RESONANCE

CONCEPT

Though people seldom witness it directly, the entire world is in a state of motion, and where solid objects are concerned, this motion is manifested as vibration. When the vibrations produced by one object come into alignment with those of another, this is called resonance. The power of resonance can be as gentle as an adult pushing a child on a swing, or as ferocious as the force that toppled what was once the world's third-longest suspension bridge. Resonance helps to explain all manner of familiar events, from the feedback produced by an electric guitar to the cooking of food in a microwave oven.

HOW IT WORKS

Vibration of Molecules

The possibility of resonance always exists wherever there is periodic motion, movement that is repeated at regular intervals called periods, and/or harmonic motion, the repeated movement of a particle about a position of equilibrium or balance. Many examples of resonance involve large objects: a glass, a child on a swing, a bridge. But resonance also takes place at a level invisible to the human eye using even the most powerful optical microscope.

All molecules exert a certain electromagnetic attraction toward each other, and generally speaking, the less the attraction between molecules, the greater their motion relative to one another. This, in turn, helps define the object in relation to its particular phase of matter.

A substance in which molecules move at high speeds, and therefore hardly attract one another at all, is called a gas. Liquids are materials in which the rate of motion, and hence of intermolecular attraction, is moderate. In a solid, on the other hand, there is little relative motion, and therefore molecules exert enormous attractive forces. Instead of moving in relation to one another, the molecules that make up a solid tend to vibrate in place.

Due to the high rate of motion in gas molecules, gases possess enormous internal kinetic energy. The internal energy of solids and liquids is much less than in gases, yet, as we shall see, the use of resonance to transfer energy to these objects can yield powerful results.

Oscillation

In colloquial terms, oscillation is the same as vibration, but, in more scientific terms, oscillation can be identified as a type of harmonic motion, typically periodic, in one or more dimensions. All things that oscillate do so either along a more or less straight path, like that of a spring pulled from a position of stable equilibrium; or they oscillate along an arc, like a swing or pendulum.

In the case of the swing or pendulum, stable equilibrium is the point at which the object is hanging straight downward—that is, the position to which gravitation force would take it if no other net forces were acting on the object. For a spring, stable equilibrium lies somewhere between the point at which the spring is stretched to its maximum length and the point at which it is subjected to maximum compression without permanent deformation.

CYCLES AND FREQUENCY.

A cycle of oscillation involves movement from a certain point in a certain direction, then a reversal of direction and a return to the original point. It is simplest to treat a cycle as the movement from a position of stable equilibrium to one of maximum displacement, or the furthest possible point from stable equilibrium.

The amount of time it takes to complete one cycle is called a period, and the number of cycles in one second is the frequency of the oscillation. Frequency is measured in Hertz. Named after nineteenth-century German physicist Heinrich Rudolf Hertz (1857-1894), a single Hertz (Hz)—the term is both singular and plural—is equal to one cycle per second.

AMPLITUDE AND ENERGY.

The amplitude of a cycle is the maximum displacement of particles during a single period of oscillation. When an oscillator is at maximum displacement, its potential energy is at a maximum as well. From there, it begins moving toward the position of stable equilibrium, and as it does so, it loses potential energy and gains kinetic energy. Once it reaches the stable equilibrium position, kinetic energy is at a maximum and potential energy at a minimum.

As the oscillating object passes through the position of stable equilibrium, kinetic energy begins to decrease and potential energy increases. By the time it has reached maximum displacement again—this time on the other side of the stable equilibrium position—potential energy is once again at a maximum.

OSCILLATION IN WAVE MOTION.

The particles in a mechanical wave (a wave that moves through a material medium) have potential energy at the crest and trough, and gain kinetic energy as they move between these points. This is just one of many ways in which wave motion can be compared to oscillation. There is one critical difference between oscillation and wave motion: whereas oscillation involves no net movement, but merely movement in place, the harmonic motion of waves carries energy from one place to another. Nonetheless, the analogies than can be made between waves and oscillations are many, and understandably so: oscillation, after all, is an aspect of wave motion.

A periodic wave is one in which a uniform series of crests and troughs follow one after the other in regular succession. Two basic types of periodic waves exist, and these are defined by the relationship between the direction of oscillation and the direction of the wave itself. A transverse wave forms a regular up-and-down pattern, in which the oscillation is perpendicular to the direction in which the wave is moving. On the other hand, in a longitudinal wave (of which a sound wave is the best example), oscillation is in the same direction as the wave itself.

Again, the wave itself experiences net movement, but within the wave—one of its defining characteristics, as a matter of fact—are oscillations, which (also by definition) experience no net movement. In a transverse wave, which is usually easier to visualize than a longitudinal wave, the oscillation is from the crest to the trough and back again. At the crest or trough, potential energy is at a maximum, while kinetic energy reaches a maximum at the point of equilibrium between crest and trough. In a longitudinal wave, oscillation is a matter of density fluctuations: the greater the value of these fluctuations, the greater the energy in the wave.

Parameters for Describing Harmonic Motion

The maximum value of the pressure change between waves is the amplitude of a longitudinal wave. In fact, waves can be described according to many of the same parameters used for oscillation—frequency, period, amplitude, and so on. The definitions of these terms vary somewhat, depending on whether one is discussing oscillation or wave motion; or, where wave motion is concerned, on whether the subject is a transverse wave or a longitudinal wave.

For the present purposes, however, it is necessary to focus on just a few specifics of harmonic motion. First of all, the type of motion with which we will be concerned is oscillation, and though wave motion will be mentioned, our principal concern is the oscillations within the waves, not the waves themselves. Second, the two parameters of importance in understanding resonance are amplitude and frequency.

