Cómo Funciona Un Horno Microondas

How does a microwave oven work?
Microwaves are electromagnetic waves. Electromagnetic waves can be classified by their frequencies, and include radio waves, television signals, radar beams, infrared waves, visible and ultraviolet light, X-rays and gamma rays. Electromagnetic waves which have a frequency between 300 MHz and 300 GHz are classified as microwaves. These two frequencies correspond towavelengths of 1 m and 1 mm, respectively. All domestic microwave ovens and laboratory microwave processors operate at 2.45 GHz (corresponding to a wavelength of 12.2 cm, or just over 4-3/4").
Microwave technology evolved out of the development of radar (Radio Detection And Ranging). Because microwave pulses can be very short, they can be used for distance and time measurement. The simplest formof radar measures the time for an echo to return from a certain direction. Microwaves penetrate fog and clouds, travel in straight lines, and give distinct shadows and reflections.
The original magnetron was invented by Albert Hull at the GE Research Laboratory in 1916. The microwave oven was invented in 1945 by Percy Spencer of Raytheon, who received a U.S. patent in 1950. The first commercialmicrowave oven appeared on the market in 1947. In the early 1950's, 50-100 units were sold per month, at a price of about $4000.00 each. In 1967, Amana introduced a counter-top model with a retail price of less than $500.00. Ten years later, in 1977, 3% of U.S. households owned a microwave oven. In 1987, 12.8 million microwave ovens were sold. According to some sources, in 1992, 90% of U.S.households owned a microwave oven, and the number worldwide now exceeds 100 million.
Microwave ovens provide an effective way of heating many nonconductive materials. Microwaves penetrate the material; whether or not heat is generated is determined by the specific dielectric properties of the material itself. In most materials, the microwave-power absorption is proportional to the water content of thematerial. The frequency of commercial microwave ovens (2.45 GHz) was selected so that a standard portion of food would be heated uniformly. Because the heat does not have to be conducted thermally through the food, but is generated inside the materials, microwaving reduces the time needed for heating the food to a uniform temperature.
Microwaves cause heating within a material by excitingmolecules to rotate. This rotation produces energy in the form of heat. Unlike conventional heating, this effect occurs simultaneously throughout the whole material being microwaved. This has important implications for microscopy, because the basis of much specimen preparation is the effective diffusion of fluids in and out of tissue blocks or sections. Heat increases the rate of diffusion, andmicrowave (internal) heating can enhance it even more effectively.
As an example, two 2 x 2 x 2 cm3 cubes of beef (striated muscle) were dehydrated. One cube was heated externally at 70 C in 100% ethyl alcohol for 5 minutes, the other kept at that temperature by microwave exposure. In the case of external heating, only the outer part of the cube was slightly dehydrated (hard and grey), but the microwavedcube was completely dehydrated (hard and grey all the way through), illustrating the more effective diffusion of alcohol into the interior of the material.
These same properties of microwave heating will dictate the choice of which processing fluids to use. Different substances subjected to the same amount of microwave energy heat up at different rates. For example, 100 ml of water needs 2.2times more heat to warm up than 100 ml of alcohol. The materials that heat up fastest are comprised of non-symmetrical polar molecules, which are easily rotated by microwave energy. This can have important implications for the microscopist. For example, xylene has been the clearing agent of choice for most conventional histology because of its fast diffusion rate, despite the fact that it is...