Waves and Phasors

8/18/2019 Waves and Phasors

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1Introduction:

Waves and Phasors

Overview

1-1 Dimensions

1-2 T

1-3 T

1-4 T

1-5 Review

1-6 Review

Historical Timeline

1-1 Dimensions, Units, and Notation

1-2 The Nature of Electromagnetism

1-3 Traveling Waves

1-4 The Electromagnetic Spectrum

1-5 Review of Complex Numbers

1-6 Review of Phasors

C H A P T E Rφ0 = π /4 φ0 = –π /4

T T

23T 2

y

–A

A

Leads ahead of reference wave Lags behind reference wave

Reference wave (φ0 = 0)

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OVERVIEW

Liquid crystal displays have become integral parts of many electronic gadgets, from alarm clocks and cellphones to laptop computers and television systems.LCD technology relies on special electrical and opticalproperties of a class of materials known as liquid

crystals, which are neither pure solids, nor pure liquids,but rather a hybrid of both. The molecular structure of these materials is such that when light travels throughthe material, the wave polarization of the emerging lightdepends on whether or not a voltage exists across thematerial. Consequently, when no voltage is applied,the exit surface appears bright, and conversely, whena voltage of a certain level is applied across the LCD

material, no light passes through it, resulting in a darkpixel. The in-between voltage range translates into arange of grey levels. By controlling the voltage acrosseach individual pixel in a two-dimensional array of pixels, a complete image can be displayed (Fig. 1-1).Color displays are composed of three subpixels with red,green, and blue filters. The wave-polarization behaviorin a LCD is a prime example of how electromagnetics isat the heart of electrical and computer engineering.

The subject of this book is applied electromagnetics,which encompasses the study of electric and magneticphenomena and their engineering applications, underboth static and dynamic conditions. Primary emphasisis placed on the fundamental properties of time-varying(dynamic) electromagnetic fields because of theirgreater relevance to practical problems in manyengineeringdisciplines,includingmicrowave andopticalcommunications, radar systems, bioelectromagnetics,and high-speedmicroelectronics, amongothers. We shall

C. 2-D array

UnpolarizedLight

EntrancePolarizer

ExitPolarizer

Two-DimensionalPixel Array

Figure1-1: Wave-polarization principlein a liquidcrystaldisplay (LCD).

study wave propagation in guided media, such as coaxialtransmission lines, optical fibers and waveguides; wavereflection and transmission at the interface betweendissimilar media; radiation by antennas, and several

other related topics. The concluding chapter is intendedto illustrate a few aspects of applied electromagneticsthrough an examination of design considerationsassociated with the use and operation of radar sensorsand satellite communication systems.

3






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4 CHAPTER 1 INTRODUCTION: WAVES AND PHASORS

We begin this chapter with a historical chronologyof electricity and magnetism. Next, we introduce thefundamental electric and magnetic field quantities weuse in electromagnetics, as well as their relationships toeach other and to the electric charges and currents that

generate them. The laws governing these relationshipsconstitute the basic infrastructure we use in the study of electromagnetic phenomena.Then, in preparationfor thematerialpresentedin Chapter 2, we provide short reviewsof three topics: traveling waves, complex numbers, andphasor analysis. Although the reader most likely hasencountered these topics in circuit analysis or otherengineeringdisciplines,short reviewsof the propertiesof traveling waves and of the convenience phasor notationprovides should prove useful in solving time-harmonicproblems.

Historical TimelineThe history of electromagnetics may be divided into twooverlapping eras. In the classical era, the fundamentallaws of electricity and magnetism were discovered andformulated.Buildingon thesefundamental formulations,the modern era of the past 100 years, characterizedby the introduction of a wide range of engineeringapplications, ushered the birth of the field of appliedelectromagnetics, the topic of this book.

EM in the Classical EraChronology 1-1 (pages 6 and 7) provides a timeline forthe classical era. It highlights those inventions and dis-coveries thathave impacted the historicaldevelopmentof electromagnetics in a very significant way, albeit that thediscoveries selected for inclusion represent only a smallfraction of the many scientific explorations responsible

forour current understandingof electromagnetics. As weproceed through the book, we will observe that some of the names highlighted in Chronology 1-1, such as thoseof Coulomb andFaraday, appearagainlateras we discussthe laws and formulations named after them.

The attractive force of magnetite was reported by theGreeks some2800 years ago. It was also a Greek, Thales

of Miletus, who first wrote about what we now call staticelectricity; he described how rubbing amber caused itto develop a force that could pick up light objects such

as feathers. The term electric first appeared in print in∼1600 in a treatise on the (electric) force generated byfriction, authored by the physician to Queen Elizabeth I,William Gilbert.

About a century later, in 1733, Charles-Fran¸ cois du Fay introduced the concept that electricity consists of two types of “fluids,” one positive and the other negative,and that like-fluids repel and opposite-fluids attract. Hisnotion of fluid is what we today call electric charge.Invention of the capacitor in 1745, originally called the

Leyden jar, made it possible to store significant amountsof electric charge in a single device. A few years later, in1752, Benjamin Franklin demonstrated that lightningis a form of electricity. He transferred electric chargefrom a cloud to a Leyden jar via a silk kite flown ina thunderstorm. The collective 18th century knowledgeabout electricity was integrated in 1785 by Charles-

Augustin de Coulomb, in the form of a mathematicalformulation characterizing the electrical force betweentwo charges in terms of the strengths and polarities of thecharges and the distance between them.

The year 1800 is noted for the development of thefirst electric battery, by Alessandro Volta, and 1820 wasa banner year for discoveries about how magnetism isinduced by electric currents. This knowledge was put togood use by Joseph Henry, who developed one of theearliest designs for electromagnets and electric motors.Shortly thereafter, Michael Faraday builtthefirstelectricgenerator (the converse of the electric motor). Faraday,

in essence, demonstrated that a changing magnetic fieldinduces an electric field (and hence a voltage). Theconverse relation, namely that a changing electric fieldinduces a magnetic field, was proposed by James Clerk

Maxwell in 1873 when he introduced his four (now)






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1-1 DIMENSIONS, UNITS, AND NOTATION 5

famous equations. Maxwell’s equations represent the foundation of classical electromagnetic theory.

Maxwell’s theory, which predicted a number of properties for electromagnetic waves, was not fullyaccepted by the scientific community at that time, not

until those properties were verified experimentally withradio waves by Heinrich Hertz in the 1880s. X-rays,another member of the EM family, were discovered in1895 by Wilhelm Roentgen. On the applied side, NikolaTesla was the first to develop the a-c motor, considered amajor advance over its predecessor, the d-c motor.

Despitetheadvancesmadeinthe19thcenturyinlearn-ing about electricity and magnetism and how to put themtopracticaluse,itwasnotuntil1897thatthefundamentalparticle of electric charge, the electron, was identifiedand its properties quantified (by J. J. Thomson). Theability to eject electrons from a material by shiningelectromagnetic energy, such as light, on it is known asthe photoelectric effect. To explain this effect, Albert

Einstein adopted thequantum concept of energythat hadbeen advanced a few years earlier (1900) by Max Planckin his formulation of the quantum theory of matter. Byso doing, Einstein symbolizes the bridge between theclassical and modern eras of electromagnetics.

EM in the Modern Era

In terms of engineering applications, electromagneticsplays a role in the design and operation of everyconceivable electronic device, including diodes, tran-sistors, integrated circuits, lasers, display screens, bar-code readers, cell phones, and microwave ovens, toname but a few. Given the breadth and diversity of these applications, it is far more difficult to constructa meaningful timeline for the modern era than was

possible earlier for the classical era. However, it is quitepossible to develop timelines for specific technologiesand to use them as educational tools by linking theirmilestone innovationsto electromagnetics. Chronologies1-2 (pages 8–9) and 1-3 (pages 10–11) present timelines

Table 1-1: Fundamental SI units.

