Science of Everyday Things Vol. 2 - Physics Episode 12 pptx

Magnetism 338 SCIENCE OF EVERYDAY THINGS VOLUME 2: REAL-LIFE PHYSICS would make possible a form of transport that could move large numbers of people in relative comfort, thus decreasing the environmental impact of automobiles, and do so at much higher speeds than a car could safely attain. Actually the idea of MAGLEV trains goes back to a time when trains held complete supremacy over automobiles as a mode of transportation: specifically, 1907, when rocket pioneer Robert Goddard (1882-1945) wrote a story describing a vehicle that traveled by means of magnetic levitation. Just five years later, French engineer Emile Bachelet produced a working model for a MAGLEV train. But the amount of magnetic force required to lift such a vehicle made it impractical, and the idea fell to the wayside. Then, in the 1960s, the advent of supercon- ductivity—the use of extremely low temperatures, which facilitate the transfer of electrical current through a conducting material with vir- tually no resistance—made possible electromag- nets of staggering force. Researchers began build- ing MAGLEV prototypes using superconducting coils with strong currents to create a powerful magnetic field. The field in turn created a repulsive force capable of lifting a train several inches above a railroad track. Electrical current sent through guideway coils on the track allowed for enormous propulsive force, pushing trains forward at speeds up to and beyond 250 MPH (402 km/h). Initially, researchers in the United States were optimistic about MAGLEV trains, but safe- ty concerns led to the shelving of the idea for several decades. Meanwhile, other industrialized nations moved forward with MAGLEVs: in Japan, engineers built a 27-mi (43.5-km) experimental MAGLEV line, while German designers experimented with attractive (as opposed to repulsive) force in their Transrapid 07. MAGLEV trains gained a new defender in the United States with now-retired Senator Daniel Patrick Moyni- han (D-NY), who as chairman of a Senate sub- committee overseeing the interstate highway system introduced legislation to fund MAGLEV research. The 1998 transportation bill allocated $950 million toward the Magnetic Levitation Prototype Development Program. As part of this program, in January 2001 the U.S. Department of Transportation selected projects in Maryland and Pennsylvania as the two finalists in the com- ELECTROMAGNET: A type of magnet in which an object is charged by an electrical current. Typically the object used is made of iron, which quickly loses magnetic force when current is reduced. Thus an electromagnet can be turned on or off, and its magnetic force altered, making it poten- tially much more powerful than a natural magnet. ELECTROMAGNETISM: The unified electrical and magnetic force field generated by the passage of an electric current through matter. ELECTRONS: Negatively charged sub- atomic particles whose motion relative to one another creates magnetic force. MAGNETIC FIELD: Wherever a magnetic force acts on a moving charged particle, a magnetic field is said to exist. Mag- netic fields are typically measured by a unit called a tesla. NATURAL MAGNET: A chemical ele- ment in which the magnetic fields created by electrons’ relative motion align uni- formly to create a net magnetic dipole, or unity of direction. Such elements, among them iron, cobalt, and nickel, are also known as magnetic metals. PERMANENT MAGNET: A magnetic material in which groups of atoms, known as domains, are brought into alignment, and in which magnetization cannot be changed merely by attempting to realign the domains. Permanent magnetization is reversible only at very high temperatures— for example, 1,418°F (770°C) in the case of iron. KEY TERMS set_vol2_sec9 9/13/01 1:15 PM Page 338 Magnetism petition to build the first MAGLEV train service in the United States. The goal is to have the service in place by approximately 2010. WHERE TO LEARN MORE Barr, George. Science Projects for Young People. New York: Dover, 1964. Beiser, Arthur. Physics, 5th ed. Reading, MA: Addison-Wesley, 1991. Hann, Judith. How Science Works. Pleasantville, NY: Reader’s Digest, 1991. Macaulay, David. The New Way Things Work. Boston: Houghton Mifflin, 1998. Molecular Expressions: Electricity and Magnetism: Interac- tive Java Tutorials (Web site). <http://micro.magnet. fsu.edu/electromag/java/> (January 26, 2001). Topical Group on Magnetism (Web site). <http://www.aps.org/units/gmag/> (January 26, 2001). VanCleave, Janice. Magnets. New York: John Wiley & Sons, 1993. Wood, Robert W. Physics for Kids: 49 Easy Experiments with Electricity and Magnetism. New York: Tab, 1990. 