Microwave Oven

Microwave Oven
Who invented the Microwave oven
Percy Spencer invented the first microwave oven after World War II from radar technology developed during the war. Named the "Radarange", it was first sold in 1946. Raytheon later licensed its patents for a home-use microwave oven that was first introduced byTappan in 1955, but these units were still too large and expensive for general home use. The countertop microwave oven was first introduced in 1967 by the Amana Corporation, which was acquired in 1965 by Raytheon.

Defination:
1: The term microwave also has a more technical meaning in electromagnetics and circuit theory. Apparatus and techniques may be described qualitatively as "microwave" when the frequencies used are high enough that wavelengths of signals are roughly the same as the dimensions of the equipment, so that lumped-element circuit theory is inaccurate. As a consequence, practical microwave technique tends to move away from the discrete resistors, capacitors, and inductors used with lower-frequency radio waves. Instead, distributed circuit elements and transmission-line theory are more useful methods for design and analysis. Open-wire and coaxial transmission lines used at lower frequencies are replaced by waveguides and stripline, and lumped-element tuned circuits are replaced by cavity resonatorsor resonant lines. In turn, at even higher frequencies, where the wavelength of the electromagnetic waves becomes small in comparison to the size of the structures used to process them, microwave techniques become inadequate, and the methods of optics are used.

2: The term microwave refers to electromagnetic energy having a frequencyhigher than 1 gigahertz (billions of cycles per second), corresponding towavelength shorter than 30 centimeters
Microwave signals propagate in straight lines and are affected very little by the troposphere. They are not refracted or reflected by ionized regions in the upper atmosphere. Microwave beams do not readily diffract around barriers such as hills, mountains, and large human-made structures. Someattenuation occurs when microwave energy passes through trees and frame houses. Radio-frequency (RF) energy at longer wavelengths is affected to a lesser degree by such obstacles.
The microwave band is well suited for wireless transmission of signals having large bandwidth. This portion of the RF electromagnetic radiation spectrum encompasses many thousands of megahertz. Compare this with the so-called shortwave band that extends from 3 MHz to 30 MHz, and whose total available bandwidth is only 27 MHz. In communications, a large allowable bandwidth translates into high data speed. The short wavelengths allow the use of dish antennas having manageable diameters. These antennas produce high power gain in transmitting applications, and have excellent sensitivity and directional characteristics for reception of signals.
How does it work:
Microwave ovens use radio waves at a specifically set frequency to agitate water molecules in food. As these water molecules get increasingly agitated they begin to vibrate at the atomic level and generate heat. This heat is what actually cooks food in the oven. Because all particles in the food are vibrating and generating heat at the same time, food cooked in the microwave cooks much more swiftly than food cooked in a conventional oven where heat must slowly travel from the outside surface of the food inward.
The same radio waves that cook your food pass harmlessly through plastics, glass, and ceramics. It is this characteristic that keeps plastic plates from melting and glasses from exploding. It is also this feature of microwaves that makes them so energy efficient; they heat only the food and nothing more.
Metals, on the other hand, reflect these radio waves, a characteristic very cleverly put to use in the walls of the microwave such that no waves escape and cook anyone in the kitchen!

All of the waves discussed so far are created inside a device called a magnetron. The magnetron pulls electrons (tiny negatively charged particles) off a fine heated wire and then uses magnets to rotate them around inside a vacuum (a space void of any other particles). As these electrons swirl around and around they generate radio waves that are then sent into the oven to cook food.

Magnetron technology was not invented with the intention of giving the world a quick way to heat food. It was actually first fully developed for military radar systems. The switch from enemy detection to cooking came one day in 1946 when a radar system engineer named Percy Spencer was testing a new magnetron. He felt a strange tingling sensation and suddenly noticed that the candy bar in his pocket had melted. He then placed popcorn, eggs and other foods in front of the device and they all cooked - actually the egg exploded all over his friend's face! Needless to say culinary history was made

Magnetron" redirects here. It is not to be confused with Megatron or Metatron.

Magnetron with section removed to exhibit the cavities. The cathode in the center is not visible. The waveguide emitting microwaves is at the left. The magnet producing a field parallel to the long axis of the device is not shown.