Dimension Unit Symbol

Length meter mMass kilogram kg

Time second sElectric Current ampere ATemperature kelvin KAmount of substance mole mol

for telecommunications and computers, respectively,representingtechnologies thathave become integralpartsof today’s societal infrastructure. Some of the entriesin the tables refer to specific inventions, such as thetelegraph, the transistor, and the laser. The operationalprinciples and capabilities of some of these technologiesare highlighted in special sections called Technology

Briefs, scattered throughout the book.

1-1 Dimensions, Units, and Notation

The International System of Units, abbreviated SI after its French name Système Internationale, is thestandard system used in today’s scientific literature forexpressing the units of physical quantities. Length is a

dimension and meter is the unit by which it is expressedrelative to a reference standard. The SI system is basedon the units for the six fundamental dimensions listedin Table 1-1. The units for all other dimensions areregarded as secondary because they are based on andcan be expressed in terms of the six fundamental units.Appendix A contains a list of quantities used in thisbook, together with their symbols and units.

For quantities ranging in value between 10−18

and1018, a set of prefixes, arranged in steps of 103, arecommonly used to denote multiples and submultiplesof units. These prefixes, all of which were derived fromGreek, Latin, Spanish, and Danish terms, are listed in






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ca. 900 Legend has it that while walking across a field in northern

Greece, a shepherd named Magnus experiences a pull

on the iron nails in his sandals by the black rock he isstanding on. The region was later named Magnesia and

the rock became known as magnetite [a form of iron with

permanent magnetism].

ca. 600 Greek philosopher Thalesdescribes how amber, after being

rubbed with cat fur, can pick up

feathers [static electricity].

ca. 1000 Magnetic compass used as a

navigational device.

1600 William Gilbert (English) coins the term electric after the

Greek word for amber (elektron), and observes that a

compass needle points north-south because the Earth

acts as a bar magnet.

1671 Isaac Newton (English) demonstrates that white light is a

mixture of all the colors.

1733 Charles-Francois du Fay (French) discovers thatelectric charges are of two forms, and that like charges

repel and unlike charges attract.

1745 Pieter van Musschenbroek (Dutch) invents the Leyden

jar, the first electrical capacitor.

1752 Benjamin Franklin(American) invents

the lightning rod anddemonstrates that

lightning is electricity.

1785

Charles-Augustin

de Coulomb (French)

demonstrates that the

electrical force between

charges is proportional to

the inverse of the square

of the distance between

them.

1800

Alessandro Volta (Italian)

develops the

first electric

battery.

1820

Hans Christian Oersted(Danish) demonstrates the

interconnection between

electricity and magnetism

through his discovery that

an electric current in a

wire causes a compass

needle to orient itself

perpendicular to

the wire.

1820 Andre-Marie Ampere (French)

notes that parallel currents in

wires attract each other and

opposite currents repel.

1820

Jean-Baptiste Biote (French)

and Felix Savart (French)

develop the Biot-Savart law

relating the magnetic field

induced by a wire segment

to the current flowing through it.

Chronology 1-1: TIMELINE FOR ELECTROMAGNETICS IN THE CLASSICAL ERA

Electromagnetics in the Classical Era

BC

BC






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1888 Nikola Tesla(Croation American)

invents the ac

(alternating

current) electric

motor.

1895 Wilhelm Roentgen (German)

discovers X-rays. One of

his first X-ray images was

of the bones in his wife's

hands. [1901 Nobel prize

in physics.]

1897 Joseph John Thomson (English) discovers the electron

and measures its charge-to-mass ratio. [1906 Nobel prize

in physics.]

1905 Albert Einstein (German American) explains the

photoelectric effect discovered earlier by Hertz in 1887.

[1921 Nobel prize in physics.]

1827 Georg Simon Ohm (German) formulates Ohm's law

relating electric potential to current and resistance.

1827 Joseph Henry (American) introduces the concept of

inductance, and builds one of the earliest electric motors.

He also assisted Samual Morse in the development

of the telegraph.

1831 Michael Faraday (English)

discovers that a changing

magnetic flux can induce

an electromotive force.

1873 James Clerk Maxwell

(Scottish) publishes his

Treatise on Electricity and

Magnetism in which he unites

the discoveries of Coulomb,

Oersted, Ampere, Faraday,

and others into four elegantly

constructed mathematical

equations, now known as

Maxwell’s Equations.

1887

Chronology 1-1: TIMELINE FOR ELECTROMAGNETICS IN THE CLASSICAL ERA ( continued )

Electromagnetics in the Classical Era

Heinrich Hertz(German) builds

a system that

can generate

electromagneticwaves (at radio

frequencies) and

detect them.

1835 Carl Friedrich Gauss (German) formulates Gauss's law

relating the electric flux flowing through an enclosed

surface to the enclosed electric charge.






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Chronology 1-2: TIMELINE FOR TELECOMMUNICATIONS

Telecommunications

1825

1837 Samuel Morse(American) patents the

electromagnetic telegraph,

using a code of dots and

dashes to represent letters

and numbers.

1872 Thomas Edison (American)

patents the electric

typewriter.

1876 Alexander Bell (Scottish-

American) invents the

telephone, the rotary dial

becomes available in 1890,

and by 1900, telephone

systems are installed in

many communities.

1887 Heinrich Hertz (German)

generates radio waves and

demonstrates that they

share the same properties

as light.

1887 Emil Berliner (American) invents the flat gramophone

disc, or record.

1893 Valdemar Poulsen(Danish) invents the

first magnetic sound

recorder using steel

wire as recording

medium.

Guglielmo Marconi (Italian)

files his first of many patentson wireless transmission

by radio. In 1901, he

demonstrates radio telegraphy

across the Atlantic Ocean.

[1909 Nobel prize in physics,

shared with Karl Braun

(German).]

1897 Karl Braun (German) invents the cathode ray tube (CRT).

[1909 Nobel prize with Marconi.]

1902 Reginald Fessenden (American) invents amplitude

modulation for telephone transmission. In 1906, he

introduces AM radio broadcasting of speech and music

on Christmas Eve.

1912 Lee De Forest(American)

develops the triode

tube amplifier for

wireless telegraphy.

Also in 1912, the

wireless distress

call issued by the

Titanic was heard

58 miles away by

the ocean liner

Carpathia, which

managed to rescue

705 Titanic passengers

3.5 hours later.

1919 Edwin Armstong (American) invents the

superheterodyne radio receiver.

1920 Birth of commercial radio broadcasting; Westinghouse

Corporation establishes radio station KDKA in Pittsburgh,

Pennsylvania.

1896William Sturgeon

(English) developsthe multiturn

electromagnet.






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1958 Jack Kilby (American) builds first integrated circuit (IC) on

germanium and, independently, Robert Noyce (American)

builds first IC on silicon.

Echo, the first passive

communication satellite is

launched, and successfully

reflects radio signals back

to Earth. In 1963, the first

communication satellite is

placed in geosynchronous orbit.

1969 ARPANET is established by the U.S. Department ofDefense, to evolve later into the Internet.

1979 Japan builds the first

cellular telephone network:

• 1983 cellular phone networks start in the United States.

• 1990 electronic beepers become common.

• 1995 cell phones become widely available.

• 2002 cell phone supports video and Internet.

1984 Worldwide Internet becomes operational.

1988 First transatlantic optical fiber cable between the U.S.

and Europe.

1997 Mars Pathfinder sends images to Earth.

2004 Wireless communication supported by many airports,

university campuses, and other facilities.

Vladimir Zworykin

(Russian-American)invents television. In

1926, John Baird (Scottish)

transmits TV images

over telephone wires

from London to Glasgow.

Regular TV broadcasting

began in Germany (1935),

England (1936), and the

United States (1939).

1926 Transatlantic telephone service between London and

New York.

1932 First microwave telephone link, installed (by Marconi)

between Vatican City and the Pope’s summer residence.

1933 Edwin Armstrong (American) invents frequency

modulation (FM) for radio transmission.

1935 Robert Watson Watt(Scottish) invents radar.

1938 H. A. Reeves (American)

invents pulse code

modulation (PCM).