339 SCIENCE OF EVERYDAY THINGS VOLUME 2: REAL-LIFE PHYSICS set_vol2_sec9 9/13/01 1:15 PM Page 339 340 SCIENCE OF EVERYDAY THINGS VOLUME 2: REAL-LIFE PHYSICS ELECTROMAGNETIC SPECTRUM Electromagnetic Spectrum CONCEPT One of the most amazing aspects of physics is the electromagnetic spectrum—radio waves, microwaves, infrared light, visible light, ultraviolet light, x rays, and gamma rays—as well as the relationship between the spectrum and electromagnetic force. The applications of the electromagnetic spectrum in daily life begin the moment a person wakes up in the morning and “sees the light.” Yet visible light, the only familiar part of the spectrum prior to the eighteenth and nineteenth centuries, is also its narrowest region. Since the beginning of the twentieth century, uses for other bands in the electromagnetic spectrum have proliferated. At the low-frequency end are radio, short-wave radio, and television signals, as well as the microwaves used in cooking. Higher-frequency waves, all of which can be gen- erally described as light, provide the means for looking deep into the universe—and deep into the human body. HOW IT WORKS Electromagnetism The ancient Romans observed that a brushed comb would attract particles, a phenomenon now known as static electricity and studied with- in the realm of electrostatics in physics. Yet, the Roman understanding of electricity did not extend any further, and as progress was made in the science of physics—after a period of more than a thousand years, during which scientific learning in Europe progressed very slowly—it developed in areas that had nothing to do with the strange force observed by the Romans. The fathers of physics as a serious science, Galileo Galilei (1564-1642) and Sir Isaac Newton (1642-1727), were concerned with gravitation, which Newton identified as a fundamental force in the universe. For nearly two centuries, physicists continued to believe that there was only one type of force. Yet, as scientists became increasingly aware of molecules and atoms, anomalies began to arise—in particular, the fact that gravitation alone could not account for the strong forces holding atoms and molecules together to form matter. FOUNDATIONS OF ELECTRO- MAGNETIC THEORY. At the same time, a number of thinkers conducted experiments concerning the nature of electricity and magnetism, and the relationship between them. Among these were several giants in physics and other dis- ciplines—including one of America’s greatest founding fathers. In addition to his famous (and highly dangerous) experiment with lightning, Benjamin Franklin (1706-1790) also contributed the names “positive” and “negative” to the differ- ing electrical charges discovered earlier by French physicist Charles Du Fay (1698-1739). In 1785, French physicist and inventor Charles Coulomb (1736-1806) established the basic laws of electrostatics and magnetism. He maintained that there is an attractive force that, like gravitation, can be explained in terms of the inverse of the square of the distance between objects. That attraction itself, however, resulted not from gravity, but from electrical charge, according to Coulomb. A few years later, German mathematician Johann Karl Friedrich Gauss (1777-1855) developed a mathematical theory for finding the magnetic potential of any point on Earth, and his set_vol2_sec9 9/13/01 1:15 PM Page 340 Electro- magnetic Spectrum contemporary, Danish physicist Hans Christian Oersted (1777-1851), became the first scientist to establish the existence of a clear relationship between electricity and magnetism. This led to the foundation of electromagnetism, the branch of physics devoted to the study of electrical and magnetic phenomena. French mathematician and physicist André Marie Ampère (1775-1836) concluded that magnetism is the result of electricity in motion, and, in 1831, British physicist and chemist Michael Faraday (1791-1867) published his theory of electromagnetic induction. This theory shows how an electrical current in one coil can set up a current in another through the development of a magnetic field. This enabled Faraday to develop the first generator, and for the first time in history, humans were able to convert mechanical energy systematically into electrical energy. MAXWELL AND ELECTROMAG- NETIC FORCE. A number of other figures contributed along the way; but, as yet, no one had developed a “unified theory” explaining the relationship between electricity and magnetism. Then, in 1865, Scottish physicist James Clerk Maxwell (1831-1879) published a groundbreak- ing paper,“On Faraday’s Lines of Force,” in which he outlined a theory of electromagnetic force— the total force on an electrically charged particle, which is a combination of forces due to electrical and/or magnetic fields around the particle. Maxwell had thus discovered a type of force in addition to gravity, and this reflected a “new” type of fundamental interaction, or a basic mode by which particles interact in nature. Newton had identified the first, gravitational interaction, and in the twentieth century, two other forms of fundamental interaction—strong nuclear and weak nuclear—were identified as well. In his work, Maxwell drew on the studies conducted by his predecessors, but added a new statement: that electrical charge is conserved. This statement, which did not contradict any of the experimental work done by the other physicists, was based on Maxwell’s predictions regarding what should happen in situations of electromagnetism; subsequent studies have supported his predictions. Electromagnetic Radiation So far, what we have seen is the foundation for modern understanding of electricity and mag- 341 SCIENCE OF EVERYDAY THINGS VOLUME 2: REAL-LIFE PHYSICS netism. This understanding grew enormously