Resonance and Energy Transfer

Resonance can be defined as the condition in which force is applied to an oscillator at the point of maximum amplitude. In this way, the motion of the outside force is perfectly matched to that of the oscillator, making possible a transfer of energy.

As its name suggests, resonance is a matter of one object or force "getting in tune with" another object. One literal example of this involves shattering a wine glass by hitting a musical note that is on the same frequency as the natural frequency of the glass. (Natural frequency depends on the size, shape, and composition of the object in question.) Because the frequencies resonate, or are in sync with one another, maximum energy transfer is possible.

The same can be true of soldiers walking across a bridge, or of winds striking the bridge at a resonant frequency—that is, a frequency that matches that of the bridge. In such situations, a large structure may collapse under a force that would not normally destroy it, but the effects of resonance are not always so dramatic. Sometimes resonance can be a simple matter, like pushing a child in a swing in such a way as to ensure that the child gets maximum enjoyment for the effort expended.

REAL-LIFE APPLICATIONS

A Child on a Swing and a Pendulum in a Museum

Suppose a father is pushing his daughter on a swing, so that she glides back and forth through the air. A swing, as noted earlier, is a classic example of an oscillator. When the child gets in the seat, the swing is in a position of stable equilibrium, but as the father pulls her back before releasing her, she is at maximum displacement.

He releases her, and quickly, potential energy becomes kinetic energy as she swings toward the position of stable equilibrium, then up again on the other side. Now the half-cycle is repeated, only in reverse, as she swings backward toward her father. As she reaches the position from which he first pushed her, he again gives her a little push. This push is essential, if she is to keep going. Without friction, she could keep on swinging forever at the same rate at which she begun. But in the real world, the wearing of the swing's chain against the support along the bar above the swing will eventually bring the swing itself to a halt.

TIMING THE PUSH.

Therefore, the father pushes her—but in order for his push to be effective, he must apply force at just the right moment. That right moment is the point of greatest amplitude—the point, that is, at which the father's pushing motion and the motion of the swing are in perfect resonance.

If the father waits until she is already on the downswing before he pushes her, not all the energy of his push will actually be applied to keeping her moving. He will have failed to efficiently add energy to his daughter's movement on the swing. On the other hand, if he pushes her too soon—that is, while she is on the upswing—he will actually take energy away from her movement.

If his purpose were to bring the swing to a stop, then it would make good sense to push her on the upswing, because this would produce a cycle of smaller amplitude and hence less energy. But if the father's purpose is to help his daughter keep swinging, then the time to apply energy is at the position of maximum displacement.

It so happens that this is also the position at which the swing's speed is the slowest. Once it reaches maximum displacement, the swing is about to reverse direction, and, therefore, it stops for a split-second. Once it starts moving again, now in a new direction, both kinetic energy and speed increase until the swing passes through the position of stable equilibrium, where it reaches its highest rate.

THE FOUCAULT PENDULUM.

Hanging from a ceiling in Washington, D.C.'s Smithsonian Institution is a pendulum 52 ft (15.85 m) long, at the end of which is an iron ball weighing 240 lb (109 kg). Back and forth it swings, and if one sits and watches it long enough, the pendulum appears to move gradually toward the right. Over the course of 24 hours, in fact, it seems to complete a full circuit, moving back to its original orientation.

There is just one thing wrong with this picture: though the pendulum is shifting direction, this does not nearly account for the total change in orientation. At the same time the pendulum is moving, Earth is rotating beneath it, and it is the viewer's frame of reference that creates the mistaken impression that only the pendulum is rotating. In fact it is oscillating, swinging back and forth from the Smithsonian ceiling, but though it shifts orientation somewhat, the greater component of this shift comes from the movement of the Earth itself.

This particular type of oscillator is known as a Foucault pendulum, after French physicist Jean Bernard Leon Foucault (1819-1868), who in 1851 used just such an instrument to prove that Earth is rotating. Visitors to the Smithsonian, after they get over their initial bewilderment at the fact that the pendulum is not actually rotating, may well have another question: how exactly does the pendulum keep moving?

As indicated earlier, in an ideal situation, a pendulum continues oscillating. But situations on Earth are not ideal: with each swing, the Foucault pendulum loses energy, due to friction from the air through which it moves. In addition, the cable suspending it from the ceiling is also oscillating slightly, and this, too, contributes to energy loss. Therefore, it is necessary to add energy to the pendulum's swing.

Surrounding the cable where it attaches to the ceiling is an electromagnet shaped like a donut, and on either side, near the top of the cable, are two iron collars. An electronic device senses when the pendulum reaches maximum amplitude, switching on the electromagnet, which causes the appropriate collar to give the cable a slight jolt. Because the jolt is delivered at the right moment, the resonance is perfect, and energy is restored to the pendulum.

Resonance in Electricity and Electromagnetic Waves

Resonance is a factor in electromagnetism, and in electromagnetic waves, such as those of light or radio. Though much about electricity tends to be rather abstract, the idea of current is fairly easy to understand, because it is more or less analogous to a water current: hence, the less impedance to flow, the stronger the current. Minimal impedance is achieved when the impressed voltage has a certain resonant frequency.

NUCLEAR MAGNETIC RESONANCE.

The term "nuclear magnetic resonance" (NMR) is hardly a household world, but thanks to its usefulness in medicine, MRI—short for magnetic resonance imagining—is certainly a well-known term. In fact, MRI is simply the medical application of NMR. The latter is a process in which a rotating magnetic field is produced, causing the nuclei of certain atoms to absorb energy from the field. It is used in a range of areas, from making nuclear measurements to medical imaging, or MRI. In the NMR process, the nucleus of an atom is forced to wobble like a top, and this speed of wobbling is increased by applying a magnetic force that resonates with the frequency of the wobble.