1947 William Schockley,

Walter Brattain, and

John Bardeen (all

Americans) invent the

junction transistor at Bell

Labs. [1956 Nobel prize

in physics.]

1955 Pager is introduced as a radio communication product in

hospitals and factories.

1955 Navender Kapany (Indian American) demonstrates the

optical fiber as a low-loss, light-transmission medium.

1923

1960

Chronology 1-2: TIMELINE FOR TELECOMMUNICATIONS ( continued )

Telecommunications






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1941 Konrad Zuze (German) develops the first programmable

digital computer, using binary arithmetic and electric

relays.

1945 John Mauchly and J. Presper Eckert develop the

ENIAC, the first all-electronic computer.

1950 Yoshiro Nakama (Japanese) patents the floppy disk as a

magnetic medium for storing data.

1956 John Backus (American)

develops FORTRAN, the

first major programming

language.

1958 Bell Labs develops the modem.

1960 Digital Equipment Corporation

introduces the first

minicomputer, the PDP-1,

to be followed with the

PDP-8 in 1965.

1964 IBM’s 360 mainframe

becomes the standard

computer for major

businesses.

1965 John Kemeny and

Thomas Kurtz(both American)

develop the BASIC

computer language.

Chronology 1-3: TIMELINE FOR COMPUTER TECHNOLOGY

Computer Technology

ca 1100 Abacus is the earliest known calculating device.

1614 John Napier (Scottish) develops the logarithm system.

Blaise Pascal(French) builds

the first adding

machine using

multiple dials.

Gottfried von Leibniz (German) builds calculator that can

do both addition and multiplication.

Charles de Colmar (French) builds the Arithometer, the

first mass-produced calculator.

1642

1671

1820

1885 Dorr Felt (American) invents and markets a key-operated

adding machine (and adds a printer in 1889).

1930 Vannevar Bush (American) develops the differential analyzer,

an analog computer for solving differential equations.

BC

PRINT

FOR Counter = 1 TO Items PRINT USING “##.”; Counter; LOCATE , ItemColumn PRINT Item$(Counter); LOCATE , PriceColumn PRINT Price$(Counter)NEXT Counter






10/106


Chronology 1-3: TIMELINE FOR COMPUTER TECHNOLOGY ( continued )

Computer Technology

1989 Tim Berners Lee (British) invents the World Wide Web by

introducing a networked hypertext system.

1991 Internet connects to 600,000 hosts in more than 100

countries.

1995 Sun Microsystems introduces the Java programming

language.

1996 Sabeer Bhatia (Indian American) and Jack Smith

(American) launch Hotmail, the first

webmail service.

1997 IBM’s Deep Blue computer defeats World ChessChampion Garry Kasparov.

1997 Palm Pilot becomes widely available.

1968

1971 Texas Instruments introduces the pocket

calculator.

1971 Ted Hoff (American) invents the Intel

4004, the first computer microprocessor.

1976 IBM introduces the laser printer.

1976 Apple Computer sells Apple I

in kit form, followed by

the fully assembled

Apple II in 1977 and the

Macintosh in 1984.

1980 Microsoft introduces the

MS-DOS computer disk

operating system.

Microsoft Windows

is marketed in 1985.

1981 IBM introduces

the PC.

I B M

p a l m O n e I n c .

I B M

A p p l e

I B M

T o m H o w e

K n n i g h t - R i d d e r

T e x a s I n s t r u m e n t s

Douglas Engelbart (American) demonstrates a

word-processor system, the mouse pointing deviceand the use of “windows.”






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Table 1-2: Multiple and submultiple prefixes.Prefix Symbol Magnitude

exa E 1018

peta P 1015

tera T 1012giga G 109

mega M 106

kilo k 103

milli m 10−3

micro µ 10−6nano n 10−9pico p 10−12

femto f 10−15atto a 10−18

Table 1-2. A length of 5 × 10−9 m, for example, may bewritten as 5 nm.

In electromagnetics we work with scalar and vectorquantities. In this book we use a medium-weight italic

fontforsymbols(otherthanGreekletters)denotingscalarquantities, such as R for resistance, whereas we use aboldface roman font for symbols denoting vectors, suchas E for the electric field vector. A vector consists of a magnitude (scalar) and a direction, with the directionusually denoted by a unit vector. For example,

E = x̂E, (1.1)where E is the magnitude of E and x̂ is its direction.Unit vectors areprinted in boldface witha circumflex ( ˆ )above the letter.

Throughout this book, we make extensive use of phasor representation in solving problems involvingelectromagnetic quantities that vary sinusoidally in time.

Letters denoting phasor quantities are printed with atilde (∼) over the letter. Thus, E is the phasor electricfield vector corresponding to the instantaneous electricfield vector E(t). This notation is discussed in moredetail in Section 1-6.

1-2 The Nature of Electromagnetism

Our physical universe is governed by four fundamentalforces of nature:

• The nuclear force,whichisthestrongestofthefour,but its range is limited to submicroscopic systems,such as nuclei.

• The weak-interaction force, whose strength isonly 10−14 that of the nuclear force. Its primaryrole is in interactions involving certain radioactiveelementary particles.

• The electromagnetic force, which exists betweenall charged particles. It is the dominant force in

microscopic systems, such as atoms and molecules,and its strength is on the order of 10−2 that of thenuclear force.

• The gravitational force, which is the weakest of all four forces, having a strength on the order of 10−41 that of the nuclear force. However, it is thedominant force in macroscopic systems, such as thesolar system.

Ourinterest in thisbook is withthe electromagneticforceand its consequences. Even though the electromagneticforce operates at the atomic scale, its effects can betransmitted in the form of electromagnetic waves thatcan propagate through both free space and materialmedia. The purpose of this section is to provide anoverview of the basic framework of electromagnetism,whichconsists of certain fundamental lawsgoverningtheelectric andmagneticfields induced by staticand movingelectric charges, respectively, the relations between the

electric andmagnetic fields, andhow these fields interactwith matter. As a precursor, however, we will takeadvantage of our familiarity with the gravitational forceby describingsome of itsproperties because they providea useful analogue to those of the electromagnetic force.






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1-2 THE NATURE OF ELECTROMAGNETISM 13

m1

m2

Fg12

Fg21

R12^ R12

Figure 1-2: Gravitational forces between two masses.

1-2.1 The Gravitational Force: A UsefulAnalogue

According to Newton’s law of gravity, the gravitationalforce Fg21 actingonmass m2 duetoamassm1 atadistanceR12 from m2, as depicted in Fig. 1-2, is given by

Fg21

= −ˆR12

Gm1m2

R212

(N), (1.2)

where G is the universal gravitational constant, R̂12 isa unit vector that points from m1 to m2, and the unitfor force is newton (N). The negative sign in Eq. (1.2)accounts for the fact that the gravitational force isattractive. Conversely, Fg12 = −Fg21 , where Fg12 is theforce acting on mass m1 due to the gravitational pullof mass m2. Note that the first subscript of Fg denotesthe mass experiencing the force and the second subscriptdenotes the source of the force.

The force of gravitation acts at a distance; that is, thetwo objects do not have to be in direct contact for eachto experience the pull by the other. This phenomenonof direct action at a distance has led to the concept of

fields. An object of mass m1 induces a gravitationalfield ψψψ1 (Fig. 1-3) that does not physically emanate fromthe object, but its influence exists at every point in spacesuch that if another object of mass m2 were to exist at adistance R12 from object m1 then the second object m2

m1

–R̂

1ψ ψ ψ ψ ψ ψ

Figure 1-3:Gravitational field ψψψ1 inducedby a mass m1.

would experience a force acting on it equal in strengthto that given by Eq. (1.2). At a distance R from m1, thefield ψψψ1 is a vector defined as

ψψψ1 = −R̂ Gm1R2

(N/kg), (1.3)

where R̂ is a unit vector that points in the radial directionawayfromobject m1,andtherefore −R̂ pointstowardm1.The force due to ψψψ1 acting on a mass m2 at a distanceR = R12 along the direction R̂ = R̂12 is

Fg21 = ψψψ1m2 = −R̂12Gm1m2

R212. (1.4)

The field concept may be generalized by defining thegravitational fieldψψψ at any point in space such that, whena test mass m is placed at that point, the force Fg actingon m is related to ψψψ by

ψψψ = Fgm

. (1.5)

The forceFg may bedue to a single mass ora distributionof many masses.