in the late nineteenth and early twentieth centuries, thanks both to the theoretical work of physicists, and the practical labors of inventors such as Thomas Alva Edison (1847-1931) and Serbian- American electrical engineer Nikola Tesla (1856- 1943). But our concern in the present context is with electromagnetic radiation, of which the waves on the electromagnetic spectrum are a particularly significant example. Energy can travel by conduction or convection, two principal means of heat transfer. But the energy Earth receives from the Sun—the energy conveyed through the electromagnetic spectrum—is transferred by another method, radiation. Whereas conduction of convection can only take place where there is matter, which pro- vides a medium for the energy transfer, radiation requires no medium. Thus, electromagnetic energy passes from the Sun to Earth through the vacuum of empty space. ELECTROMAGNETIC WAVES. The connection between electromagnetic radiation and electromagnetic force is far from obvi- ous. Even today, few people not scientifically trained understand that there is a clear relationship between electricity and magnetism—let alone a connection between these and visible light. The breakthrough in establishing that connection can be attributed both to Maxwell and to German physicist Heinrich Rudolf Hertz (1857- 1894). Maxwell had suggested that electromagnetic force carried with it a certain wave phenomenon, and predicted that these waves traveled at a certain speed. In his Treatise on Electricity and Mag- netism (1873), he predicted that the speed of these waves was the same as that of light— 186,000 mi (299,339 km) per second—and theo- rized that the electromagnetic interaction included not only electricity and magnetism, but light as well. A few years later, while studying the behavior of electrical currents, Hertz confirmed Maxwell’s proposition regarding the wave phenomenon by showing that an electrical current generated some sort of electromagnetic radiation. In addition, Hertz found that the flow of electrical charges could be affected by light under certain conditions. Ultraviolet light had already been identified, and Hertz shone an ultraviolet beam on the negatively charged side of a gap in a set_vol2_sec9 9/13/01 1:15 PM Page 341 Electro- magnetic Spectrum 342 SCIENCE OF EVERYDAY THINGS VOLUME 2: REAL-LIFE PHYSICS THE HUBBLE SPACE TELESCOPE INCLUDES AN ULTRAVIOLET LIGHT INSTRUMENT CALLED THE GODDARD HIGH RES- OLUTION SPECTROGRAPH THAT IT IS CAPABLE OF OBSERVING EXTREMELY DISTANT OBJECTS. (Photograph by Roger Ress- meyer/Corbis. Reproduced by permission.) current loop. This made it easier for an electrical spark to jump the gap. Hertz could not explain this phenomenon, which came to be known as the photoelectric effect. Indeed, no one else could explain it until quantum theory was developed in the early twentieth century. In the mean- time, however, Hertz’s discovery of electromagnetic waves radiating from a current loop led to the invention of radio by Italian physicist and engineer Guglielmo Marconi (1874-1937) and others. Light: Waves or Particles? At this point, it is necessary to jump backward in history, to explain the progression of scientists’ understanding of light. Advancement in this area took place over a long period of time: at the end of the first millennium A.D., the Arab physicist Alhasen (Ibn al-Haytham; c. 965-1039) showed that light comes from the Sun and other self-illu- minated bodies—not, as had been believed up to that time—from the eye itself. Thus, studies in optics, or the study of light and vision, were— compared to understanding of electromagnetism itself—relatively advanced by 1666, when New- ton discovered the spectrum of colors in light. As Newton showed, colors are arranged in a sequence, and white light is a combination of all colors. set_vol2_sec9 9/13/01 1:15 PM Page 342 Electro- magnetic Spectrum Newton put forth the corpuscular theory of light—that is, the idea that light is made up of particles—but his contemporary Christiaan Huygens (1629-1695), a Dutch physicist and astronomer, maintained that light appears in the form of a wave. For the next century, adherents of Newton’s corpuscular theory and of Huygens’s wave theory continued to disagree. Physicists on the European continent began increasingly to accept wave theory, but corpuscular theory remained strong in Newton’s homeland. Thus, it was ironic that the physicist whose work struck the most forceful blow against corpuscular theory was himself an Englishman: Thomas Young (1773-1829), who in 1801 demonstrated interference in light. Young direct- ed a beam of light through two closely spaced pinholes onto a screen, reasoning that if light truly were made of particles, the beams would project two distinct points onto the screen. Instead, what he saw was a pattern of interference—a wave phenomenon. By the time of Hertz, wave theory had become dominant; but the photoelectric effect also exhibited aspects of particle behavior. Thus, for the first time in more than a century, particle theory gained support again. Yet, it was clear that light had certain wave characteristics, and this raised the question—which is it, a wave or a set of particles streaming through