The principles of NMR were first developed in the late 1930s, and by the early 1970s they had been applied to medicine. Thanks to MRI, physicians can make diagnoses without the patient having to undergo either surgery or x rays. When a patient undergoes MRI, he or she is made to lie down inside a large tube-like chamber. A technician then activates a powerful magnetic field that, depending on its position, resonates with the frequencies of specific body tissues. It is thus possible to isolate specific cells and analyze them independently, a process that would be virtually impossible otherwise without employing highly invasive procedures.

LIGHT AND RADIO WAVES.

One example of resonance involving visible and invisible light in the electromagnetic spectrum is resonance fluorescence. Fluorescence itself is a process whereby a material absorbs electromagnetic radiation from one source, then re-emits that radiation on a wavelength longer than that of the illuminating radiation. Among its many applications are the fluorescent lights found in many homes and public buildings. Sometimes the emitted radiation has the same wavelength as the absorbed radiation, and this is called resonance fluorescence. Resonance fluorescence is used in laboratories for analyzing phenomena such as the flow of gases in a wind tunnel.

Though most people do not realize that radio waves are part of the electromagnetic spectrum, radio itself is certainly a part of daily life, and, here again, resonance plays a part. Radio waves are relatively large compared to visible light waves, and still larger in comparison to higher-frequency waves, such as those in ultraviolet light or x rays. Because the wavelength of a radio signal is as large as objects in ordinary experience, there can sometimes be conflict if the size of an antenna does not match properly with a radio wave. When the sizes are compatible, this, too, is an example of resonance.

MICROWAVES.

Microwaves occupy a part of the electromagnetic spectrum with higher frequencies than those of radio waves. Examples of microwaves include television signals, radar—and of course the microwave oven, which cooks food without applying external heat. Like many other useful products, the microwave oven ultimately arose from military-industrial research, in this case, during World War II. Introduced for home use in 1955, its popularity grew slowly for the first few decades, but in the 1970s and 1980s, microwave use increased dramatically. Today, most American homes have microwaves ovens.

Of course there will always be types of food that cook better in a conventional oven, but the beauty of a microwave is that it makes possible the quick heating and cooking of foods—all without the drying effect of conventional baking. The basis for the microwave oven is the fact that the molecules in all forms of matter are vibrating. By achieving resonant frequency, the oven adds energy—heat—to food. The oven is not equipped in such a way as to detect the frequency of molecular vibration in all possible substances, however; instead, the microwaves resonant with the frequency of a single item found in nearly all types of food: water.

Emitted from a small antenna, the microwaves are directed into the cooking compartment of the oven, and, as they enter, they pass a set of turning metal fan blades. This is the stirrer, which disperses the microwaves uniformly over the surface of the food to be heated. As a microwave strikes a water molecule, resonance causes the molecule to align with the direction of the wave. An oscillating magnetron, a tube that generates radio waves, causes the microwaves to oscillate as well, and this, in turn, compels the water molecules to do the same. Thus, the water molecules are shifting in position several million times a second, and this vibration generates energy that heats the water.

Microwave ovens do not heat food from the inside out: like a conventional oven, they can only cook from the outside in. But so much energy is transferred to the water molecules that conduction does the rest, ensuring relatively uniform heating of the food. Incidentally, the resonance between microwaves and water molecules explains why many materials used in cooking dishes—materials that do not contain water—can be placed in a microwave oven without being melted or burned. Yet metal, though it also contains no water, is unsafe.

Metals have free electrons, which makes them good electrical conductors, and the presence of these free electrons means that the microwaves produce electric currents in the surfaces of metal objects placed in the oven. Depending on the shape of the object, these currents can jump, or arc, between points on the surface, thus producing sparks. On the other hand, the interior of the microwave oven itself is in fact metal, and this is so precisely because microwaves do bounce back and forth off of metal. Because the walls are flat and painted, however, currents do not arc between them.

Resonance of Sound Waves

A highly trained singer can hit a note that causes a wine glass to shatter, but what causes this to happen is not the frequency of the note, per se. In other words, the shattering is not necessarily because of the fact that the note is extremely high; rather, it is due to the phenomenon of resonance. The natural, or resonant, frequency in the wine glass, as with all objects, is determined by its shape and composition. If the singer's voice (or a note from an instrument) hits the resonant frequency, there will be a transfer of energy, as with the father pushing his daughter on the swing. In this case, however, a full transfer of energy from the voice or musical instrument can overload the glass, causing it to shatter.

Another example of resonance and sound waves is feedback, popularized in the 1960s by rock guitarists such as Jimi Hendrix and Pete Townsend of the Who. When a musician strikes a note on an electric guitar string, the string oscillates, and an electromagnetic device in the guitar converts this oscillation into an electrical pulse that it sends to an amplifier. The amplifier passes this oscillation on to the speaker, but if the frequency of the speaker is the same as that of the vibrations in the guitar, the result is feedback.

Both in scientific terms and in the view of a music fan, feedback adds energy. The feedback from the speaker adds energy to the guitar body, which, in turn, increases the energy in the vibration of the guitar strings and, ultimately, the power of the electrical signal is passed on to the amp. The result is increasing volume, and the feedback thus creates a loop that continues to repeat until the volume drowns out all other notes.

How Resonance Can Break a Bridge

The power of resonance goes beyond shattering a glass or torturing eardrums with feedback; it can actually destroy large structures. There is an old folk saying that a cat can destroy a bridge if it walks across it in a certain way. This may or may not be true, but it is certainly conceivable that a group of soldiers marching across a bridge can cause it to crumble, even though it is capable of holding much more than their weight, if the rhythm of their synchronized footsteps resonates with the natural frequency of the bridge. For this reason, officers or sergeants typically order their troops to do something very unmilitary—to march out of step—when crossing a bridge.