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1-2.2 Electric Fields

The electromagnetic force consists of an electrical forceFe and a magnetic force Fm. The electrical force Feis similar to the gravitational force, but with a majordifference. The source of the gravitational field is mass,andthe source of theelectricalfield iselectriccharge,andwhereas both types of fields vary inversely as the squareof the distance from their respective sources, electriccharge may have positive or negative polarity, whereasmass does not exhibit such a property.

We know from atomic physics that all matter containsa mixture of neutrons, positively charged protons, andnegatively charged electrons, with the fundamentalquantity of charge being that of a single electron, usuallydenoted by the letter e. The unit by which electric chargeis measured is the coulomb (C), named in honor of theeighteenth-century French scientist Charles Augustin deCoulomb (1736–1806). The magnitude of e is

e = 1.6 × 10−19 (C). (1.6)The charge of a single electronis qe = −e and that of a proton is equal in magnitude but opposite in polarity:qp = e. Coulomb’s experiments demonstrated that:(1) two like charges repel one another, whereas two

charges of opposite polarity attract,

(2) the force acts along theline joiningthe charges,and

(3) its strength is proportional to the product of

the magnitudes of the two charges and inversely

proportional to the square of the distance between

them.

These properties constitute what today is calledCoulomb’s law, which can be expressed mathematically

by the following equation:

Fe21 = R̂12q1q2

4πε 0R212(N) (in free space), (1.7)

+q1

+q2

Fe12

Fe21

R12^

R12

Figure 1-4: Electric forces on two positive point chargesin free space.

where Fe21 is the electrical force acting on charge q2 dueto charge q1, R12 is the distance between the twocharges,R̂12 is a unit vector pointing from charge q1 to charge q2(Fig.1-4), and ε0 is a universal constant called theelectrical permittivity of free space [ε0 = 8.854 × 10−12farad per meter (F/m)]. The two charges are assumed

to be in free space (vacuum) and isolated from allother charges. The force Fe12 acting on charge q1 due tocharge q2 isequaltoforce Fe21 in magnitude,but oppositein direction; Fe12 = −Fe21 .

The expression given by Eq. (1.7) for the electricalforce is analogous to that given by Eq. (1.2) for thegravitational force,and wecan extend theanalogy furtherby defining the existence of an electric field intensity Edue to any charge q as follows:

E = R̂ q4π ε0R2

(V/m) (in free space), (1.8)

where R is the distance between the charge and the

observation point, and R̂ is the radial unit vectorpointing away from the charge. Figure 1-5 depicts theelectric-field lines due to a positive charge. For reasonsthat will become apparent in later chapters, the unit for Eis volt per meter (V/m).






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charge in the absence of the material. To extend Eq. (1.8)from the free-space case to any medium, we replace thepermittivity of free space ε0 with ε , where ε is now thepermittivity of the material in which the electric field ismeasured and is therefore characteristic of that particular

material. Thus,

E = R̂ q4πε R2

(V/m). (1.10)

Often, ε is expressed in the form

ε = εrε0 (F/m), (1.11)where ε r is a dimensionless quantity called the relative

permittivity or dielectric constant of the material. Forvacuum, εr = 1; forair near Earth’s surface, εr = 1.0006;and for materials that we will have occasion to use in thisbook, their values of εr are tabulated in Appendix B.

Inadditionto theelectric field intensityE,wewilloftenfind it convenient to also use a related quantity called theelectric flux density D, given by

D = εE (C/m2), (1.12)

and its unit is coulomb per square meter (C/m2). Thesetwo electrical quantities, E and D, constitute one of twofundamental pairs of electromagnetic fields. The secondpair consists of the magnetic fields discussed next.

1-2.3 Magnetic Fields

As early as 800 B.C., the Greeks discovered that certainkinds of stones exhibit a force that attracts pieces of iron.These stones are now called magnetite (Fe3O4) and thephenomenon they exhibit is magnetism. In the thirteenth

century, French scientists discovered that, when a needlewas placed on the surface of a spherical natural magnet,the needle oriented itself along different directions fordifferent locations on the magnet. By mapping thedirections taken by the needle, it was determined that the

S

N

B

Figure 1-7: Pattern of magnetic field lines around a barmagnet.

magnetic forceformedmagnetic-field lines thatencircledthe sphere and appeared to pass through two pointsdiametrically opposite each other. These points, calledthe north and south poles of the magnet, were foundto exist for every magnet, regardless of its shape. Themagnetic-field pattern of a bar magnet is displayed inFig. 1-7. It was also observed that like poles of differentmagnets repel each other and unlike poles attract eachother. This attraction–repulsion property is similar to theelectric force between electric charges, except for oneimportant difference: electric charges can be isolated,but magnetic poles always exist in pairs. If a permanentmagnet is cutinto small pieces, no matterhow small eachpiece is, it will always have a north and a south pole.

The magnetic lines encircling a magnet are called magnetic-field lines and represent the existence of a

magnetic field called the magnetic flux density B.A magnetic field not only exists around permanentmagnets but can also be created by electric current.This connection between electricity and magnetism wasdiscovered in 1819 by the Danish scientist Hans Oersted






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1-2.4 Static and Dynamic Fields

Because the electric field E is governed by the charge qand the magnetic field H is governed by I = dq/dt , andsince q and dq/dt are independentvariables, the induced

electric and magnetic fields are independent of oneanother as long as I remains constant. To demonstratethe validity of this statement, consider for example asmall section of a beam of charged particles that aremoving at a constant velocity. The moving chargesconstitute a d-c current. The electric field due to thatsection of the beam is determined by the total charge qcontained in that section. The magnetic field does notdepend on q , but rather on the rate of charge (current)flowing through that section. Few charges moving veryfast can constitute the same current as many chargesmoving slowly. In these two cases the induced magneticfield will be the same because the current I is the same,but the induced electric field will be quite differentbecause the numbers of charges are not the same.

Electrostatics and magnetostatics, corresponding tostationary charges and steady currents, respectively,are special cases of electromagnetics. They representtwo independent branches, so characterized becausethe induced electric and magnetic fields are uncoupledto each other. Dynamics, the third and more generalbranch of electromagnetics, involves time-varying fieldsinduced by time-varying sources, that is, currents andcharge densities. If the current associated with the beamof moving charged particles varies with time, then theamount of charge present in a given section of the beamalso varies with time, and vice versa. As we will seein Chapter 6, the electric and magnetic fields becomecoupled to each other in that case. In fact, a time-varying

electric field will generate a time-varying magnetic field,andvice versa.Table1-3providesasummaryofthethreebranches of electromagnetics.

The electric and magnetic properties of materials arecharacterizedbythetwoparameters ε and µ,respectively.

A third fundamental parameter is also needed, the conductivity of a material σ , which is measured insiemens per meter (S/m). The conductivity characterizesthe ease with which charges (electrons) can move freelyin a material. If σ = 0, the charges do not movemore than atomic distances and the material is said tobe a perfect dielectric, and if σ = ∞, the chargescan move very freely throughout the material, which isthen called a perfect conductor. The material parametersε, µ, and σ are often referred to as the constitutive

parameters of a material (Table 1-4). A medium is saidto be homogeneous if its constitutive parameters areconstant throughout the medium.

REVIEW QUESTIONS

Q1.1 What are the four fundamental forces of natureand what are their relative strengths?

Q1.2 What is Coulomb’s law? State its properties.

Q1.3 What are the two important properties of electriccharge?

Q1.4 What do the electrical permittivity and magneticpermeability of a material account for?