space? The work of German physicist Max Planck (1858-1947), father of quantum theory, and of Albert Einstein (1879-1955), helped resolve this apparent contradiction. Using Planck’s quantum principles, Einstein, in 1905, showed that light appears in “bundles” of energy, which travel as waves but behave as particles in certain situations. Eighteen years later, American physicist Arthur Holly Compton (1892-1962) showed that, depending on the way it is tested, light appears as either a particle or a wave. These particles he called photons. Wave Motion and Electro- magnetic Waves The particle behavior of electromagnetic energy is beyond the scope of the present discussion, though aspects of it are discussed elsewhere. For the present purposes, it is necessary only to view the electromagnetic spectrum as a series of waves, and in the paragraphs that follow, the rudiments of wave motion will be presented in short form. A type of harmonic motion that carries energy from one place to another without actually moving any matter, wave motion is related to oscillation, harmonic—and typically periodic— motion in one or more dimensions. Oscillation involves no net movement, but only movement in place; yet individual waves themselves are oscillating, even as the overall wave pattern moves. The term periodic motion, or movement repeated at regular intervals called periods, describes the behavior of periodic waves: waves in which a uniform series of crests and troughs follow each other in regular succession. Periodic waves are divided into longitudinal and transverse waves, the latter (of which light waves are an example) being waves in which the vibration or motion is perpendicular to the direction in which the wave is moving. Unlike longitudinal waves, such as those that carry sound energy, transverse waves are fairly easy to visualize, and assume the shape that most people imagine when they think of waves: a regular up-and-down pattern, called “sinusoidal” in mathematical terms. PARAMETERS OF WAVE MO- TION. A period (represented by the symbol T) is the amount of time required to complete one full cycle of the wave, from trough to crest and back to trough. Period is mathematically related to several other aspects of wave motion, including wave speed, frequency, and wavelength. Frequency (abbreviated f) is the number of waves passing through a given point during the interval of one second. It is measured in Hertz (Hz), named after Hertz himself: a single Hertz (the term is both singular and plural) is equal to one cycle of oscillation per second. Higher frequencies are expressed in terms of kilohertz (kHz; 10 3 or 1,000 cycles per second); megahertz (MHz; 10 6 or 1 million cycles per second); and gigahertz (GHz; 10 9 or 1 billion cycles per second.) Wavelength (represented by the symbol λ, the Greek letter lambda) is the distance between a crest and the adjacent crest, or a trough and an adjacent trough, of a wave. The higher the frequency, the shorter the wavelength; and, thus, it is possible to describe waves in terms of either. According to quantum theory, however, electromagnetic waves can also be described in terms of 343 SCIENCE OF EVERYDAY THINGS VOLUME 2: REAL-LIFE PHYSICS set_vol2_sec9 9/13/01 1:15 PM Page 343 Electro- magnetic Spectrum photon energy level, or the amount of energy in each photon. Thus, the electromagnetic spectrum, as we shall see, varies from relatively long- wavelength, low-frequency, low-energy radio waves on the one end to extremely short-wavelength, high-frequency, high-energy gamma rays on the other. The other significant parameter for describing a wave—one mathematically independent from those so far discussed—is amplitude. Defined as the maximum displacement of a vibrating material, amplitude is the “size” of a wave. The greater the amplitude, the greater the energy the wave contains: amplitude indicates intensity. The amplitude of a light wave, for instance, determines the intensity of the light. A RIGHT-HAND RULE. Physics textbooks use a number of “right-hand rules”: devices for remembering certain complex physical interactions by comparing the lines of movement or force to parts of the right hand. In the present context, a right-hand rule makes it easier to visualize the mutually perpendicular direc- tions of electromagnetic waves, electric field, and magnetic field. A field is a region of space in which it is possible to define the physical properties of each point in the region at any given moment in time. Thus, an electrical field and magnetic field are simply regions in which electrical and magnetic components, respectively, of electromagnetic force are exerted. Hold out your right hand, palm perpendicular to the floor and thumb upright. Your fingers indicate the direction that an electromagnetic wave is moving. Your thumb points in the direction of the electrical field, as does the heel of your hand: the electrical field forms a plane perpendicular to the direction of wave propagation. Simi- larly, both your palm and the back of your hand indicate the direction of the magnetic field, which is perpendicular both to the electrical field and the direction of wave propagation. The Electromagnetic Spectrum As stated earlier, an electromagnetic