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These days, in many homes, it is virtually impossible to have a so called modern kitchen, without having a microwave oven in it. The microwave oven could probably occupy the second place (after the refrigerator), in terms of dependence, in the kitchen. This magical piece of technology has revolutionized the way people, and the food industries cook food. The microwave oven is only a natural evolution of technology; to fit into this fast-paced world. As in every evolutionary process, one must look back at the history of how it all began. Then for the scientifically-inclined; the heart of the microwave oven will be revealed, by exposing the physics that make it work. And last, we will present to the reader how these microwaves actually cook the food.
Like many of today’s greatest inventions, the microwave oven was a by-product of another technology. The microwave oven did not come about as a result of someone trying to find a better, faster way to cook, but it was an accidental invention! In 1939, Two British scientists, H A H Boot and J T Randall developed the pulse type magnetron tube for radar, a tube that produces microwaves. Installing magnetrons in Britain’s radar system, the microwaves were able to spot Nazi warplanes on their way to bomb the British Isles.
A few years later, around 1945, during a radar-related research project; Dr. Percy Spencer, an engineer with the Raytheon Corporation, noticed something very unusual. Dr. Spencer was conducting some tests on the magnetron, when he discovered that the candy bar in his pocket had melted. This intrigued Dr. Spencer, so he tried another experiment. This time he placed some popcorn kernels near the magnetron and he watched as the popcorn cracked and popped all over his lab. An idea was born!
The following day, Dr. Spencer decided to put the magnetron tube near an egg. And he watched as the egg began to tremor and quake. The rapid temperature rise within the egg was causing tremendous internal pressure. Evidently, after a couple of minutes, the egg exploded and splattered hot yolk all over the table. The face of Spencer lit up with a logical scientific conclusion: the melted candy bar, the popcorn, and now the exploding egg, were all attributable to exposure to low-density microwave energy. If an egg can be cooked that fast why not other foods? At that point, it was only a natural conclusion that microwave heating could raise the internal temperature of many foods far more rapidly than a conventional oven.
Dr. Spencer fashioned a metal box with an opening into which he fed microwave power. The energy entering the box was unable to escape, thereby creating a higher density electromagnetic field. When food was placed in the box and microwave energy fed in, the temperature of the food rose very rapidly. Dr. Spencer had invented what was to revolutionize cooking, and form the basis of a multimillion dollar industry; the microwave oven.
Figure 1: Original Microwave Oven Patent by Dr. Spencer
At this point, engineers went to work on Spencer’s hot new idea, developing and refining it for practical use. By late 1945, the Raytheon Company had filed a patent proposing that microwaves be used to cook food. At last, in 1947, the first commercial microwave oven hit the market. Raytheon demonstrated the world’s first microwave oven and called it the “Radarange”. These primitive units where gigantic and enormously expensive, standing 5 1/2 feet tall, weighing over 750 pounds, and costing about $5000 each. The magnetron tube had to be water-cooled, so plumbing installations were also required.
Not surprisingly, initial reactions were unfavourable; consumers were highly reluctant about these first units. The first units were big, ugly and scary! And so they found only limited acceptance. Initial sales were disappointing, but not for long. Further improvements and refinements soon produced a more reliable and lightweight 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.
Sometime around 1952 and 1955, a company by the name of Tappan introduced the first home model priced at $1295. In 1967 Raytheon introduced the first countertop, domestic oven. It was a 100-volt microwave oven, which cost just under $500 and was smaller, safer and more reliable than previous models.
The microwave oven had reached a new level of acceptance, particularly with regard to certain industrial applications. By having a microwave oven available, restaurants and vending companies could now keep products refrigerator-fresh up to the point of service, then heat to order. The result: fresher food, less waste, and money saved. Now the invention boomed!
As the food industry began to recognize the potential and versatility of the microwave oven, its usefulness was put to new tests. Industries began using microwaves to dry potato chips and roast coffee beans and peanuts. Meats could be defrosted, precooked and tempered. Even the shucking of oysters was made easier by microwaves. Other industries found the diverse applications of microwave heating quite advantageous. In time, microwaves were being used to dry cork, ceramics, paper, leather, tobacco, textiles, pencils, flowers, wet books and match heads. The microwave oven had become a necessity in the commercial market and the possibilities seemed endless.
Technological advances and further developments led to a microwave oven that was polished and priced for the consumer kitchen. However, there were many myths and fears surrounding these mysterious new electronic “radar ranges.” By the seventies, more and more people were finding the benefits of microwave cooking to outweigh the feared risks, and no one was dying of radiation poisoning, going blind, or becoming sterile! As fears faded, a growing wave of acceptance began filtering into the kitchens of America and other countries.
By 1975, sales of microwave ovens would, for the first time, exceed that of gas ranges. Before long, though, microwave ovens were enhancing the kitchens in million of homes in the United States. In 1976, the microwave oven became a more commonly owned kitchen appliance than the dishwasher. America’s cooking habits were being drastically changed by the time and energy-saving convenience of the microwave oven. Once considered a luxury, the microwave oven had developed into a practical necessity for a fast-paced world.
The magnetron is the heart of every microwave oven because it generates the required microwave energy to heat or cook food. It is a diode-type electron tube which is used to produce the required 2450 MHz of microwave energy. (Frequency of around 2.5 GHz has a wavelength of around 12 cm. It is also about the same frequency used for some cordless home phones, and some wireless routers used, in homes, schools and in companies). The external components of the magnetron vary between different models of microwave ovens, but the internal parts are basically the same, which are the anode, the filament/cathode, the antenna, and the magnets.