A similar magnetron with a different section removed. Central cathode is visible; antenna conducting microwaves at the top; magnet is not shown.

Obsolete 9 GHz magnetron tube and magnets from a Soviet aircraft radar. The tube is embraced between the poles of two horseshoe-shapedalnico magnets (top, bottom), which create a magnetic field along the axis of the tube. The microwaves are emitted from the waveguide aperture (top)which in use is attached to a waveguide conducting the microwaves to the radar antenna. Modern tubes use rare earth magnets which are much less bulky.
The cavity magnetron is a high-powered vacuum tube that generates microwaves using the interaction of a stream of electrons with amagnetic field while moving past a series of open metal cavities (cavity resonators). Bunches of electrons passing by the openings to the cavities excite radio wave oscillations in the cavity, much as a guitar's strings excite sound in its sound box. The frequency of the microwaves produced, the resonant frequency, is determined by the cavities' physical dimensions. Unlike other microwave tubes, such as the klystron and traveling-wave tube (TWT), the magnetron cannot function as an amplifier, increasing the power of an applied microwave signal, it serves solely as an oscillator, generating a microwave signal from direct current power supplied to the tube.
The first form of magnetron tube, the split-anode magnetron, was invented by Albert Hull in 1920, but it wasn't capable of high frequencies and was little used. Similar devices were experimented with by many teams through the 1920s and 30s. The modern 'resonant' cavity magnetron tube was invented by John Randall and Harry Boot in 1940 at the University of Birmingham, England.[1] The high power of pulses from their device made centimeter-band radar practical, with shorter wavelength radars allowing detection of smaller objects from smaller antennas. The compact cavity magnetron tube drastically reduced the size of radar sets[2] so that they could be installed in anti-submarine aircraft[3] and escort ships.[2]
In the post-war era the magnetron became less widely used in the radar role. This was due to the fact that the magnetron's output changes from pulse to pulse, both in frequency and phase. This makes the signal unsuitable for pulse-to-pulse comparisons, which is widely used for detecting and removing "clutter" from the radar display.[4] The magnetron remains in use in some radars, but has become much more common as a low-cost microwave source for microwave ovens. In this form, approximately one billion magnetrons are in use today.[4][5]

How enery is converted in a microwave
The Magnetron
The microwave radiation of microwave ovens and some radar applications is produced by a device called a magnetron.

The magnetron is called a "crossed-field" device in the industry because both magnetic andelectric fields are employed in its operation, and they are produced in perpendicular directions so that they cross. The applied magnetic field is constant and applied along the axis of the circular device illustrated. The power to the device is applied to the center cathode which is heated to supply energetic electrons which would, in the absence of the magnetic field, tend to move radially outward to the ring anode which surrounds it.

Electrons are released at the center hot cathode by the process of thermionic emission and have an accelerating field which moves them outward toward the anode. The axial magnetic field exerts a magnetic force on these charges which is perpendicular to their initially radial motion, and they tend to be swept around the circle. In this way, work is done on the charges and therefore energy from the power supply is given to them. As these electrons sweep toward a point where there is excess negative charge, that charge tends to be pushed back around the cavity, imparting energy to the oscillation at the natural frequency of the cavity. This driven oscillation of the charges around the cavities leads to radiation of electromagnetic waves, the output of the magnetron.

Advantages:
(i) Microwave oven cooks many foods in about 1/4th of the time necessary on a gas burner. There is no wastage of energy.
(ii) It saves time in heating frozen foods. Thawing can be done in minutes or seconds.
(iii) Only the food is heated during cooking. The oven or the utensil does not get heated except under prolonged heating periods.
(iv) Flavour and texture do not change when reheated in a microwave oven.
(v) Loss of nutrients is minimized. (3-carotene and vitamin C are better retained by microwave cooking compared to pressure cooking and saucepan cooking.
(vi) After cooking in a microwave oven washing dishes is much easier as food does not stick to the sides of the vessels.
(vii) Food gets cooked uniformly.
(viii) Preserves the natural colour of vegetables and fruits.
(ix) No fat or low fat cooking can be made.
Disadvantages:
(i) Due to short period of cooking, food does not become brown unless the microwave has a browning unit.
(ii) It is not possible to make chapati or tandoori roti in it. It cannot cook soft or hard boiled eggs. Deep frying necessary for puris, jalebis, pakoda, vadas cannot be done in it.
(iii) Sometimes unwanted chemicals migrate to food from plastic cookware or food packages. Only "microwave safe" should be used.
(iv) The short cooking time may not give a chance of blending of flavours as in conventional methods.
(v) The operator should be careful in operating the microwave oven since any exposure to microwave oven causes physiological abnormalities.
(vi) If the food is greater than 80 mm the central portion is out of range of the microwave radiation will only heat by the normal slow process of conduction. It will be relatively uncooked while the exterior accessible to microwave is cooked in minutes or seconds.

