Are You a Head Master, Teacher, Parent or Student? we appreciate your help, Please register and add materials to this site or mail to us for all quiries - info@myschoolvision.com
 Home>>MICROWAVE OVEN

Microwave Oven

A microwave oven, or simply a microwave, is a kitchen appliance that heats food by dielectric heating. This is accomplished by using microwave radiation to heat polarized molecules within the food. This excitation is fairly uniform, leading to food being more evenly heated throughout (except in dense objects) than generally occurs in other cooking techniques.

The first personal microwave was introduced in 1967 by the Amana Corporation. The first microwave for home use was introduced by Tappan in 1955.

Basic microwave ovens heat foods quickly and efficiently, but, unlike conventional ovens, do not brown or bake food. This makes them unsuitable for cooking certain foods, or to achieve certain culinary effects. Additional kinds of heat sources can be added to microwave packaging, or into combination microwave ovens, to add these additional effects.

Principles

A microwave oven works by passing non-ionizing microwave radiation, usually at a frequency of 2.45 gigahertz (GHz)—a wavelength of 122 millimetres (4.80 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 heating. Many molecules (such as those of water) are electric dipoles, meaning that they have a partial positive charge at one end and a partial 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 represents heat which is then dispersed as the rotating molecules hit other molecules and put them into motion.

Microwave heating is more efficient on liquid water (than on frozen water, where the molecules are not free to rotate) and on fats and sugars (which have a smaller molecular dipole moment).[8] 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.[9] Moreover, large industrial/commercial microwave ovens operating at the common large industrial-oven microwave heating frequency of 915 MHz—wavelength 328 millimetres (12.9 in)—also heat water and food perfectly well.[10]

Microwave heating can cause localized thermal runaways in some materials with low thermal conductivity, where dielectric constant increases with temperature. Under certain conditions, glass can exhibit thermal runaway in a microwave to the point of melting.[citation needed]

A common misconception is that microwave ovens cook food "from the inside out," meaning from the center of the entire mass of food outwards. 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 further. Microwaves cook from the inside out only in the sense that each molecule is generating heat from "inside" and radiating it "outward". 

  • Joy of reading maths
  • Joy of reading physics
  • Joy of reading chemistry
  • Joy of reading biology
  • Joy of reading maths
  • Joy of reading physics
  • Joy of reading chemistry
  • Joy of reading biology
Web Site Directory Information
Top
  • Follows us our servcies