wave is transverse, meaning that even as it moves forward, it oscillates in a direction perpendicular to the line of propagation. An electromagnetic wave can thus be defined as a transverse wave with mutually perpendicular electrical and magnetic fields that emanate from it. The electromagnetic spectrum is the complete range of electromagnetic waves on a continuous distribution from a very low range of frequencies and energy levels, with a correspondingly long wavelength, to a very high range of frequencies and energy levels, with a correspondingly short wavelength. Included on the electromagnetic spectrum are radio waves and microwaves; infrared, visible, and ultraviolet light; x rays, and gamma rays. Though each occupies a definite place on the spectrum, the divisions between them are not firm: as befits the nature of a spectrum, one simply “blurs” into another. FREQUENCY RANGE OF THE ELECTROMAGNETIC SPECTRUM. The range of frequencies for waves in the electromagnetic spectrum is from approximately 10 2 Hz to more than 10 25 Hz. These numbers are an example of scientific notation, which makes it possible to write large numbers without having to include a string of zeroes. Without scientific notation, the large numbers used for discussing properties of the electromagnetic spectrum can become bewildering. The first number given, for extremely low- frequency radio waves, is simple enough—100— but the second would be written as 1 followed by 25 zeroes. (A good rule of thumb for scientific notation is this: for any power n of 10, simply attach that number of zeroes to 1. Thus 10 6 is 1 followed by 6 zeroes, and so on.) In any case, 10 25 is a much simpler figure than 10,000,000,000,000,000,000,000,000—or 10 trillion trillion. As noted earlier, gigahertz, or units of 1 billion Hertz, are often used in describing extremely high frequencies, in which case the number is written as 10 16 GHz. For simplicity’s sake, however, in the present context, the simple unit of Hertz (rather than kilo-, mega-, or gigahertz) is used wherever it is convenient to do so. WAVELENGTHS ON THE ELEC- TROMAGNETIC SPECTRUM. The range of wavelengths found in the electromagnetic spectrum is from about 10 8 centimeters to less than 10 -15 centimeters. The first number, equal to 1 million meters (about 621 mi), obvi- ously expresses a great length. This figure is for radio waves of extremely low frequency; ordinary radio waves of the kind used for actual radio 344 SCIENCE OF EVERYDAY THINGS VOLUME 2: REAL-LIFE PHYSICS set_vol2_sec9 9/13/01 1:15 PM Page 344 Electro- magnetic Spectrum broadcasts are closer to 10 5 centimeters (about 328 ft). For such large wavelengths, the use of centimeters might seem a bit cumbersome; but, as with the use of Hertz for frequencies, centimeters provide a simple unit that can be used to measure all wavelengths. Some charts of the electromagnetic spectrum nonetheless give figures in meters, but for parts of the spectrum beyond microwaves, this, too, can become challenging. The ultra-short wavelengths of gamma rays, after all, are equal to one-trillionth of a centimeter. By comparison, the angstrom—a unit so small it is used to measure the diameter of an atom—is 10 million times as large. ENERGY LEVELS ON THE ELEC- TROMAGNETIC SPECTRUM. Finally, in terms of photon energy, the unit of measurement is the electron volt (eV), which is used for quan- tifying the energy in atomic particles. The range of photon energy in the electromagnetic spectrum is from about 10 -13 to more than 10 10 electron volts. Expressed in terms of joules, an electron volt is equal to 1.6 • 10 -19 J. To equate these figures to ordinary language would require a lengthy digression; suffice it to say that even the highest ranges of the electromagnetic spectrum possess a small amount of energy in terms of joules. Remember, however, that the energy level identified is for a photon— a light particle. Again, without going into a great deal of detail, one can just imagine how many of these particles, which are much smaller than atoms, would fit into even the smallest of spaces. Given the fact that electromagnetic waves are traveling at a speed equal to that of light, the amount of photon energy transmitted in a single second is impressive, even for the lower ranges of the spectrum. Where gamma rays are concerned, the energy levels are positively staggering. REAL-LIFE APPLICATIONS The Radio Sub-Spectrum Among the most familiar parts of the electromagnetic spectrum, in modern life at least, is radio. In most schematic representations of the spectrum, radio waves are shown either at the left end or the bottom, as an indication of the fact that these are the electromagnetic waves with the lowest frequencies, the longest wavelengths, and the smallest levels of photon energy. Included in this broad sub-spectrum, with frequencies up to about 10 7 Hertz, are long-wave radio, short-wave radio, and microwaves. The areas of communication affected are many: broadcast radio, television, mobile phones, radar—and even highly specific forms of technology such as baby monitors. Though the work of Maxwell and Hertz was foundational to the harnessing of radio waves for human use, the practical use of radio had its beginnings