Environmental impact
Microwaves do have a few other environmental advantages. For one thing, they produce a lot less indoor air pollution than other cooking methods. Plus, they don't heat up your house the way an oven can, which means lower energy costs associated with both your A/C and your refrigerator. Heating up a meal on the plate you intend to eat off of also means fewer dishes to wash—although regular use of your microwave might encourage higher consumption of ready-to-eat convenience meals and all their extra packaging.
Whichever cooking method you choose, there are plenty of little things you can do to save energy. With a conventional oven, avoid preheating and turn the heat off a few minutes before your food is done. If you switch from metal pans to heat-retaining glass or ceramic ones, you'll be able to turn your oven down by 25 degrees. Microwave ovens can be made more efficient by keeping them clean—the appliance can't tell the difference between your dinner and old cheese splatters, so it'll expend energy heating up both. Also remember to keep the appliance unplugged when you're not using it. (Over its lifetime, the average microwave will use as much energy in standby mode as it will in actually cooking your food.)
In the end, though, the greenest way to cook your food is the one that produces the tastiest meal. As we've discussed here before, food waste is an important environmental consideration: Not only does it mean more garbage, it also means squandering all the resources that went into growing, storing, and preparing that food. Let's say you roasted your Sunday-night chicken dinners in the microwave rather than the oven for a whole year. Over 52 weeks you'd save 15.6 kilowatt-hours—or about 0.14 percent of your home's annual energy use. If saving that teeny bit of energy means you'd be more likely to toss your soggy leftovers, then by all means, fire up the big boy and cook that chicken right.

A fixture in office break rooms, convenience stores and homes for decades, the microwave oven has been heating frozen foods, leftovers and even more elaborate meals for decades. In fact, some hip urban restaurants employ the familiar device to cook all their meals, from apps to entrees. Not only does this save energy and allow the restaurants to cope with small square footages in space-constrained districts, but it also offers a new retro-novelty, giving a wow factor to those who aren't familiar with the appliance's true versatility.
Yet Google "are microwave ovens safe," and you'll get a barrage of hits from concerned mothers and others who are worried that the handy device might have a dark, even dangerous side. Of course, the prevailing consensus among scientists, public health experts, government agencies and the general public is that microwave ovens are overwhelmingly safe when used as directed. However, it's also true that there may be some legitimate questions about the safety of certain aspects of the technology, beyond the paranoia of the tin-foil hat crowd.
Let's take a closer look at some myths, facts and misconceptions about microwave ovens, which are estimated to be used in at least 90% of American homes.
1. Microwave Ovens Were Discovered Accidentally
Status: Fact
Apparently no one thought of cooking food with microwaves until the 1940s, when a self-taught engineer named Percy Spencer was building radar equipment in a lab for Raytheon, and noticed that a chocolate bar he had in his pocket started to melt. He had been building magnetrons, and realized that microwaves can be directed at food to heat it up rapidly. He tested his idea by popping popcorn and exploding an egg. Not long after we were all happily scarfing down TV dinners.
2. There Is Dissent Over How Microwaves Actually Heat Food
Status: Fact
Microwave radiation is a form of non-ionizing radiation (meaning it can't directly break up atoms or molecules) that lies between common radio and infrared frequencies. So it is not thought to damage DNA of living things, the way X and gamma rays do. Still, microwaves can obviously cause heating effects, and can harm or kill at high energies. That's why microwave ovens on the market must operate at or below strict limits set by the federal government.
Most microwave ovens hit food with microwaves at a frequency of 2.45 gigahertz (GHz) (a wavelength of 12.24 centimetres (4.82 in)). The prevailing belief is that molecules in the food, particularly water, absorb energy from the waves through dielectric heating. That is, since water molecules are polar, having a positive end and negative end, they begin to rotate rapidly as the alternating electric field passes through. That rotation is thought to add heat to the food.
However, there are some scientists who have dissented with this view, suggesting thatother interactions between the particles may be responsible for the heating.

