History
were falling rapidly. Often called "electronic ovens" in the 1960s, the name "microwave ovens" later became standardized, often now referred to informally as simply "microwaves." Formerly found only in large industrial applications, microwave ovens were increasingly becoming a standard fixture of most kitchens. The rapidly falling price of microprocessors also helped by adding electronic controls to make the ovens easier to use. By 1986, roughly 25% of households in the U.S. owned a microwave, up from only about 1% in 1971. Current estimates hold that over 90% of American households have a microwave.
Principles
A microwave oven works by passing non-ionizing microwave radiation, usually at a frequency of 2.45 gigahertz (GHz) (a wavelength of 12.24 cm(4.82 in), through the food. Microwave radiation is between common radio and infrared frequencies. Water, fat, and other substances in the food absorb energy from the microwaves in a process called dielectric heting. Many molecules (such as those of water) are electric dipoles, meaning that they have a positive charge at one end and a negative charge at the other, and therefore rotate as they try to align themselves with the alternating electric field of the microwaves. This molecular movement creates heat as the rotating molecules hit other molecules and put them into motion.
Microwave heating is more efficient on liquid water than on fats and sugars (which have a smaller molecular dipole moment), and also more efficient than on frozen water (where the molecules are not free to rotate).Microwave heating is sometimes explained as a resonance of water molecules, but this is incorrect: such resonance only occurs in water vapor at much higher frequencies, at about 20 GHz. Moreover, large industrial/commercial microwave ovens operating at the common large industrial-oven microwave heating frequency of 915 MHz (0.915 GHz), also heat water and food perfectly well.
A common misconception is that microwave ovens cook food from the "inside out". In reality, microwaves are absorbed in the outer layers of food in a manner somewhat similar to heat from other methods. The misconception arises because microwaves penetrate dry non-conductive substances at the surfaces of many common foods, and thus often induce initial heat more deeply than other methods. Depending on water content, the depth of initial heat deposition may be several centimetres or more with microwave ovens, in contrast to broiling (infrared) or convection heating, which deposit heat thinly at the food surface. Penetration depth of microwaves is dependent on food composition and the frequency, with lower microwave frequencies (longer wavelengths) penetrating better.
Design
A microwave oven consists of:
a high voltage transformer, which passes energy to the magnetron
a cavity magnetron, which converts high-voltage electric energy to microwave radiation
a magnetron control circuit (usually with a microcontroller)
a waveguide
a cooking chamber
The frequencies used in microwave ovens were chosen based on two constraints. The first is that they should be in one of the ISM bands set aside for