with Marconi. During the 1890s, he made the first radio transmissions, and, by the end of the century, he had succeeded in trans- mitting telegraph messages across the Atlantic Ocean—a feat which earned him the Nobel Prize for physics in 1909. Marconi’s spark transmitters could send only coded messages, and due to the broad, long- wavelength signals used, only a few stations could broadcast at the same time. The development of the electron tube in the early years of the twentieth century, however, made it possible to transmit narrower signals on stable frequencies. This, in turn, enabled the development of technology for sending speech and music over the airwaves. Broadcast Radio THE DEVELOPMENT OF AM AND FM. A radio signal is simply a carrier: the process of adding information—that is, complex sounds such as those of speech or music—is called modulation. The first type of modulation developed was AM, or amplitude modulation, which Canadian-American physicist Reginald Aubrey Fessenden (1866-1932) demonstrated with the first United States radio broadcast in 1906. Amplitude modulation varies the instanta- neous amplitude of the radio wave, a function of the radio station’s power, as a means of transmit- ting information. By the end of World War I, radio had emerged as a popular mode of communication: for the first time in history, entire nations could hear the same sounds at the same time. During the 1930s, radio became increasingly important, both for entertainment and information. Fami- lies in the era of the Great Depression would gather around large “cathedral radios”—so named for their size and shape—to hear comedy programs, soap operas, news programs, and 345 SCIENCE OF EVERYDAY THINGS VOLUME 2: REAL-LIFE PHYSICS set_vol2_sec9 9/13/01 1:15 PM Page 345 Electro- magnetic Spectrum speeches by important public figures such as President Franklin D. Roosevelt. Throughout this era—indeed, for more than a half-century from the end of the first World War to the height of the Vietnam Conflict in the mid-1960s—AM held a dominant position in radio. This remained the case despite a number of limitations inherent in amplitude modulation: AM broadcasts flickered with popping noises from lightning, for instance, and cars with AM radios tended to lose their signal when going under a bridge. Yet, another mode of radio transmission was developed in the 1930s, thanks to American inventor and electrical engineer Edwin H. Armstrong (1890-1954). This was FM, or frequency modulation, which varied the radio signal’s frequency rather than its amplitude. Not only did FM offer a different type of modulation; it was on an entirely different frequency range. Whereas AM is an example of a long-wave radio transmission, FM is on the microwave sector of the electromagnetic spectrum, along with television and radar. Due to its high frequency and form of modulation, FM offered a “clean” sound as compared with AM. The addition of FM stereo broadcasts in the 1950s offered still further improvements; yet despite the advantages of FM, audiences were slow to change, and FM did not become popular until the mid- to late 1960s. SIGNAL PROPAGATION. AM signals have much longer wavelengths, and smaller frequencies, than do FM signals, and this, in turn, affects the means by which AM signals are prop- agated. There are, of course, much longer radio wavelengths; hence, AM signals are described as intermediate in wavelength. These intermediate- wavelength signals reflect off highly charged lay- ers in the ionosphere between 25 and 200 mi (40-332 km) above Earth’s surface. Short-wavelength signals, such as those of FM, on the other hand, follow a straight-line path. As a result, AM broadcasts extend much farther than FM, particularly at night. At a low level in the ionosphere is the D layer, created by the Sun when it is high in the sky. The D layer absorbs medium-wavelength signals during the day, and for this reason, AM signals do not travel far during daytime hours. After the Sun goes down, however, the D layer soon fades, and this makes it possible for AM signals to reflect off a much higher layer of the ionosphere known as the F layer. (This is also some- times known as the Heaviside layer, or the Ken- nelly-Heaviside layer, after English physicist Oliver Heaviside and British-American electrical engineer Arthur Edwin Kennelly, who independ- ently discovered the ionosphere in 1902.) AM signals “bounce” off the F layer as though it were a mirror, making it possible for a listener at night to pick up a signal from halfway across the country. The Sun has other effects on long-wave and intermediate-wave radio transmissions. Sunspots, or dark areas that appear on the Sun in cycles of about 11 years, can result in a heavier buildup of the ionosphere than normal, thus impeding radio-signal propagation. In addition, occasional bombardment of Earth by charged particles from the Sun can also disrupt transmissions. Due to the high frequencies of FM signals, these do not reflect off the ionosphere; instead, they are received as direct waves. For this reason, an FM station has a fairly short broadcast range, and this varies little with regard to day or night. The limited range of FM stations as compared to AM means