Microwave ovens, or microwaves, are kitchen appliances which use microwave radiation to cook, heat or defrost food.
Properties of a microwave:
Microwaves are electromagnetic waves with wavelengths longer than those of Terahertz (THz) wavelengths, but relatively short for radio waves. Microwaves have wavelengths approximately in the range of 30 cm (frequency = 1 GHz) to 1 mm (300 GHz). The word microwave generally refers to "alternating current signals with frequencies between 300 MHz (3 x 108 Hz) and 300 GHz (3 x 1011 Hz)."
History of the microwave oven:
The microwave oven was first invented by Dr. Percy Spencer who was a self taught engineer in 1946. He discovered that a magnetron tube which was a new type of vacuum tube, could melt things. He conducted an experiment by placing some popcorn kernels near the tube and found that the kernels near the tube cracked and popped. He attributed this result to exposure to low-density microwave energy. Dr. Spencer created a metal box with an opening into which he fed microwave power, thus creating a higher density electromagnetic field. When food was placed in the box and microwave energy powered up, the temperature of the food rose rapidly. In 1947, the first commercial microwave oven hit the market. These first models were said to be gigantic and enormously expensive, standing 1.67 meters tall, weighing over 340 kilograms and costing about $5000USD each.
Further improvements and refinements soon produced a more reliable and lightweight microwave oven that was not only less expensive, but with the development of a new air cooled magnetron, there was no longer any need for a plumber.
The microwave ovens used in domestic and commercial kitchens are vastly superior to the earlier models. A microwave oven consists of a:
•magnetron control circuit (usually with a microcontroller) •waveguide
Magnetron: It is a highly powered vacuum tube that emits a beam of microwaves. It consists of a ring of resonant electromagnetic cavities around a hot central filament. Electric charges swing back and forth inside the resonant cavity at a frequency of 2.45 to 2.5 Gigahertz. The filament produces the electrons that will be used in creating the microwaves. A combination of magnetic fields and electric charges on the walls of the resonant cavities forces large amounts of energy to build up. Some of this energy is tapped by an antenna, which is radiated away from the magnetron. This tapped energy is what makes up the microwave. The energy passes through a metal tube called the wave-guide, before entering the cooking chamber. Waveguide: The waveguide is a metal tube that transports the microwave radiation to the cooking chamber. The magnetron antenna is located at the mouth of the tube. The metal of the tube reflects the radiation, on its way to the chamber and prevents it from escaping into the environment.
Cooking Chamber: This is a metal box that reflects any radiation from the magnetron. This box usually has a paint covering which serves as an aesthetic purpose on the appliance. Almost every commercial and residential microwave has a viewing window. In order to keep the microwaves from leaking into the environment, the metal box must be completely enclosed on all sides. The viewing window contains a sheet of metal that has small holes punched into it. This sheet completes the box, and allows maximum efficiency in heating your food. Digital timer: It controls the amount of time the microwave is allowed to remain on or off. This allows the user to control how much time is spent cooking the food. After the specified time has elapsed the digital timer sends an electrical signal to the power relay. Power relay: Allows the magnetron microwave source to be turned on or off. Power transformer: Increases the incoming voltage to 3000 volts. This large amount of voltage is needed for the huge energy requirements of the magnetron. Power rectifier: This changes the alternating current that is fed into the appliance, into the direct current power needed by the magnetron.
Capacitor: Used to smooth out the current before it reaches the magnetron. A microwave oven works by passing microwave radiation, usually at a frequency of 2450 MHz (a wavelength of 12.24 cm), through food. Microwave radiation, in sufficient intensity will cause water molecules to vibrate, thereby causing friction, which produces the heat that cooks the food in a process called dielectric heating. Many molecules such as water are electric dipoles which mean that they have a positive charge at one end and a negative charge at the other. They rotate as they try to align themselves with the alternating electric field induced by the microwave beam. This movement causes heat as the rotating molecules hit other molecules and put them into motion. Microwave heating works best on liquid water and less on fats and sugars. A common misconception is that microwave ovens cook food from the "inside out". However, microwaves are absorbed in the outer layers of food in a manner similar to heat from other methods. Depth of penetration of microwaves is dependent on food composition and the frequency, with lower microwave frequencies being more penetrating. Most microwave ovens allow the user to choose between several power levels, including one or more defrosting levels. In most ovens, however, there is no change in the intensity of the microwave radiation; instead, the magnetron is turned on and off in cycles of several seconds at a time.
Microwave ovens are generally used for time efficiency in both commercial and domestic applications. They have revolutionized food preparation and cooking methods since their use became widespread in the 1970s. In fact, almost everyone household has one. In America, 95% of houses have a microwave oven.