Q1.5 What are the three branches and associatedconditions of electromagnetics?

1-3 Traveling Waves

Waves are a natural consequence of many physicalprocesses: waves and ripples on oceans and lakes; soundwaves that travel through air; mechanical waves onstretched strings; electromagnetic waves that constitutelight; earthquake waves; and many others. All thesevarious types of waves exhibit a number of commonproperties, including the following:

• Moving waves carry energy from one point toanother.






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1-3 TRAVELING WAVES 19

Table 1-3: The three branches of electromagnetics.

Branch Condition Field Quantities (Units)

Electrostatics Stationary charges Electric field intensity E (V/m)(∂q/∂t

= 0) Electric flux density D (C/m2)

D = εEMagnetostatics Steady currents Magnetic flux density B (T)

(∂I/∂t = 0) Magnetic field intensity H (A/m)B = µH

Dynamics Time-varying currents E, D, B, and H(Time-varying fields) (∂I/∂t = 0) (E, D) coupled to (B, H)

Table 1-4: Constitutive parameters of materials.

Parameter Units Free-space Value

Electrical permittivity ε F/m ε0 = 8.854 × 10−12 (F/m)

136π

× 10−9 (F/m)

Magnetic permeability µ H/m µ0 = 4π × 10−7

(H/m)

Conductivity σ S/m 0

• Waveshave velocity; ittakestimeforawavetotravelfrom one point to another. In vacuum, light wavestravel at a speed of 3 × 108 m/s and sound waves inair travel at a speed approximately a million timesslower, specifically 330 m/s.

• Some waves exhibit a property called linearity.Waves that do not affect the passage of other wavesare called linear because they pass right througheach other, and the total of two linear waves is

simply the sum of the two waves as they wouldexist separately. Electromagnetic waves are linear,as are sound waves. When two people speak to oneanother, their sound waves do not reflect from oneanother, but simply pass through independently of

each other. Water waves are approximately linear;the expanding circles of ripples caused by twopebbles thrown into two locations on a lake surfacedo not affect each other. Although the interactionof the two circles may exhibit a complicatedpattern, it is simply the linear superposition of twoindependent expanding circles.

Waves are of two types: transient waves caused by

a short-duration disturbance and continuous harmonicwaves generated by an oscillating source. We willencounter both types of waves in this book, but mostof our discussion will deal with the propagation of continuous waves that vary sinusoidally with t ime.






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(a) Circular waves (c) Spherical wave(b) Plane and cylindrical waves

Plane wavefrontTwo-dimensional wave

Cylindrical wavefront Spherical wavefront

Figure 1-10: Examples of two-dimensional and three-dimensional waves: (a) circular waves on a pond, (b) a plane light waveexciting a cylindricallightwavethroughthe useof a long narrow slit in an opaque screen, and(c) a slicedsection of a sphericalwave.

u

Figure1-9: Aone-dimensionalwavetravelingonastring.

An essential feature of a propagating wave is that itis a self-sustaining disturbance of the medium through

which it travels. If this disturbance varies as a functionof one space variable, such as the vertical displacementof the string shown in Fig. 1-9, we call the wave a

one-dimensional wave. The vertical displacement varieswith time and with the location along the length of the string. Even though the string rises up into asecond dimension, the wave is only one-dimensionalbecause the disturbance varies with only one spacevariable. A two-dimensional wave propagates out acrossa surface, like the ripples on a pond [Fig. 1-10(a)], andits disturbance can be described by two space variables.And by extension, a three-dimensional wave propagatesthrough a volume and its disturbance may be a functionof all three space variables. Three-dimensional wavesmay take on many different shapes; they include planewaves, cylindrical waves, and spherical waves. A planewave is characterized by a disturbance that at a givenpoint in time has uniform properties across an infiniteplane perpendicular to the direction of wave propagation






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[Fig.1-10(b)] and, similarly, for cylindricaland sphericalwavesthedisturbancesareuniformacrosscylindricalandspherical surfaces, as shown in Figs. 1-10(b) and (c).

In the material that follows, we will examine some of thebasicpropertiesofwavesbydevelopingmathematical

formulationsthatdescribetheirfunctionaldependenceontimeandspacevariables.Tokeepthepresentationsimple,we will limit our present discussion to sinusoidallyvarying waves whose disturbances are functions of onlyone space variable, and we will defer discussion of morecomplicated waves to later chapters.

1-3.1 Sinusoidal Wave in a Lossless Medium

Regardless of the mechanism responsible for generatingthem, all waves can be described mathematically incommon terms. By way of an example, let us considera wave traveling on a lake surface. A medium is said tobe lossless if it does not attenuate the amplitude of the

wave traveling within it or on its surface. Let us assume

for the time being that frictional forces can be ignored,therebyallowingawavegeneratedonthewatersurfacetotravel indefinitely with no loss in energy. If y denotes theheight of the water surface relative to the mean height(undisturbed condition) and x denotes the distance of wave travel, the functional dependence of y on time t and the spatial coordinate x has the general form

y(x,t) = A cos

2πt

T − 2π x

λ+ φ0

(m), (1.17)

where A is the amplitude of the wave, T is its time period , λ is its spatial wavelength, and φ0 is a reference phase. The quantity y(x,t ) can also be expressed in theform

y(x,t) = A cos φ(x,t), (1.18)where

φ(x,t) =

2π t

T − 2πx

λ+ φ0

(rad). (1.19)

–A0 2 3λ 2

A

y( x , 0)

x

(a) y( x , t ) versus x at t = 0

–A

0 T 2

T 3T 2

A

T

y(0, t )

t

(b) y( x , t ) versus t at x = 0

λ

λ λ

Figure 1-11: Plots of y(x, t) =

A cos2πt T − 2πxλ as afunction of (a) x at t = 0 and (b) t at x = 0.The angle φ(x,t ) is called the phase of the wave, andit should not be confused with the reference phase φ0,which is constant with respect to both time and space.Phase is measured by the same units as angles, that is,radians (rad) or degrees, with 2π radians = 360◦.

Let us first analyze the simple case when φ0 = 0:

y(x,t) = A cos

2π t T

− 2πxλ

(m). (1.20)

The plots in Fig. 1-11 show the variation of y(x,t ) withx at t = 0 and with t at x = 0. The wave pattern repeatsitself at a spatial period λ along x and at a temporal

period T along t .If we take time snapshots of the water surface, the

height profile y(x) would exhibit the sinusoidal patternsshown in Fig. 1-12. For each plot, corresponding to aspecific value of t , the spacing between peaks is equal






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λ λ

23λ 2

3λ 2

3λ 2

λ λ 2

λ λ 2

y( x, 0)

y( x, T /4)

y( x, T /2)

A

– A

A

– A

A

– A

(a) t = 0

(b) t = T /4

(c) t = T /2

x

x

x

P

P

P

up

Figure 1-12: Plots of y(x,t) = A cos

2πt T − 2πx

λ

as

a function of x at (a) t = 0, (b) t = T /4, and (c) t =T /2. Note that the wave moves in the +x-direction witha velocity up = λ/ T .

to the wavelength λ, but the patterns are shifted relativeto one another because they correspond to differentobservation times. Because the pattern advances along

the +x-direction at progressively increasing values of t ,the height profile behaves like a wave traveling in thatdirection. If we choose any height level, such as thepeak P , and follow it in time, we can measure the phasevelocity of the wave. The peak corresponds to when the

phase φ(x,t) of the wave is equal to zero or multiplesof 2π radians. Thus,

φ(x,t)= 2πt T

− 2πxλ

=2nπ, n = 0, 1, 2, . . . (1.21)

Hadwechosenanyotherfixedheightofthewave,say y0,and monitored its movement as a function of t and x, thisagain is equivalent to setting the phase φ(x,t ) constantsuch that

y(x,t) = y0 = A cos

2π t

T − 2π x

λ

, (1.22)

or

2πt

T − 2πx

λ= cos−1

y0A

= constant. (1.23)

The apparent velocity of that fixed height is obtained bytaking the time derivative of Eq. (1.23),

2π

T − 2π

λ

dx

dt = 0, (1.24)

which gives the phase velocity up as

up = dx

dt = λ

T (m/s). (1.25)

The phase velocity, also called the propagation velocity,is the velocity of the wave pattern as it moves acrossthe water surface. The water itself mostly moves up anddown; when the wave moves from one point to another,the water does not move physically along with it.