that there is much less interference on the FM dial than for AM. Distribution of Radio Frequencies In the United States and most other countries, one cannot simply broadcast at will; the airwaves are regulated, and, in America, the governing authority is the Federal Communications Com- mission (FCC). The FCC, established in 1934, was an outgrowth of the Federal Radio Commis- sion, founded by Congress seven years earlier. The FCC actually “sells air,” charging companies a fee to gain rights to a certain frequency. Those companies may in turn sell that air to others for a profit. At the time of the FCC’s establishment, AM was widely used, and the federal government assigned AM stations the frequency range of 535 kHz to 1.7 MHz. Thus, if an AM station today is called, for instance, “AM 640,” this means that it operates at 640 kHz on the dial. The FCC assigned the range of 5.9 to 26.1 MHz to short- wave radio, and later the area of 26.96 to 27.41 MHz to citizens’ band (CB) radio. Above these are microwave regions assigned to television sta- 346 SCIENCE OF EVERYDAY THINGS VOLUME 2: REAL-LIFE PHYSICS set_vol2_sec9 9/13/01 1:15 PM Page 346 Electro- magnetic Spectrum tions, as well as FM, which occupies the range from 88 to 108 MHz. The organization of the electromagnetic spectrum’s radio frequencies—which, of course, is an entirely arbitrary, humanmade process—is fascinating. It includes assigned frequencies for everything from garage-door openers to deep- space radio communications. The FCC recog- nizes seven divisions of radio carriers, using a system that is not so much based on rational rules as it is on the way that the communications industries happened to develop over time. THE SEVEN FCC DIVISIONS. Most of what has so far been described falls under the heading of “Public Fixed Radio Ser- vices”: AM and FM radio, other types of radio such as shortwave, television, various other forms of microwave broadcasting, satellite systems, and communication systems for federal departments and agencies. “Public Mobile Ser- vices” include pagers, air-to-ground service (for example, aircraft-to-tower communications), offshore service for sailing vessels, and rural radio-telephone service. “Commercial Mobile Radio Services” is the realm of cellular phones, and “Personal Communications Service” that of the newer wireless technology that began to chal- lenge cellular for market dominance in the late 1990s. “Private Land Mobile Radio Service” (PMR) and “Private Operational-Fixed Microwave Ser- vices” (OFS) are rather difficult to distinguish, the principal difference being that the former is used exclusively by profit-making businesses, and the latter mostly by nonprofit institutions. An example of PMR technology is the dispatching radios used by taxis, but this is only one of the more well-known forms of internal electronic communications for industry. For instance, when a film production company is shooting a picture and the director needs to speak to someone at the producer’s trailer a mile away, she may use PMR radio technology. OFS was initially designated purely for nonprofit use, and is used often by schools; but banks and other profit-making institutions often use OFS because of its low cost. Finally, there is the realm of “Personal Radio Services,” created by the FCC in 1992. This branch, still in its infancy, will probably one day include video-on-demand, interactive polling, online shopping and banking, and other activi- ties classified under the heading of Interactive Video and Data Services, or IVDS. Unlike other types of video technology, these will all be wireless, and, therefore, represent a telecommunica- tions revolution all their own. Microwaves MICROWAVE COMMUNICATION. Though microwaves are treated separately from radio waves, in fact, they are just radio signals of a very short wavelength. As noted earlier, FM signals are actually carried on microwaves, and, as with FM in particular, microwave signals in gen- eral are very clear and very strong, but do not extend over a great geographical area. Nor does microwave include only high-frequency radio and television; in fact, any type of information that can be transmitted via telephone wires or coaxial cables can also be sent via a microwave circuit. Microwaves have a very narrow, focused beam: thus, the signal is amplified considerably when an antenna receives it. This phenomenon, known as “high antenna gain,” means that microwave transmitters need not be highly powerful to produce a strong signal. To further the reach of microwave broadcasts, transmitters are often placed atop mountain peaks, hilltops, or tall buildings. In the past, a microwave-transmit- ting network such as NBC (National Broadcast- ing Company) or CBS (Columbia Broadcasting System) required a network of ground-based relay stations to move its signal across the continent. The advent of satellite broadcasting in the 1960s, however, changed much about the way signals are beamed: today, networks typically replace, or at least augment, ground-based relays with satellite relays. The first worldwide satellite TV broadcast, in the summer of 1967, featured the Beatles singing their latest song “All You Need Is Love.” Due to the international character of the broadcast, with an estimated 200 million viewers, John Lennon and Paul McCartney wrote a song with simple, universal lyrics, and the result was just another example of electronic communication uniting large populations. Indeed, the phenomenon of rock music, and of superstardom as people know it today, would be impossible without many of the forms of technology discussed here. Long before the TV broadcast, the Beatles had come to fame through the playing of their music 347 SCIENCE OF EVERYDAY THINGS VOLUME 2: REAL-LIFE PHYSICS set_vol2_sec9 9/13/01 1:15 PM Page 347 [...]