The resonance between vibrations produced by wind and those of the structure itself brought down a powerful bridge in 1940, a highly dramatic illustration of physics in action that was captured on both still photographs and film. Located on Puget Sound near Seattle, Washington, the Tacoma Narrows Bridge was, at 2,800 ft (853 m) in length, the third-longest suspension bridge in the world. But on November 7, 1940, it gave way before winds of 42 mi (68 km) per hour.

It was not just the speed of these winds, but the fact that they produced oscillations of resonant frequency, that caused the bridge to twist and, ultimately, to crumble. In those few seconds of battle with the forces of nature, the bridge writhed and buckled until a large segment collapsed into the waters of Puget Sound. Fortunately, no one was killed, and a new, more stable bridge was later built in place of the one that had come to be known as "Galloping Gertie." The incident led to increased research and progress in understanding of aerodynamics, harmonic motion, and resonance.

WHERE TO LEARN MORE

Beiser, Arthur. Physics, 5th ed. Reading, MA: Addison-Wesley, 1991.

Berger, Melvin. The Science of Music. Illustrated by Yvonne Buchanan. New York: Crowell, 1989.

"Bridges and Resonance" (Web site). <http://instruction.ferris.edu/loub/media/BRIDGE/Bridge.html> (April 23, 2001).

"Resonance" (Web site). <http://hyperphysics.phyastr.gsu.edu/hbase/sound/reson.html> (April 26, 2001).

"Resonance" (Web site). <http://www.exploratorium.edu/xref/phenomena/resonance.html> (April 23, 2001).

"Resonance." The Physics Classroom (Web site). <http://www.glenbrook.k12.il.us/gbssci/phys/Class/sound/u11l5a.html> (April 26, 2001).

"Resonance Experiment" (Web site). <http://131.123.17.138/> (April 26, 2001).

"Resonance, Frequency, and Wavelength" (Web site). <http://www.cpo.com/CPOCatalog/SW/sw_b1.html> (April 26, 2001).

Suplee, Curt. Everyday Science Explained. Washington, D.C.: National Geographic Society, 1996.

"Tacoma Narrows Bridge Disaster" (Web site). <http://www.enm.bris.ac.uk/research/nonlinear/tacoma/tacoma.html> (April 23, 2001).

KEY TERMS

AMPLITUDE:

The maximum displacement of particles from their normal position during a single period of oscillation.

CYCLE:

One full repetition of oscillation.

FREQUENCY:

For a particle experiencing oscillation, frequency is the number of cycles that take place during one second. Frequency is measured in Hertz.

HARMONIC MOTION:

The repeated movement of a particle about a position of equilibrium, or balance.

HERTZ:

A unit for measuring frequency, named after nineteenth-century German physicist Heinrich Rudolf Hertz (1857-1894). Higher frequencies are expressed in terms of kilohertz (kHz; 10³ or 1,000 cycles per second) or megahertz (MHz; 10⁶ or 1 million cycles per second.)

KINETIC ENERGY:

The energy that an object possesses due to its motion, as with a sled when sliding down a hill. This is contrasted with potential energy.

LONGITUDINAL WAVE:

A wave in which the movement of vibration is in the same direction as the wave itself. This is contrasted to a transverse wave.

MAXIMUM DISPLACEMENT:

For an object in oscillation, maximum displacement is the furthest point from stable equilibrium.

OSCILLATION:

A type of harmonic motion, typically periodic, in one or more dimensions.

PERIOD:

The amount of time required for one cycle in oscillating motion.

PERIODIC MOTION:

Motion that is repeated at regular intervals. These intervals are known as periods.

PERIODIC WAVE:

A wave in which a uniform series of crests and troughs follow one after the other in regular succession.

POTENTIAL ENERGY:

The energy that an object possesses due to its position, as, for instance, with a sled at the top of a hill. This is contrasted with kinetic energy.

RESONANCE:

The condition in which force is applied to an object in oscillation at the point of maximum amplitude.

RESONANT FREQUENCY:

A frequency that matches that of an oscillating object.

STABLE EQUILIBRIUM:

A position in which, if an object were disturbed, it would tend to return to its original position. For an object in oscillation, stable equilibrium is in the middle of a cycle, between two points of maximum displacement.

TRANSVERSE WAVE:

A wave in which the vibration or motion is perpendicular to the direction in which the wave is moving. This is contrasted to a longitudinal wave.

WAVE MOTION:

A type of harmonic motion that carries energy from one place to another without actually moving anymatter.

Microwave Oven

Background

Microwaves are actually a segment of the electromagnetic wave spectrum, which comprises forms of energy that move through space, generated by the interaction of electric and magnetic fields. The spectrum is commonly broken into subgroups determined by the different wavelengths (or frequencies) and emission, transmission, and absorption behaviors of various types of waves. From longest to shortest wavelengths, the spectrum includes electric and radio waves, microwaves, infrared (heat) radiation, visible light, ultraviolet radiation, X-rays, gamma rays, and electromagnetic cosmic rays. Microwaves have frequencies between approximately .11 and 1.2 inches (0.3 and 30 centimeters).

Microwaves themselves are used in many different applications such as telecommunication products, radar detectors, wood curing and drying, and medical treatment of certain diseases. However, certain of their properties render them ideal for cooking, by far the most common use of microwave energy. Microwaves can pass through plastic, glass, and paper materials; metal surfaces reflect them, and foods (especially liquids) absorb them. A meal placed in a conventional oven is heated from the outside in, as it slowly absorbs the surrounding air that the oven has warmed. Microwaves, on the other hand, heat food much more quickly because they penetrate all layers simultaneously. Inside a piece of food or a container filled with liquid, the microwaves agitate molecules, thereby heating the substance.