Thedirectionofwavepropagationiseasilydeterminedby inspecting the signs of the t and x terms in theexpressionforthephase φ(x,t) givenbyEq.(1.19): ifoneof the signs is positive and the other is negative, then the

wave is traveling in the positive x -direction, and if both

signs are positive or both are negative, then the wave is

traveling in the negative x-direction. The constant phasereference φ0 has no influence on either the speed or thedirection of wave propagation.






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The frequency of a sinusoidal wave, f , is thereciprocal of its time period T :

M 1.9

D 1.1

f = 1T

(Hz). (1.26)

Combiningthe precedingtwo equationsgives therelation

M1.1-1.3 up = f λ (m/s). (1.27)

The wave frequency f , which is measured in cycles persecond, has been assigned the unit (Hz) (pronounced“hertz”), named in honor of the German physicist Hein-rich Hertz (1857–1894), who pioneered the developmentof radio waves.

Using Eq. (1.26), Eq. (1.20) can be rewritten in theshortened form as

y(x,t)

=A cos2πf t −

2π

λx

= A cos(ωt − βx), (1.28)

where ω is the angular velocity of the wave and β is its phase constant (or wavenumber), defined as

ω = 2πf (rad/s), (1.29a)

β = 2πλ

(rad/m). (1.29b)

In terms of these two quantities,

D1.2 up = f λ =

ω

β. (1.30)

Sofar,wehaveexaminedthebehaviorofawavetravelingin the +x-direction. To describe a wave traveling in the−x-direction, we reverse the sign of x in Eq. (1.28):

y(x,t) = A cos(ωt + βx). (1.31)

We now examine the role of the phase reference φ0given previously in Eq. (1.17). If φ0 is not zero, thenEq. (1.28) should be written as

y(x,t) = A cos(ωt − βx + φ0). (1.32)

A plot of y(x,t ) as a function of x at a specified t or as a function of t at a specified x will be shifted inspace or time, respectively, relative to a plot with φ0 = 0by an amount φ0. This is illustrated by the plots shownin Fig. 1-13. We observe that when φ0 is positive, y(t)reaches its peak value, or any other specified value,soonerthan when φ0 = 0. Thus, the wavewith φ0 = π/4is said to lead the wave with φ0 = 0 by a phase lead of π/4; and similarly, the wave with φ0 = −π/4 issaid to lag the wave with φ0 = 0 by a phase lag of π/4. A wave function with a negative φ0 takes longer toreach a given value of y(t) than the zero-phase referencefunction. When its value is positive, φ0 signifies a phaselead in time, and when it is negative, it signifies aphase lag.

M1.7-1.81-3.2 Sinusoidal Wave in a Lossy Medium

Ifawaveistravelinginthe x-directionina lossymedium,its amplitude will decrease as e−αx . This factor is calledthe attenuation factor, and α is called the attenuation

constant of the medium and its unit is neper per meter(Np/m). Thus, in general,

y(x,t) = Ae−αx cos(ωt − βx + φ0). (1.33)The wave amplitude is now Ae−αx , and not just A. Fig-ure1-14showsaplotof y(x,t) asa function of x at t = 0for A = 10m, λ = 2 m, α = 0.2Np/m,and φ0 = 0.Notethat the envelope of the wave pattern decreases as e−αx .

The real unit of α is (1/m); the neper (Np) part is

a dimensionless, artificialadjectivetraditionally usedas areminder that the unit (Np/m) refers to the attenuationconstant of the medium, α. A similar practice is appliedto the phase constant β by assigning it the unit (rad/m)instead of just (l/m).

M1.4-1.6 and D1.3






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φ0 = π /4 φ0 = –π /4

T t

T 2

3T 2

y

–A

A

Leads ahead of reference wave Lags behind reference wave

Reference wave (φ0 = 0)

Figure 1-13: Plots of y(0, t) = A cos[(2π t / T ) + φ0] for three different values of the reference phase φ0.

–10 m

–5 m

0

5 m

10 m

y( x )

y( x )

10e–0.2 x

x (m)1 2 3 4 5 6 7 8

Figure1-14: Plot of y(x) = (10e−0.2x cos π x) meters. Note that theenvelope is bounded between thecurve given by10e−0.2xand its mirror image.

REVIEW QUESTIONS

Q1.6 How can you tell if a wave is traveling in thepositive x-direction or the negative x-direction?

Q1.7 How does the envelope of the wave pattern vary

with distance in (a) a lossless medium and (b) a lossymedium?

Q1.8 Why does a negative value of φ0 signify a phaselag?






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Example 1-1 Sound Wave in Water

An acoustic wave traveling in the x-direction in a fluid(liquid or gas) is characterized by a differential pressurep(x,t). The unit for pressure is newton per square meter(N/m2). Find an expression for p(x,t) for a sinusoidalsound wave traveling in the positive x-direction in water,given that the wave frequency is 1 kHz, the velocity of sound in water is 1.5 km/s, the wave amplitude is 10N/m2, and p(x,t) was observed to be at its maximumvalue at t = 0 and x = 0.25 m. Treat water as a losslessmedium.

Solution: According to the general form given byEq. (1.17)for a wavetraveling in thepositive x-direction,

p(x,t) = A cos

2πT

t − 2πλ

x + φ0

(N/m2).

The amplitude A = 10 N/m2, T = 1/f = 10−3 s, andfrom up = f λ,

λ = upf

= 1.5 × 103103

= 1.5 m.

Hence,

p(x,t) = 10 cos

2π × 103t − 4π3

x + φ0

(N/m2).

Since at t = 0 and x = 0.25 m, p(0.25, 0) = 10 N/m2,we have

10 = 10cos−4π

3 0.25 + φ0

= 10cos

−π3

+ φ0

,

which yields the result (φ0 −

π/3) =

cos−1(1), orφ0 = π /3. Hence,

p(x,t) = 10 cos

2π × 103t − 4π3

x + π3

(N/m2).

Example 1-2 Power Loss

A laser beam of light propagating through theatmosphere is characterized by an electric field intensitygiven by

E(x,t) =150e−0.03x cos(3 × 1015t − 107x) (V/m),

where x is the distance from the source in meters. Theattenuation is due to absorption by atmospheric gases.Determine (a) the direction of wave travel, (b) the wavevelocity, and (c) the wave amplitude at a distance of 200 m.

Solution: (a) Since the coefficients of t and x in theargument of the cosine function have opposite signs, thewave must be traveling in the +x-direction.(b)

up = ω

β= 3 × 10

15

107 = 3 × 108 m/s,

which is equal to c, the velocity of light in free space.(c) At x = 200 m, the amplitude of E(x,t ) is

150e−0.03×200 = 0.37 (V/m).

EXERCISE 1.1 The electric field of a traveling electro-magnetic wave is given by

E(z,t) = 10 cos(π × 107t + π z/15 + π/6) (V/m).Determine (a) the direction of wave propagation, (b) thewave frequency f , (c) its wavelength λ, and (d) its phasevelocity up.

Ans. (a) −z-direction, (b) f = 5 MHz, (c) λ = 30 m,(d) up

= 1.5

×108 m/s. (See C D

RO M )

EXERCISE 1.2 An electromagnetic wave is propagatingin the z-direction in a lossy medium with attenuationconstant α = 0.5 Np/m. If the wave’s electric-fieldamplitude is 100 V/m at z = 0, how far can the wave






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travel before its amplitude will have been reduced to (a)10 V/m, (b) 1 V/m, (c) 1 µV/m?