Figure 2: Typical Magnetron Structure
Anode: The anode is an iron cylinder from which an even number of anode vanes extend inward. The open trapezoidal shaped areas between each of the vanes are resonant cavities that serve as tuned circuits and determine the output frequency of the tube. The anode operates in such a way that alternate segments must be connected, or strapped, so that each segment is opposite in polarity to the segment on either side.
Filament/Cathode: The filament/cathode is located in the center of the magnetron, and is supported by the large and rigid filament leads, which are carefully sealed into the tube and shielded.
Antenna: The antenna is a probe or loop that is connected to the anode and extends into one of the tuned cavities, which use to transmit RF (Radiofrequency) energy.
Magnetic Field: The magnetic field is provided by strong permanent magnets, which are mounted around the magnetron so that the magnetic field is parallel with the axis of the cathode.
The magnetron is called a “crossed-field” device because both magnetic and electric fields are employed in its operation, and they are produced in perpendicular directions so that they cross. The applied magnetic field is constant. The power to the device is applied to the center cathode which is heated to supply energetic electrons. Those electrons, in the absence of the magnetic field, will tend to move radially outward to the ring anode which surrounds it.
Oscillating magnetic and electric fields are produced in the resonant cavity. This cavity creates a resonance similar to a parallel LC resonant circuit, where flowing currents around the cavity act as inductor and charge at ends of cavity act as capacitor, thus producing a natural resonant frequency. This frequency will keep pumping in the cavity by a process called thermionic emission. Electrons are released at the hot filament/cathode and have an accelerating field which moves them toward the anode. They tend to be swept as the magnetic field exerts a magnetic force on them. When they sweep at a point that there is excess negative charge, this negative charge will push back around the cavity, and impart energy to the oscillation at the natural resonant frequency of the cavity. This will lead to the radiation of electromagnetic waves, which is also known as microwave radiation.
Microwave Oven is a high-voltage system. It inputs a regular AC voltage (~115V) and outputs a very-high DC voltage (>2000V). To transform a low AC voltage to a high DC voltage the diode (rectifier), with the capacitor must function together to effectively double the already-high voltage. This is called a voltage-doubler circuit.
Figure 3: Voltage-Doubler circuit in a Microwave Oven
The voltage-doubler circuit acts as a sine wave shown in the above figure. During the positive half-wave, the capacitor is charging through the diode as the current flows. Initially there is no voltage because the capacitor is not charging up yet, but at T1, with the flowing current; the voltage across the capacitor will rise to the transformer’s secondary voltage to the maximum of 2800 V. As the transformer secondary voltage begins to decrease from its maximum positive value T2, the capacitor will discharge back through the diode, but it remains at 2800 V. At T3 the transformer secondary (output) voltage swings into the negative half-cycle and increases in a negative direction to a negative 2800 V. The transformer secondary and the charged capacitor will then become two energy sources in series, which applied a sum voltage of 5600 V to the magnetron. Since a voltage doubler is a rectifier, the output voltage will be DC. This DC power will then be converted to RF (Radiofrequency) energy in the magnetron in order to produce microwaves.
Now, with all this knowledge in our possession, how does all of this cook the food? That’s a very good question!
Figure 4: Typical Microwave Oven
As mentioned previously the magnetron produces the microwaves. Theses waves enter the oven area by traveling through a tube called a waveguide. The waveguide is a tube consisting of a hollow metallic conductor, maybe containing some kind of dielectric material, all for the purpose of sending out most efficiently those microwaves.
The inner wall of the oven is made of metal, coated with epoxy paint, to reflect the waves, and eventually, those waves will hit the food, even if they miss it the first time. The microwave door has a special screen on it that stops microwaves from escaping outside.
When the waveguide tube injects microwaves into the inside of a microwave oven, there are always some areas that get hit with fewer waves than other areas. If food was warmed up like that, there would be parts that are hot and some that are cold. So using turntables to spin the food inside microwave ovens solves this problem conveniently.
Why do microwave ovens produce these electromagnetic waves with a frequency of 2.5GHz? What is important with this frequency value is that these waves are absorbed most readily by water, as well as by fats and sugars, the main components of everyday food! So these chemical compounds when hit by these electromagnetic waves, absorb them which are directly converted into atomic motion; namely the atomic molecules start to move or rotate. This movement is present in all the molecules which start to move around and into each other, also known as friction. One should notice that this whole process of heating is different than the conventional gas or electric oven process in that in the former, one is exciting the atoms making them hot wherever there is H2O, whereas in the latter, one is conducting heat from the outside towards the inside. Interestingly as well, is that with that frequency range, waves are not absorbed by glass, paper, ceramics, and certain plastics. The heat developed in these materials is from the heat of the food only.
One could clearly see that electromagnetic theory has its place everywhere around us, from telephone, to microwave ovens. Those ovens use three fundamental characteristics of electromagnetic waves: reflection, transmission, and absorption. Those characteristics are present around us on a regular basis; even in this very course (Elec351). Reflection: microwaves are reflected by bouncing off the metal walls inside the oven. Transmission: microwaves pass harmlessly through materials such as glass, and some plastics. Absorption: anything that is moist will absorb microwaves. Consequently, molecules of the moist object will vibrate causing heat by friction, hence heating the food.
Even though the initial reactions toward the microwave oven were unfavourable, the expanding market has now produced a style to suit every taste; a size, shape, and color to fit any kitchen, and a price to please almost every wallet. Options and features, such as the addition of convection heat, probe and sensor cooking, meet the needs of virtually every cooking, heating or drying application. Today, the magic of microwave cooking has radiated around the globe, becoming an international phenomenon.
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