The ability of microwave energy to cook food was discovered in the 1940s by Dr. Percy Spencer, who had conducted research on radar vacuum tubes for the military during World War II. Spencer's experiments revealed that, when confined to a metal enclosure, high-frequency radio waves penetrate and excite certain type of molecules, such as those found in food. Just powerful enough to cook the food, the microwaves are not strong enough to alter its molecular or genetic structure or to make it radioactive.

Raytheon, the company for which Dr. Spencer was conducting this research, patented the technology and soon developed microwave ovens capable of cooking large quantities of food. Because manufacturing costs rendered them too expensive for most consumers, these early ovens were used primarily by hospitals and hotels that could more easily afford the $3,000 investment they represented. By the late 1970s, however, many companies had developed microwave ovens for home use, and the cost had begun to come down. Today, microwaves are a standard household appliance, available in a broad range of designs and with a host of convenient features: rotating plates for more consistent cooking; digital timers; autoprogramming capabilities; and adjustable levels of cooking power that enable defrosting, browning, and warming, among other functions.

Design

The basic design of a microwave oven is simple, and most operate in essentially the same manner. The oven's various electronic motors, relays, and control circuits are located on the exterior casing, to which the oven cavity is bolted. A front panel allows the user to program the microwave, and the door frame has a small window to enable the cook to view the food while it is cooking.

Near the top of the steel oven cavity is a magnetron—an electronic tube that produces high-frequency microwave oscillations—which generates the microwaves. The microwaves are funneled through a metal waveguide and into a stirrer fan, also positioned near the top of the cavity. The fan distributes the microwaves evenly within the oven. Manufacturers vary the means by which they disburse microwaves to achieve uniform cooking patterns: some use dual stirrer fans located on opposite walls to direct microwaves to the cavity, while others use entry ports at the bottom of the cavity, allowing microwaves to enter from both the top and bottom. In addition, many ovens rotate food on a turntable.

Raw Materials

The cover or outer case of the microwave oven is usually a one-piece, wrap-around metal enclosure. The oven's inside panels and doors are made of galvanized or stainless steel and are given a coating of acrylic enamel, usually light in color to offer good visibility. The cooking surface is generally made of ceramic or glass. Inside the oven, electromechanical components and controls consist of timer motors, switches, and relays. Also inside the oven are the magnetron tube, the waveguide, and the stirrer fan, all made of metal. The hardware that links the various components consists of a variety of metal and plastic parts such as gears, pulleys, belts, nuts, screws, washers, and cables.

The Manufacturing
Process

Oven cavity and door manufacture

• 1 The process of manufacturing a microwave oven starts with the cavity and the door. First, the frame is formed using automatic metal-forming presses that make about 12 to 15 parts per minute. The frame is then rinsed in alkaline cleaner to get rid of any dirt or oil and further rinsed with water to get rid of the alkaline solution.
• 2 Next, each part is treated with zinc phosphate, which prepares it for electro-deposition. Electro-deposition consists of immersing the parts in a paint tank at 200 volts for 2.5 minutes. The resulting coating is about 1.5 mils thick. The parts are then moved through a paint bake operation where the paint is cured at 300 degrees Fahrenheit (149 degrees Celsius) for 20 minutes.
• 3 After the door has been painted, a perforated metal plate is attached to its window aperture. The plate reflects microwaves but allows light to enter the cavity (the door will not be attached to the cavity until later, when the chassis is assembled).

The magnetron tube subassembly

• 4 The magnetron tube assembly consists of a cathode cylinder, a filament heater, a metal anode, and an antenna. The filament is attached to the cathode, and the cathode is enclosed in the anode cylinder; this cell will provide the electricity that will help to generate the microwaves. Metal cooling fins are welded to the anode cylinder, and a powerful magnet is placed around the anode to provide the magnetic field in which the microwaves will be generated. A metal strap holds the complete assembly together. A thermal protector is mounted directly on the magnetron to prevent damage to the tube from overheating.
• 5 An antenna enclosed in a glass tube is mounted on top of the anode, and the air within the tube is pumped out to create a vacuum. The waveguide is connected to the magnetron on top of the protruding antenna, while a blower motor used to cool the metal fins of the magnetron is attached directly to the tube. Finally, a plastic fan is attached to the motor, where it will draw air from outside the oven and direct it towards the vanes. This completes the magnetron subassembly.

Main chassis assembly

• 6 The chassis assembly work is performed on a pallet—a work-holding device used in conjunction with other tools—located at the station. First, the main chassis is placed on the pallet, and the cavity is screwed on to the chassis. Next, the door is attached to the cavity and chassis by means of hinges. The magnetron tube is then bolted to the side of the cavity and the main chassis.
• 7 The circuit that produces the voltage required to operate the magnetron tube consists of a large transformer, an oil-based capacitor, and a high voltage rectifier. All of these components are mounted directly on the chassis, close to the magnetron tube.

Stirrer fan

• 8 The stirrer fan used to circulate the microwaves is mounted on top of the cavity. Some manufacturers use a pulley to drive the fan from the magnetron blower motor; others use a separate stirrer motor attached directly to the fan. Once the stirrer fan is attached, a stirrer shield is screwed on top of the fan assembly. The shield prevents dirt and grease from entering the waveguide, where they could produce arcing and damage the magnetron.

Control switches, relays, and motors

• 9 The cook switch provides power to the transformer by energizing a relay and a timer. The relay is mounted close to the power transformer, while the timer is mounted on the control board. The defrost switch works like the cook switch, activating a motor and timer to operate the defrost cycle. Also mounted on the control board are a timer bell that rings when the cooking cycle is complete and a light switch that allows viewing of the cavity. A number of interlocking switches are mounted near the top and bottom of the door area. The interlocking switches are sometimes grouped together with a safety switch that monitors the other switches and provides protection if the door accidently opens during oven operation.