Ans. (a) 4.6 m, (b) 9.2 m, (c) 37 m. (See C DRO M )

1-4 The Electromagnetic Spectrum

Visible light belongs to a family of waves called theelectromagnetic spectrum (Fig. 1-15). Other membersof this family include gamma rays, X rays, infraredwaves, and radio waves. Generically, they all are calledelectromagnetic (EM) waves because they share thefollowing fundamental properties:

• An EM wave consists of electric and magnetic fieldintensities that oscillate at the same frequency f .

• The phase velocity of an EM wave propagating invacuum is a universal constant given by the velocityof light c, defined earlier by Eq. (1.14).

• In vacuum, the wavelength λ of an EM wave isrelated to its oscillation frequency f by

λ = cf

. (1.34)

Whereas all EM waves share these properties, each isdistinguished by its own wavelength λ , or equivalentlyby its own oscillation frequency f .

Thevisible part ofthe EMspectrumshownin Fig. 1-15covers a very narrow wavelength range extendingbetween λ = 0.4 µm (violet) and λ = 0.7 µm (red). Aswe move progressively toward shorter wavelengths, weencounter the ultraviolet, X-ray, and gamma-ray bands,

each so named because of historical reasons associatedwith the discovery of waves with those wavelengths. Ontheother sideof the visiblespectrumlie the infrared bandand then the radio region. Because of the link between λand f given by Eq. (1.34), each of these spectral ranges

may be specified in terms of its wavelength range oralternatively in terms of its frequency range. In practice,however, a wave is specified in terms of its wavelength λif λ < 1 mm, which encompasses all parts of the EMspectrum except for the radio region, and the wave is

specified in terms of its frequency f if λ > 1 mm (i.e.,in the radio region). A wavelength of 1 mm correspondsto a frequency of 3 × 1011 Hz = 300 GHz in free space.

The radio spectrum consists of several individualbands, as shown in the chart of Fig. 1-16. Each bandcovers one decade of the radio spectrum and has aletter designation based on a nomenclature defined bythe International Telecommunication Union. Differentfrequencies have different applications because they areexcited by different mechanisms, and the propertiesof an EM wave propagating in a material may varyconsiderably from one band to another. The extremelylow frequency (ELF) band from 3 to 30 Hz is usedprimarilyfor thedetectionof buriedmetalobjects.Lowerfrequencies down to 0.1 Hz are used in magnetotelluricsensing of the structure of the earth, and frequenciesin the range from 1 Hz to 1 kHz sometimes are usedfor communications with submerged submarines and forcertain kinds of sensing of Earth’s ionosphere. The verylowfrequency(VLF)regionfrom3to30kHzisusedbothfor submarine communications and for position locationby the Omega navigation system. The low-frequency(LF) band, from 30 to 300 kHz, is used for some formsof communication and for the Loran C position-locationsystem. Some radio beacons and weather broadcaststations used in air navigation operate at frequencies inthe higher end of the LF band. The medium-frequency(MF) region from 300 kHz to 3 MHz contains thestandard AM broadcast band from 0.5 to 1.5 MHz.

Long-distance communicationsandshort-wave broad-casting over long distances use frequencies in thehigh-frequency (HF) band from 3 to 30 MHz becausewaves in this band are strongly affected by reflectionsby the ionosphere and least affected by absorption in






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1-4 THE ELECTROMAGNETIC SPECTRUM 27

1 fm 1 pm 1 nm1 Å

1 EHz 1 PHz 1 THz 1 GHz 1 MHz 1 kHz 1 Hz

1 µm 1 mm 1 km 1 Mm1 m

10–15

1023 1021 1018 1015 1012 109 106 103 1

10–12 10–10 10–9 10–6 10–3 103 106 1081

Frequency (Hz)

Wavelength (m)

visible

Gamma rays

Cancer therapy

X-rays

Medical diagnosis

Ultraviolet

Sterilization

Infrared

Heating,Night vision

Radio spectrum

Communication, radar, radio and TV broadcasting,radio astronomy

Atmospheric opacity

100%

0

Atmosphere opaque

Opticalwindow

Infraredwindows Radio window

Ionosphere opaque

Figure 1-15: The electromagnetic spectrum.

the ionosphere. The next frequency region, the veryhigh frequency (VHF) band from 30 to 300 MHz, isused primarily for television and FM broadcasting overline-of-sight distances and also for communicating withaircraft and other vehicles. Some early radio-astronomyresearch was also conducted in this range. The ultrahighfrequency (UHF) region from 300 MHz to 3 GHz isextensively populated with radars, although part of thisband also is used for television broadcasting and mobilecommunications with aircraft and surface vehicles. Theradarsinthisregionofthespectrumarenormallyusedforaircraft detection and tracking. Some parts of this regionhave been reserved for radio astronomical observation.

Many point-to-point radio communication systemsand various kinds of ground-based radars and ship radarsoperate at frequencies in the superhigh frequency (SHF)rangefrom3 to30 GHz. Some aircraftnavigationsystemsoperate in this range as well.

Most of the extremely high frequency (EHF) bandfrom 30 to 300 GHz is used less extensively, primarilybecause the technology is not as well developed andbecause of excessive absorption by the atmosphere insome parts of this band. Some advanced communicationsystems are being developed for operation at frequenciesin the “atmospheric windows,” where atmosphericabsorption is not a serious problem, as are automobilecollision-avoidance radars and some military imagingradar systems. These atmospheric windows include theranges from 30 to 35 GHz, 70 to 75 GHz, 90 to 95 GHz,and 135 to 145 GHz.

Although no precise definition exists for the extentof the microwave band , it is conventionally regardedto cover the full ranges of the UHF, SHF, and EHFbands, with the EHF band sometimes referred to as the

millimeter-wave band , because the wavelength rangecovered by this band extends from 1 mm (300 GHz) to1 cm (30 GHz).






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Radar, advanced communication systems,

remote sensing, radio astronomy

Extremely High Frequency

EHF (30 - 300 GHz)Radar, satellite communication systems, aircraftnavigation, radio astronomy, remote sensing

Super High FrequencySHF (3 - 30 GHz)

TV broadcasting, radar, radio astronomy,microwave ovens, cellular telephone

Ultra High FrequencyUHF (300 MHz - 3 GHz)

TV and FM broadcasting, mobile radiocommunication, air traffic control

Very High FrequencyVHF (30 - 300 MHz)

Short wave broadcastingHigh FrequencyHF (3 - 30 MHz)

AM broadcastingMedium FrequencyMF (300 kHz - 3 MHz)

Radio beacons, weather broadcast stationsfor air navigation

Low FrequencyLF (30 - 300 kHz)

Navigation and position locationVery Low FrequencyVLF (3 - 30 kHz)

Audio signals on telephoneUltra Low Frequency

ULF (300 Hz - 3 kHz)

Ionospheric sensing, electric powerdistribution, submarine communication

Super Low FrequencySLF (30 - 300 Hz)

Detection of buried metal objectsExtremely Low FrequencyELF (3 - 30 Hz)

f < 3 Hz) Magnetotelluric sensing of the

earth's structure

1012

109

106

103

300 GHz

1 GHz

1 MHz

1 kHz

1 Hz

Microwave

Frequency (Hz)

Band Applications

Figure 1-16: Individual bands of the radio spectrum and their primary applications.

REVIEW QUESTIONSQ1.9 What are the three fundamental properties of EMwaves?Q1.10 What is the range of frequencies covered by the

microwave band?Q1.11 What is the wavelength range of the visiblespectrum? What are some of the applications of theinfrared band?

1-5 Review of Complex Numbers

A complex number z is written in the form

z

= x

+jy , (1.35)

where x and y arethe real (Re)and imaginary (Im)partsof z, respectively, and j = √ −1. That is,

x = Re(z), y = Im(z). (1.36)






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1-5 REVIEW OF COMPLEX NUMBERS 29

θ ℜe( z)

ℑm( z)

y z

x

| z|

x = | z| cos θ y = | z| sin θ

θ = tan–1 ( y / x )

| z| = x 2 + y2+

Figure1-17: Relation between rectangularand polar rep-resentations of a complex number z = x + jy = |z|ejθ .