... Kurtus, Ron “Visible Light” (Web site) (May 2, 20 01) “Light Waves and Color.” The Physics Classroom (Web site) (May 2, 20 01) Miller-Schroeder, Patricia The Science and Light of Color Milwaukee, WI: Gareth Stevens Publishing, 20 00 Nassau, Kurt Experimenting with Color New York:... VOLUME 2: REAL-LIFE PHYSICS 363 Light KEY TERMS SPECTRUM: The continuous distribu- CONTINUED cular to the direction in which the wave is tion of properties in an ordered arrange- moving ment across an unbroken range Examples VACUUM: of spectra (the plural of “spectrum”) matter, including air An area of space devoid of include the colors of visible light, or the WAVELENGTH: electromagnetic spectrum of which... preferred scientific notation is 7.5 12 • 107 To visualize the value of very large A particle of electromagnet- multiples of 10, it is helpful to remember ic radiation carrying a specific amount of that the value of 10 raised to any power n is energy, measured in electron volts (eV) the same as 1 followed by that number of For parts of the electromagnetic spectrum zeroes Hence 1 025 , for instance, is simply... TRANSVERSE and an adjacent trough, of a wave The WAVE: A wave in which the vibration or motion is perpendi- than the old-fashioned ways of producing phonograph records Lasers used in the production of CD-ROM (Read-Only Memory) disks are able to condense huge amounts of information—a set of encyclopedias or the New York metropolitan phone book—onto a disk one can hold in the palm of one’s hand Laser etching... as that of the pot of gold at the end of a rainbow In fact, a rainbow, like many other “magical” aspects of daily life, can be explained in terms of physics A rainbow, in fact, is simply an illustration of the visible light spectrum Rain drops perform the role of tiny prisms, dispersing white sunlight, much as scientists before Newton had learned to do But if there is a pot of gold at the end of the... inversely related to frequency Luminescence Extremely low-energy, long-wavelength radio waves have frequencies of around 1 02 Hz, while the highest-energy, shortest-wavelength gamma rays can have frequencies of up to 1 025 Hz This means that these gamma rays are oscillating at the rate of 10 trillion trillion times a second! MODERN MUCH TO UNDERSTANDING OF LUMINESCENCE OWES MARIE CURIE pies a definite place... high range of frequencies The repeated HERTZ: A unit for measuring frequency, named after nineteenth-century German physicist Heinrich Rudolf Hertz and energy levels, with a correspondingly (185 7-1 894) High frequencies are ex- short wavelength Included on the electro- pressed in terms of kilohertz (kHz; 103 or magnetic spectrum are long-wave and 1,000 cycles per second); megahertz (MHz; short-wave radio;... and mag- which a wave moves energy per unit of netic fields that emanate from it The direc- cross-sectional area tions of these fields are perpendicular to OSCILLATION: one another, and both are perpendicular to motion, typically periodic, in one or more the line of propagation for the wave itself dimensions ELECTROMAGNETISM: The branch PERIOD: A type of harmonic For wave motion, a period is of physics. .. William Abney (184 3-1 920 ) developed infrared photography, a method of capturing infrared radiation, rather than visible light, on film By the mid-twentieth century, infrared photography had come into use for a variety of purposes Military forces, for instance, may use infrared to S C I E N C E O F E V E RY DAY T H I N G S VOLUME 2: REAL-LIFE PHYSICS 349 skin A suntan, as a matter of fact, is actually... nothing between “on” and “off.” A dimmer switch, on the other hand, is a spectrum, because a very large number of gradations exist between the two extremes represented by a light switch S E V E N C O LO R S O R S I X ? The distribution of colors across the spectrum is as follows: red-orange-yellow-green-blue-violet The reasons for this arrangement, explained below in the context of the electromagnetic . described in terms of 343 SCIENCE OF EVERYDAY THINGS VOLUME 2: REAL-LIFE PHYSICS set _vol2 _sec9 9/13/01 1:15 PM Page 343 Electro- magnetic Spectrum photon energy level, or the amount of energy in each. waves of extremely low frequency; ordinary radio waves of the kind used for actual radio 344 SCIENCE OF EVERYDAY THINGS VOLUME 2: REAL-LIFE PHYSICS set _vol2 _sec9 9/13/01 1:15 PM Page 344 Electro- magnetic Spectrum broadcasts. account for this difference. Hence, the distance to the target can be cal- 348 SCIENCE OF EVERYDAY THINGS VOLUME 2: REAL-LIFE PHYSICS set _vol2 _sec9 9/13/01 1:15 PM Page 348 Electro- magnetic Spectrum culated

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