The Engineering Behind the Microwave Oven

In today’s technology-driven world, almost everyone, at some point in their lives, has either used or had some sort of contact with a microwave oven (see Fig. 1). Today, these familiar kitchen appliances have found a niche for themselves in as many as ninety-percent of American homes [1]. However, popular opinion was not always so overwhelmingly supportive of this breakthrough technology, as the microwave oven initially struggled to gain acceptance since its inception over fifty years ago.

Like many of the great inventions of our past, the idea behind the microwave oven was accidentally stumbled upon in 1946. Dr. Percy Spencer was working as an engineer with the Raytheon Corporation at the time, when he discovered something very unusual one day while working on a radar-related research project. While testing a new vacuum tube known as a magnetron, he discovered that a candy bar in his pocket had melted. Intrigued as he was, Spencer decided upon further experimentation. Later on, having pointed the tube at such objects as a bag of popcorn kernels and an egg, with similar results in both experiments (the popcorn popped and the egg exploded), he correctly concluded that the observed effects in each case were all attributed to exposure to low-density microwave energy [2]. Shortly after the accidental discovery, engineers at Raytheon went to work on Spencer’s new idea, developing and refining it to be of practical use.

In late 1946, this resulted in the Raytheon Company’s first patent proposing that microwaves be used to cook food. The following year, the first commercial microwave oven, dubbed the “Radarange,” hit the market with a cost between $2,000 and $3,000. Finally in 1965, Raytheon introduced the first countertop domestic oven, much improved in the area of size, safety and reliability than older models with a cost of $500. As fears and myths of these mysterious new “radar ranges” began to fade during the 1970’s, public demand began to swell with acceptance until the sales of microwave ovens eventually surpassed those of gas ranges in 1975. Furthermore, in 1976 the microwave became a more common household appliance than the dishwasher as it found its home in nearly fifty-two million U.S. households, or 60% of U.S. homes [2].

Why does our food go in cold and come out hot? The answer to this question is a multi-faceted one, involving both physics and engineering. In discovering how a microwave oven works, we must first understand the basic physical concept of electromagnetic waves. All electromagnetic (EM) waves are characterized by both a wavelength and a frequency. To help in visualizing this concept, envision yourself standing on a curb somewhere, watching an oscillating EM wave pass before you. The wavelength (in meters) can be found by measuring the length of one complete cycle of the wave, while the frequency (in seconds^-1) can be determined by keeping track of how often those cycles pass in front of you.

The relationship that forms results in the creation of an electromagnetic spectrum, composed of a wide variety of different wavelengths and corresponding frequency values. However, while each electromagnetic wave has a different corresponding wavelength and frequency, the product of these two components always equals the speed of light (roughly, 3.0 x 10⁸ meters/second) [3]. Microwaves correspond to a region in the EM spectrum defined by having wavelengths between approximately 1 meter and 1 millimeter, corresponding to frequencies between 300 MHz (Mega = 10⁶ Hz = 10⁶ sec^-1) and 300 GHz (Giga = 109).

Used extensively in communications due to their relatively short wavelengths, microwaves are often used to transmit data from satellites in space to satellite dishes on Earth. A satellite dish reflects microwaves because it is made of metal. A tighter grasp of how this “reflection” works lies in understanding the interactions that occur between the two mediums. As an EM wave hits the surface of the metal, mobile charges inherent within the metal itself are accelerated by the EM wave’s electric field, thereby preventing the wave from entering the surface and reflecting it instead [3]. As we will see, this concept, among others, readily contributes to the design of the microwave oven.

Now that we understand the essence of microwaves, we can focus our attention specifically on how a microwave oven heats food. The underlying principle behind the technology that makes microwave ovens a reality depends heavily on the fact that water molecules are electrically polar in nature-they have both positively and negatively charged ends. These polar characteristics stem from the quantum mechanical structure of water as well as the tendency for oxygen to pull electrons away from the hydrogen atoms. Having a “bent” geometry, the water molecule looks similar to Mickey Mouse’s head with its two hydrogens sticking out from the lone oxygen. As the oxygen pulls electrons away from the hydrogens a partial negative charge begins to form on the oxygen end of the molecule, while the hydrogen ends change to accommodate a partial positive charge. Thus water can be considered a polar molecule. In ice, the movement of water molecules is very constrained due to the organization of the molecules into rigid structures and orientations. But in its liquid phase, the molecules move around much more freely, with orientations being much more random in nature.

When water is placed in the presence of a strong electric field, the water molecules tend to rotate themselves into alignment with their positive ends in the direction of the field. Consequently, in their rotation they often “bump” into other water molecules, which in turn transfers some of the molecule’s electrostatic potential energy into thermal energy. An analogy would be a very crowded room, when everyone is told to turn and face the stage. In doing so, people brush up against one another as they turn and friction causes the conversion of some of their energy into thermal energy. If this action were to happen over and over, people would get extremely warm. The same idea is true for water. By reversing the direction of the electric field many times, water molecules spin backwards and forward, getting hotter and hotter each time. It is this thermal energy that cooks the food. Microwave ovens use 2.45 GHz microwaves to flip water molecules back and forth at a rate of over a billion times per second. This particular frequency was chosen because it was not in use for communications and because it provided just enough time to allow a water molecule to flip, before the field reverses its direction [3].