Alternatively, z may be written in polar form as

z = |z|ejθ = |z|∠θ (1.37)where |z| is the magnitude of z, θ is its phase angle,and the form ∠θ is a useful shorthand representationcommonly used in numerical calculations. Applying

Euler’s identity,

ejθ = cos θ + j sin θ, (1.38)

we can convert z from polar form, as in Eq. (1.37), intorectangular form, as in Eq. (1.35),

z = |z|ejθ = |z| cos θ + j |z| sin θ, (1.39)which leads to the relations

x = |z| cos θ, y = |z| sin θ, (1.40)|z| = +

x2 + y2 , θ = tan−1(y/x). (1.41)

The two forms are illustrated graphically in Fig. 1-17.

When using Eq. (1.41), care should be taken to ensurethat θ is in the proper quadrant. Also note that, since |z|is a positive quantity, only the positive root in Eq. (1.41)is applicable. This is denoted by the + sign above thesquare-root sign.

The complex conjugate of z, denoted with a starsuperscript (or asterisk), is obtained by replacing j (wherever it appears) with −j , so that

z∗ =

(x +

jy )∗ =

x −

jy = |

z|e−jθ

= |z|∠

−θ . (1.42)

The magnitude |z| is equal to the positive square root of the product of z and its complex conjugate:

|z| = +√ z z∗ . (1.43)

We now highlight some of the properties of complexalgebra that we will likely encounter in future chapters.

Equality: If two complex numbers z1 and z2 are given by

z1 = x1 + jy1 = |z1|ejθ 1 , (1.44)z

2 =x

2 +jy

2 = |z

2|ejθ 2 , (1.45)

then z1 = z2 if and only if x1 = x2 and y1 = y2 or,equivalently, |z1| = |z2| and θ 1 = θ 2.

Addition:

z1 + z2 = (x1 + x2) + j (y1 + y2). (1.46)

Multiplication:

z1z2 = (x1 + jy1)(x2 + jy2)= (x1x2 − y1y2) + j (x1y2 + x2y1), (1.47a)

or

z1z2 = |z1|ejθ 1 · |z2|ejθ 2= |z1||z2|ej(θ 1+θ 2)= |z1||z2|[cos(θ 1 + θ 2) + j sin(θ 1 + θ 2)]. (1.47b)






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1-6 REVIEW OF PHASORS 31

(c) V I ∗ = 5e−j 53.1◦ × 3.61e−j 236.3◦= 18.05e−j 289.4◦ = 18.05ej 70.6◦.

(d)V

I = 5e

−j 53.1◦

3.61ej 236.3◦

=1.39e−j 289.4

◦

= 1.39ej 70.6

◦.

(e)√

I = √ 3.61ej 236.3◦= ±

√ 3.61 ej 236.3

◦/2 = ±1.90ej 118.15◦.

EXERCISE 1.3 Express the following complex functionsin polar form:

z1 = (4 − j 3)2,z2 = (4 − j 3)1/2.

Ans. z1 = 25∠−73.7◦ , z2 = ±√

5∠−18.4◦ . (See C DRO M )

EXERCISE 1.4 Show that√

2j = ±(1 + j ). (See

C DR

O M )

1-6 Review of Phasors

Phasor analysis is a useful mathematical tool forsolving problems involving linear systems in which theexcitation is a periodic time function. Many engineeringproblemsarecast in the formof linearintegro-differentialequations.Iftheexcitation,morecommonlyknownasthe

forcing function, varies sinusoidally with time, the useof phasor notation to represent time-dependent variablesallows us to convert the integro-differential equation intoa linear equation with no sinusoidal functions, therebysimplifying the method of solution. After solving forthe desired variable, such as the voltage or current ina circuit, conversion from the phasor domain back to the

time domain provides the desired result.The phasor technique can also be used for analyzinglinear systems when the forcing function is any arbitrary(nonsinusoidal) periodic time function, such as a squarewave or a sequence of pulses. By expanding the

C

R

i

vs(t )

+

–

Figure 1-19: R C circuit connected to a voltage sourcevs(t) .

forcing function into a Fourier series of sinusoidalcomponents, we can solve for the desired variable usingphasor analysis for each Fourier component of theforcing function separately. According to the principleof superposition, the sum of the solutions due to all of the Fourier components gives the same result as onewould obtain had the problem been solved entirely in thetime domain without the aid of Fourier representation.

The obvious advantage of the phasor–Fourier approachis simplicity. Moreover, in thecase of nonperiodic sourcefunctions, such as a single pulse, the functions can beexpressed as Fourier integrals, and a similar applicationof the principle of superposition can be used as well.

The simple R C circuit shown in Fig. 1-19 contains asinusoidally time-varying voltage source given by

vs(t) = V 0 sin(ωt + φ0), (1.55)where V 0 is the amplitude, ω is the angular frequency,and φ0 is a reference phase. Application of Kirchhoff’svoltage law gives the following loop equation:

R i (t)

+

1

C i(t) dt = vs(t ) (time domain). (1.56)Our objective is to obtain an expression for the currenti(t). We can do this by solving Eq. (1.56) in the timedomain, which is somewhat cumbersome because the






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forcing function vs(t) is a sinusoid. Alternatively, we cantake advantage of the phasor technique as follows.

Step 1: Adopt a cosine reference

This means that we should express the forcing function

as a cosine, if not already in that form, and hence alltime-varying functions, such as the current in the circuitand the voltages across R and C, will also have a cosinereference. Thus,

vs(t) = V 0 sin(ωt + φ0)= V 0 cos

π2 − ωt − φ0

= V 0 cos

ωt + φ0 − π

2

, (1.57)

where we used the properties sin x = cos(π/2 − x) andcos(−x) = cos x.

Step 2: Express time-dependent variables as phasors

Any cosinusoidally time-varying function z(t) can beexpressed in the form

z(t) = Re Z ejωt , (1.58)where Z is a time-independent function called the

phasor ofthe instantaneous function z(t). To distinguishinstantaneous quantities from their phasor counterparts,a letter denoting a phasor is given a tilde (∼) over theletter. The voltage vs(t) given by Eq. (1.57) can be castin the form

vs(t) =Re

V 0ej(ωt +φ0−π/2)

=Re V 0ej( φ0−π/2)ejωt =Re

V se

jωt

, (1.59)

where V s = V 0ej (φ0−π/2). (1.60)The phasor V s, corresponding to the time functionvs(t), contains amplitude and phase information but is

independent of the time variable t . Next we define theunknown variable i(t) in terms of a phasor Ĩ ,

i(t) = Re(Ĩ ejωt ), (1.61)and if the equation we are trying to solve contains

derivatives or integrals, we use the following twoproperties:

di

dt = d

dt

Re(Ĩ ejωt )

=Re

d

dt (Ĩ ejωt )

=Re[j ωĨ ejωt ], (1.62)

and i dt =

Re(Ĩ ejωt ) d t

=Re

Ĩ ejωt dt

=Re Ĩ j ω

ejωt . (1.63)Thus, differentiation of the time function i(t) isequivalent to multiplication of its phasor Ĩ by j ω, andintegration is equivalent to division by j ω.

Step 3: Recast the differential / integral equation in

phasor form

Upon using Eqs. (1.59), (1.61), and (1.63) in Eq. (1.56),we have

RRe(Ĩ ejωt )+ 1CRe

Ĩ j ω

ejωt

= Re(V sejωt ). (1.64)

Since both R and C are real quantities and the Re( )

operation is distributive, Eq. (1.64) simplifies to

Ĩ

R + 1

j ωC

= V s (phasor domain). (1.65)






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1-6 REVIEW OF PHASORS 33

The time factor ejωt has disappeared because it wascontained in all three terms. Equation (1.65) is thephasor-domain equivalent of Eq. (1.56).

Step 4: Solve the phasor-domain equation

From Eq. (1.65) the phasor current Ĩ �

Waves and Phasors

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