Front panel

• 10 A front panel that allows the operator to select the various settings and features available for cooking is attached to the chassis. Behind the front panel, the control circuit board is attached. The board, which controls the various programmed operations in their proper sequence when the switches are pushed on the front panel, is connected to the various components and the front panel by means of plug-in sockets and cables.

Making and assembling the case

• 11 The outer case of the microwave is made of metal and is assembled on a roll former. The case is slipped onto the preassembled microwave oven and bolted to the main chassis.

Testing and packaging the oven

• 12 The power cords and dial knobs are now attached to the oven, and it is sent for automatic testing. Most manufacturers run the oven from 50-100 hours continuously as part of the testing process. After testing is complete, a palletizer robot records the model and serial data of the oven for inventory purposes, and the oven is sent for packaging. This completes the manufacturing process.

Quality Control

Extensive quality control during the manufacture of microwave ovens is essential, because microwave ovens emit radiation that can burn anyone exposed at high levels for prolonged periods. Federal regulations, applied to all ovens made after October 1971, limit the amount of radiation that can leak from an oven to 5 milliwatts of radiation per square centimeter at approximately 2 inches from the oven surface. The regulations also require all ovens to have two independent, interlocking switches to stop the production of microwaves the moment the latch is released or the door is opened.

In addition, a computer controlled scanner is used to measure emission leaks around the door, window, and back of the oven. Other scanners check the seating of the magnetron tube and antenna radiation. Each scanner operation relays data to the next-on-line operation so that any problems can be corrected.

The Future

Because of their speed and convenience, microwave ovens have become an indispensable part of modern kitchens. Many developments in the microwave market and allied industries are taking place fairly rapidly. For example, foods and utensils designed specially for microwave cooking have become a huge business. New features will also be introduced in microwaves themselves, including computerized storage of recipes that the consumer will be able to recall at the touch of a button. The display and programmability of the ovens will also be improved, and combination ovens capable of cooking with microwaves as well as by conventional methods will become a standard household product.

Where To Learn More

Books

Davidson, Homer L. Microwave Oven Repair, 2nd edition. Tab Books Inc., 1991.

Gallawa, J. Carlton. The Complete Microwave Oven Service Handbook: Operation, Maintenance. Prentice Hall, 1989.

Microwave Oven Radiation. U.S. department of Health and Human Services, 1986.

Pickett, Amold and John Ketterer. Household Equipment in Residential Design. John Wiley and Sons, 1986.

Raytheon Company. Appliance Manufacturer. Cahners Publishing, 1985.

Periodicals

Klenck, Thomas. "How It Works: Microwave Oven." Popular Mechanics. September, 1989, p. 78.

Roman, Mark. "The Little Waves That Could." Discover. November, 1989, p. 54.

—Rashid Riaz

Resonance

Resources

Generally, resonance is a long, intense sound produced by acoustical vibration. In physics, it is defined as the greatly amplified oscillations of a mechanical system when an internal or external source is subjected to vibration. There are many instances in which scientists want to add energy to the motion of an object that is oscillating. In order for this transfer to be efficient, the oscillation and the source of new energy have to be matched in a very specific way. When this match occurs, scientists say that the oscillation and source are in resonance.

A simple example of an oscillation that people have all seen is that of a child on a playground swing. The motion starts when someone pulls the swing to a position away from the point of stable equilibrium and lets go. The child then moves back and forth, but gradually slows down as the energy of the motion is lost due to friction in the joint where the rope or chain of the swing attaches to its support. Of course, the child wants to continue moving, usually higher and faster, and this requires the addition of more energy. It is easy to accomplish this by pushing the swing, but people all know from experience that the timing is critical. Even a small push can add energy efficiently if it occurs just at the instant when the swing has moved to its highest position and begins to move back to the point of stable equilibrium. If the push occurs a little too late, not all of the energy of the push is added (inefficient). Even worse, if the push occurs too soon, the result will be to slow down the swing (removing energy instead of adding it). In addition, it obviously does no good to push at other times when the swing has moved away (it looks strange and anyway, there is zero efficiency since no energy is transferred into the motion). The trick is to push at the correct instant during every repetition of the swinging motion. When this occurs, the adult’s push (the energy source in this case) and the oscillation are in resonance.

The feature of the motion that must be matched in resonance is the frequency. For any oscillation, the motion takes a specific amount of time to repeat itself (its period for one cycle). Therefore, a certain number of cycles occurs during each second (the frequency). The frequency tells scientists how often the object returns to its position of maximum displacement, and as people know for the swing: that is the best location at which to add energy. Resonance occurs when the rhythm of the energy source matches the natural, characteristic frequency of the oscillation. For this reason, the latter is often called the resonant frequency. It is common to say that the source of energy provides a driving force, as in the case where a push is needed to add energy to the motion of a swing.

In a way, resonance is just a new name for a familiar situation. However, resonance is also important in other instances that are less obvious, like lasers and electronic circuits. A particularly interesting example is the microwave oven, which cooks food without external heat. Even if an object like a book (or a steak) appears to be stationary, it is composed of microscopic atoms that are oscillating around positions of stable equilibrium. Those motions are too small to see, but people can feel them since the temperature of an object is related to their amplitudes—the larger the amplitudes, the hotter the object. This is very similar to the motion of the child on the swing in which larger amplitude means more energy. If one can add energy to the motion of a swing by a driving force in resonance, then one should be able to add energy (heat) to a steak very efficiently. Conventional ovens cook food from the outside, for example by heating air molecules that bump into atoms at the surface of the food. However, the microwave oven uses resonance to cook from the inside.

