are dissipated as heat in the magnetron tube, which will not end up in the food but rather in the air in the kitchen. See Electrical efficiency.
Most of the actual microwave power will end up heating the food inside the oven, unless the microwave oven is loaded with a very small amount of absorbing food. In that case, the magnetron element will reabsorb the microwaves, which can lead to overheating.
Safety and controversy
Microwaving food is fast and popular, but there are potential hazards. See also: Microwaving.
Food is heated for so short a time that it is often cooked unevenly. Microwave ovens are frequently used for reheating previously cooked food, and bacterial contamination may not be killed by the reheating, resulting in foodborne illness. The uneven heating is partly due to the uneven distribution of microwave energy inside the oven, and partly due to the different rates of energy absorption in different parts of the food. The first problem is reduced by a stirrer, a type of fan that reflects microwave energy to different parts of the oven as it rotates, and by a turntable that turns the food. The second problem must be addressed by the cook, who should arrange the food so that it absorbs energy evenly, and periodically test and shield any parts of the food that overheat.
Water or wet foods, when heated in a microwave oven in a container with a smooth surface, can superheat, that is, reach temperatures that are a many degrees Celsius above the normal boiling point, but without actually boiling. The boiling process can start explosively if a tiny bubble should form, or even if the container is bumped when the operator removes it. Explosive boiling can result in severe burns. Closed containers and eggs can explode when heated in a microwave oven due to the pressure build-up of steam. Products that are heated too long can catch fire. Manuals of microwave ovens typically warn for these hazards. Tin foil, aluminium foil, ceramics decorated with metal, and products containing other metals can cause sparks when they are cooked in a microwave (some find it entertaining to place a compact disc in a microwave for this reason, however this may produce toxic fumes).
Large flame-like electric arcs can be triggered by metal objects in the oven chamber. If a metal utensil should brush against the metal wall of the oven, or if it should touch lightly against another conductive object (such as a metal bowl or piece of foil,) an electric arc can leap between the metal surfaces. This arc consumes several hundred watts of the oven's output, is as hot as a flame, and can char food or even set it on fire. Electric arcs can also be triggered if sharp metal edges are present, and if at the same time very little food is in the chamber. With too little food, the electric field in the oven grows intense enough that corona discharges appears on sharp metal objects such as foil, edges of metal cans, or the sharp tips of forks or knives. As before, these arc discharges are similar to flames and can cause fires.
Human body heating
Intense radio waves are more dangerous than intense visible light because the radio waves can heat a volume of flesh, while light heats mostly the body surface. While intense light can only cause surface burns, intense microwaves could cause very large internal burns. Radio waves are particularly dangerous to human eyes because the transparent parts of our eyes lack the cooling blood flow present in all other tissue. Intense EM waves can cause "RF cataracts" or "microwave cataracts."
There exists an anti-microwave lobby that claims that there exist more subtle dangers than the ones listed above associated with cooking in a microwave oven. It is claimed that microwave cooking causes more loss of nutrients than conventional cooking and that unlike conventional cooking, microwave radiation leads to toxic chemical reactions in the food. Still, these claims have not been proven scientifically. These issues are presented in the following websites:
Some people are concerned with being exposed to the radio waves present inside the oven. The USA legal limit for this frequency of electromagnetic waves is 1 mW/cm2 at 5 cm from a new oven (for a used oven, it is 5 times higher). It is rare for an oven to exceed these limits. As a comparison, a GSM cellular phone may emit up to 1 W at 1800 MHz, which is 2 mW/cm2 at 5 cm. Whether or not cellular phones are hazardous to the health is also controversial. Note also that radio waves do not resemble nuclear radiation, also called ionizing radiation. The energy per photon for ionizing radiation is higher than that for visible light, while the energy for microwaves is lower than that of light.
It is a common myth that metallic kitchen equipment, like forks and knives, can somehow repel the microwaves back into the magnetron and cause it to catch fire and maybe even explode. This is pure fantasy, and metal objects present no danger to the magnetron tube.
However, metal objects conduct heat better than food and are more likely to burn a person than food at the same temperature. Metal objects that have been in an operating microwave oven can burn a person even if the food is not hot enough to burn a person.
- Article from How Stuff Works
- Health physics society: microwave oven safety
- How Microwaves Work Article by Professor Lee, Colorado University
- How Microwaves Work Article by Microtech
- "Fun Things to Do with Microwave Ovens" by Guy Dunphy
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