3. Microwave Ovens Cook Food from the Inside Outside
Status: Myth
Although many people believe this to be the case, microwaves actually work on the outer layers of food, heating it by exciting the water molecules there. The inner parts of food are warmed as heat transfers from the outer layers inward. This is why a microwave can only cook a big hunk of meat to a depth of about one inch inward.
4. Metals Get Dangerously Hot in Microwaves
Status: Myth
Metals reflect microwaves, whereas plastic, glass and ceramics allow them to pass through. That means metals don't appreciably heat up in a microwave oven. However, thin pieces of metal, such as foils or the tines of a fork, can act as antenna, and the waves can arc off them, forming dramatic sparks.
5. Microwave Ovens are Energy Efficient Ways to Cook
Status: Fact
A complete analysis of cooking efficiency depends on a number of factors, including what you are trying to prepare and the cost -- and greenness -- of your local supply of electricity, gas or other fuel. Typically though, a microwave uses less energy to heat food than conventional ovens or ranges, because it works faster and more of the energy is focused directly on the food, versus heating containers or surrounding air. In fact, Energy Star calculated that cooking or re-heating small portions of food in the microwave saves as much as 80% of the energy needed for an oven.
Don't get too excited, however. Consumer advocate Michael Bluejay pointed out toEarth Talk that even for someone who bakes three hours a week, using the cheapest cooking method would save only an estimated $2.06/month compared to the most expensive method. "Focusing on cooking methods is not the way to save electricity [at home]," says Bluejay. "You should look at heating, cooling, lighting and laundry instead."
6. You Can't Heat Oils in a Microwave
Status: Fact
Oils such as olive oil do not heat well in microwaves because their molecules lack the polarity found in water. It's also true that frozen butter is hard to thaw in a microwave, because the bulk of the substance is oil, and the portion of water present is in the form of ice, which keeps the molecules locked up in crystal form, making oscillation more difficult.

7. Heating Plastics in a Microwave Can Be Dangerous
Status: Fact
The safest course of action is to avoid putting any plastics in the microwave. When the Milwaukee Journal Sentinel tested plastics labeled microwave-safe and advertised for infants, even those were found to release "toxic doses" of Bisphenol Awhen heated in a microwave. "The amounts detected were at levels that scientists have found cause neurological and developmental damage in laboratory animals," the paper reports.
In fact, the term "microwave safe" is not regulated by the government, so it has no verifiable meaning. According to the Journal Sentinel's testing, BPA "is present in frozen food trays, microwaveable soup containers and plastic baby food packaging." It is often found in plastics marked No. 7, but may also be present in some plastics labeled with Nos. 1, 2 and 5 as well, according to the report. Better to stick to glass or ceramics.
8. Boiling a Cup of Water in a Microwave Can Cause It to Explode
Status: Fact
One potential danger of microwave ovens is getting scalded by over heated water. What can happen is that when plain water is heated in a microwave in a clean ceramic or glass container for too long, it can prevent bubbles from forming, which normally cool it down. The water can become superheated, past its boiling point. So when it is disturbed, say by moving it or dropping something in it, the heat releases violently, erupting boiling water out of the cup.
To avoid this risk heat water only the minimum amount of time needed. Or place a wooden spoon or stick in it (you should be fine with a metal spoon too, as we discussed above. Don't use a metal fork though, which could spark.)
9. Microwave Cooking Can Be Unsafe Because It Doesn't Heat Evenly
Status: Fact
As we learned from Jim Gaffigan, microwaves don't always heat food evenly, sometimes leaving cold pockets next to hot pockets. If you're working with raw meat, this can be dangerous, since it could leave harmful bacteria.
The Center for Science in the Public Interest has warned that consumers should follow heating instructions carefully, including the standing time needed for additional cooking (in other words don't try to cool it off before you touch it).