The water molecule is made of one oxygen atom and two hydrogen atoms that are held together, not in a straight line, but in a V shape. The oxygen atom is located at the bottom of the V and the hydrogen atoms are at ends of the arms. It should not be too surprising to learn that water molecules and even the oxygen and hydrogen atoms within them can oscillate. However, experiments discovered a specific oscillation (a rotation of the entire molecule) that is particularly important. The characteristic frequency of that oscillation falls within the same range as the microwave type of electromagnetic radiation. Microwaves are commonly used in radar, so a large amount of work had already been done to develop dependable, relatively compact devices to produce them. The breakthrough was in realizing that a good steak (even a bad one) contains a large amount of water. If one places a steak within a microwave oven and turns it on, microwaves are produced within the interior of the oven at the resonant frequency of the water molecule. The microwaves act as the driving force to add energy by making the molecules oscillate with greater amplitude. This heats the steak, cooking it from within.

There are many other situations when resonance is important. For example, a rock guitarist must be careful when playing in front of a powerful speaker. When a string vibrates (oscillates) after being struck, an electromagnetic pick-up converts that motion into an electrical pulse, which is then sent to an amplifier and on to the speaker. If the sound vibration from the speaker (with the same frequency as that of the string oscillation) happens to match a resonant frequency of the guitar body, feedback can occur. Actually, this is an example of positive feedback. The sound adds energy to the guitar body, which also vibrates; this

KEY TERMS

Cycle— One repetition of an oscillation as an object travels from any point (in a certain direction) back to the same point and begins to move again in the original direction.

Frequency— The number of cycles of an oscillating motion, which occur per second. One cycle per second is called a Hertz, abbreviated as Hz.

Positive feedback— This occurs when an oscillation is reinforced to continually increase its amplitude. The added energy comes from some external source, like a guitar amplifier, which produces a driving force at the same frequency as that of the original oscillation.

Resonant frequency— A particular frequency that is characteristic of an oscillation. A driving force can efficiently add energy to an oscillation when tuned to the resonant frequency.

adds energy to the string to produce a larger electrical signal, and even more sound. This pattern can repeat until the volume at this resonant frequency grows to drown out other notes, and the rest of the band. Similarly, resonance can have destructive consequences. A famous case is that of the Tacoma Narrows Bridge in Washington State, where winds managed to act as a driving force to make the bridge sway wildly until it collapsed by adding energy to an oscillation at the resonant frequency.

See also Oscillations.

Resources

BOOKS

Bloomfield, Louis. How Things Work: The Physics of Everyday Life. New York: Wiley, 2005.

Editors of McDougal Littell. Matter and Energy. Evanston, IL: McDougal Littell, 2005.

Griffith, W. Thomas. The Physics of Everyday Phenomena: A Conceptual Introduction to Physics. Boston, MA: McGraw-Hill, 2004.

Halliday, David. Fundamentals of Physics. Hoboken, NJ: Wiley, 2005.

Young, Hugh D. Sears and Zemansky’s University Physics. San Francisco, CA: Pearson Addison Wesley, 2004.

James J. Carroll

Resonance

There are many instances in which we want to add energy to the motion of an object which is oscillating. In order for this transfer to be efficient, the oscillation and the source of new energy have to be "matched" in a very specific way. When this match occurs, we say that the oscillation and source are in resonance.

A simple example of an oscillation that we have all seen is that of a child on a playground swing. The motion starts when someone pulls the swing to a position away from the point of stable equilibrium and lets go. The child then moves back and forth, but gradually slows down as the energy of the motion is lost due to friction in the joint where the rope or chain of the swing attaches to its support. Of course, the child wants to continue moving, usually higher and faster, and this requires the addition of more energy. It is easy to accomplish this by pushing the swing, but we all know from experience that the timing is critical. Even a small push can add energy efficiently if it occurs just at the instant when the swing has moved to its highest position and begins to move back to the point of stable equilibrium. If the push occurs a little too late, not all of the energy of the push is added (inefficient). Even worse, if the push occurs too soon, the result will be to slow down the swing (removing energy instead of adding it). Also, it obviously does no good to push at other times when the swing has moved away (it looks strange and anyway, there is zero efficiency since no energy is transferred into the motion). The trick is to push at the "right" instant during every repetition of the swinging motion. When this occurs, the adult's push (the energy source in this case) and the oscillation are in resonance.

The feature of the motion that must be matched in resonance is the frequency . For any oscillation, the motion takes a specific amount of time to repeat itself (its period for one cycle). Therefore, a certain number of cycles occurs during each second (the frequency). The frequency tells us how often the object returns to its position of maximum displacement, and as we know for the swing, that is the best location at which to add energy. Resonance occurs when the rhythm of the energy source matches the natural, characteristic frequency of the oscillation. For this reason, the latter is often called the resonant frequency. It is common to say that the source of energy provides a driving force, as in the case where a push is needed to add energy to the motion of a swing.

In a way, resonance is just a new name for a familiar situation. However, resonance is also important in other instances which are less obvious, like lasers and electronic circuits. A particularly interesting example is the microwave oven, which cooks food without external heat . Even if an object like a book (or a steak) appears to be stationary, it is composed of microscopic atoms which are oscillating around positions of stable equilibrium. Those motions are too small to see, but we can feel them since the temperature of an object is related to their amplitudes—the larger the amplitudes, the hotter the object. This is very similar to the motion of the child on the swing in which a larger amplitude means more energy. If we can add energy to the motion of a swing by a driving force in resonance, then we should be able to add energy (heat) to a steak very efficiently. Conventional ovens cook food from the outside, for example by heating air molecules that bump into atoms at the surface of the food. However, the microwave oven uses resonance to cook from the inside.