In its most basic form, the microwave oven comprises of several key components, each playing an important role in the overall functionality of the unit. To create the specific EM waves needed, with an exact frequency of 2.45 GHz, microwave ovens utilize a special vacuum tube called a magnetron. In short, a magnetron allows for streams of electrons to make charges (positive and negative) “slosh” in several microwave “tank” circuits that have the necessary resonant frequency, 2.45 GHz, to produce the target microwaves. Enlisting the help of a short antenna, the magnetron emits the microwaves that cook the food. Arranged in a circle, the microwave tank circuits, comprising of both an inductor and a capacitor, form the outer edge of the magnetron. Each C-shaped circuit is oriented in such a way as to resemble several people spaced evenly (yet close together, to stay warm!) around a blazing campfire on a cold night.

The capacitor section of the circuit consists of the two “arms” where separated charges initially reside (positive and negative charges, respectively on each arm), while the curved part of the circuit plays the role of the inductor, which resists changes in the circuit’s current. To illustrate the process by which this “circle of C’s” operates, let us simply envision one tank circuit by itself (picture a giant “C”). Under initial conditions, charge separation is in place with positive charges residing on the top arm, while negative charges occupy the bottom one. The charge begins to flow producing current from the positive end to the negative end. This current produces a magnetic field that flows in an upward, perpendicular direction in reference to the movement of electrons. For our purposes, in relation to our “C,” the orientation of the field would be upward and out of the plane of our paper, as if threatening to poke us in the nose. The strength of the field then grows until the separated charge at the capacitor side is eventually all gone. At this point, harnessing the potential energy stored within the magnetic field and wanting to keep the current constant, the inductor begins to propel charges through the strip even after the initial charge separation found on the capacitor end has completely dissipated. Eventually the magnetic field dies away, but not before the initial conditions of charge separation are once again realized, only this time-upside-down (our “C” would now have a negatively charge top arm, instead of a positive one, etc.). Thus the process is allowed to repeat while reversing direction [3].

This oscillation of currents at a resonant frequency of 2.45 GHz creates an environment of alternating electric and magnetic fields within the magnetron. Due to this characteristic, the microwave tank circuit is known as a resonant cavity or resonator [3]. In a typical microwave oven, the magnetron contains eight resonators, assembled in a ring, with each of their tips touching the tips of their neighbor’s (remember the campfire analogy). Another important factor arises in the discussion of the importance of materials used in microwave construction that contribute to its efficiency and functionality. As is such in any case of design or creation, the selection of materials must be carefully considered in light of the various advantages and disadvantages that are inherent within each substance. Due to the limitations of copper as an electrical conductor, a portion of the generated energy is lost during the “microwave wave-making” process in the form of heat. To make up for this loss, as well as adjust for the energy expended in cooking the food, power is supplied to the resonators in the form of a stream of electrons [3]. At the center of the ring of resonators lies the source of that stream in the form of a cathode, a negatively charged filament that is connected to a high voltage power supply. The power supply electrically “pumps” the filament with negative charges, inducing a strong electric field that originates from four surrounding, positive resonator tips. The direction of the field is established by convention to point towards the direction that positively charged particles flow when subjected to the field.

There also exists within the magnetron a strong magnetic field, generated by a nearby large permanent magnet. Much like our earlier example, the field would extend upward, and out of the plane of a piece of paper if we were looking at the magnetron from above. If left to operate by itself, the magnetic field would undoubtedly accelerate the many electrons contained on the “hot” cathode in a counter-clockwise direction, never approaching the resonators. In real life, both the electric field and the magnetic field are present at the same time.

Since both of these fields apply forces to the moving electrons, the ensuing stream of charges is rather complicated in nature. Merging the two initial forces, outward and circular, the resultant force takes the form of something resembling a spinning bicycle wheel, with four outwardly bent electron beams rotating in a counter-clockwise motion [3]. The main difference is that the electron beams now reach the resonators, not at their positively charged tips (which would be the case without the magnetic field), but at their negatively charged tips. Therefore, the net effect is one of addition to the charge separation in the resonators [3].

With each oscillation of charge that takes place on the resonators, the electron beams rotate with perfect synchronization, such that they always land on a negatively charged tip. By helping to increase the charge separation, they in turn boost the power needed for oscillations in the resonators and allow the transfer of energy to the food to continue The power of the oscillating charges are harnessed by a small wire coil placed within the cavity of the magnetron, from which a 2.45 GHz alternating current is induced due to the changing magnetic field. This current is then translated to a small antenna, which emits microwaves into a metal pipe attached to the cooking chamber. The waves then reflect along until they reach the chamber where they proceed to cook the food.

The microwave oven has taken its time in establishing a place for itself among the many other appliances that adorn the kitchen countertops of today. Yet while its usefulness and capabilities are often well known, the intricacies of its design and inner workings are not. Hopefully in the future, this “imbalance” of understanding will shift more towards enlightenment as more and more people come to realize that the only real “magic” within a microwave oven is the engineering behind it.