The water molecule is made of one oxygen atom and two hydrogen atoms which are held together, not in a straight line, but in a "V" shape. The oxygen atom is located at the bottom of the "V" and the hydrogen atoms are at ends of the arms. It should not be too surprising to learn that water molecules and even the oxygen and hydrogen atoms within them can oscillate. However, experiments discovered a specific oscillation (really a rotation of the entire molecule) that is particularly important. The characteristic frequency of that oscillation falls within the same range as the microwave type of electro-magnetic radiation . Microwaves are commonly used in radar , so a large amount of work had already been done to develop dependable, relatively compact devices to produce them. The breakthrough was in realizing that a good steak (even a bad one) contains a large amount of water. If we place a steak within a microwave oven and turn it on, microwaves are produced within the interior of the oven at the resonant frequency of the water molecule. The microwaves act as the driving force to add energy by making the molecules oscillate with greater amplitude. This heats the steak, cooking it from within.

There are many other situations when resonance is important. For example, a rock guitarist must be careful when playing in front of a powerful speaker. When a string vibrates (oscillates) after being struck, an electro-magnetic pick-up converts that motion into an electrical pulse which is then sent to an amplifier and on to the speaker. If the sound vibration from the speaker (same frequency as that of the string oscillation) happens to match a resonant frequency of the guitar body, feedback can occur. Actually, this is an example of positive feedback. The sound adds energy to the guitar body, which also vibrates; this adds energy to the string to produce a larger electrical signal, and even more sound. This pattern can repeat until the volume at this resonant frequency grows to drown out other notes, and the rest of the band. Similarly, resonance can have destructive consequences. A famous case is that of the Tacoma Narrows Bridge in Washington State, where winds managed to act as a driving force to make the bridge sway wildly until it collapsed by adding energy to an oscillation at the resonant frequency.

See also Oscillations.

Resources

books

clark, j. matter and energy: physics in action. new york: oxford university press, 1994.

ehrlich, r. turning the world inside out, and 174 other simple physics demonstrations. princeton, nj: princeton university press, 1990.

epstein, l.c. thinking physics: practical lessons in critical thinking. 2nd ed. san francisco: insight press, 1994.

James J. Carroll

KEY TERMS

Cycle

—One repetition of an oscillation as an object travels from any point (in a certain direction) back to the same point and begins to move again in the original direction.

Frequency

—The number of cycles of an oscillating motion which occur per second. One cycle per second is called a Hertz, abbreviated as Hz.

Positive feedback

—This occurs when an oscillation "feeds back" to continually increase its amplitude. The added energy comes from some external source, like a guitar amplifier, which produces a driving force at the same frequency as that of the original oscillation.

Resonant frequency

—A particular frequency that is characteristic of an oscillation. A driving force can efficiently add energy to an oscillation when tuned to the resonant frequency.

MICROWAVE TECHNOLOGY

MICROWAVE TECHNOLOGY. Microwave detection and communications systems have come to play a major, but little appreciated, role in American life since 1940. Perhaps the application best known to the public is the microwave oven, but microwaves have also made possible live television from space and between continents. Microwave technology is also essential for safe all weather operation of commercial and military aircraft as well as for intercity telephone traffic. Before the 1970s the high cost of microwave systems tended to limit their use for mass-produced consumer products. This situation changed with the introduction of comparatively inexpensive solid-state microwave sources suitable for such applications as counter-top ovens, collision-avoidance devices for automobiles, burglar alarms, mobile telephones, and health-data telemetry.

Microwave technology has gone through several stages. European physicists did some theoretical and experimental work in the late nineteenth century, but interest languished because of the dominance of long waves in early radio communication. George C. Southworth, John R. Carson, and others at the Bell Telephone Laboratories made some fundamental advances in the transmission of microwaves during the 1930s. Researchers at Stanford University in 1939 developed an important new microwave generator known as the klystron. The wave-guide, klystron, and cavity magnetron, brought to the United States in a famous "black box" by a British team in 1940, became key elements in a wide variety of radar systems developed by several groups, including the Radiation Laboratories formed at the Massachusetts Institute of Technology in 1941.

Bell System installed the prototype for a major microwave communications system by means of repeating stations separated by distances of about thirty miles between New York and Boston in 1947. By 1960, microwave chains carried about 40 percent of Bell's intercity traffic; the proportion of domestic communications handled through microwave networks increased steadily thereafter. Similar apparatus was adapted for use in satellite repeating stations, beginning with the launch of Pioneer 3 in 1958. Further innovations and the increasing congestion of the electromagnetic spectrum aroused renewed interest in the use of Southworth's hollow pipes for communications by 1970. (A single circular pipe is believed capable of carrying 250,000 simultaneous conversations over long distances.) The discovery in the 1960s by J. B. Gunn of IBM and others that semiconductor devices such as the Gunn oscillator and IMPATT diode can generate and amplify microwave signals stimulated a variety of consumer and industrial applications of microwave technology.

In the 1940s Dr. Percy Spencer, an engineer with the Raytheon Corporation researching radars, noticed that microwaves emitted by a new vacuum tube called a magnetron caused food sitting nearby to heat up. The first commercial microwave oven—weighing more than 750 pounds and standing over five feet tall—hit the market in 1947. In the late 1960s, microwave ovens began appearing in stores as domestic appliances, and by 1975 their sales surpassed those of gas ranges. Although culinary purists avoided them, microwave ovens spurred a new industry in frozen prepared foods and vastly reduced the amount of time needed to cook food, further reducing Americans' dependence on a primary domestic laborer.

BIBLIOGRAPHY

Cowan, Ruth Schwartz. More Work for Mother: The Ironies of Household Technology from the Open Hearth to the Microwave. New York: Basic Books, 1983.

Veley, Victor F. C. Modern Microwave Technology. Englewood Cliffs, N.J.: Prentice Hall, 1987.

James E.Brittain/a. r.

See alsoPhysics: Solid-State Physics ; Radar .