Practical Electricity · 2013-07-17 · Electric and Magnetic Circuits 19 Understanding Electric...

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AudelTM

Practical Electricity

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AudelTM

Practical Electricity

All New 5th Edition

Paul RosenbergRobert Middleton

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Vice President and Executive Group Publisher: Richard SwadleyVice President and Executive Publisher: Robert IpsenVice President and Publisher: Joseph B. WikertExecutive Editor: Carol A. LongEditorial Manager: Kathryn A. MalmSenior Production Manager: Fred BernardiDevelopment Editors: Emilie Herman and Erica WeinsteinProduction Editor: Vincent KunkemuellerText Design & Composition: TechBooks

Copyright C© 2004 by Wiley Publishing, Inc. All rights reserved.

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Library of Congress Cataloging-in-Publication Data:

Rosenberg, Paul.Practical electricity / Paul Rosenberg, Robert Middleton.—All new 5th ed.

p. cm.At head of title: Audel.Rev. ed. of: Practical electricity / by Robert G. Middleton. c1988.Includes index.

1. Electric engineering. I. Middleton, Robert. Practical electricity.II. Title

TK146.R539 2004621.319′24—dc22

2004005525

Printed in the United States of America

10 9 8 7 6 5 4 3 2 1

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eISBN: 0-7645-7407-8

www.wiley.com

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Contents

Introduction xiii

Chapter 1 Magnetism and Electricity 1Magnetic Poles 2Experiments with Magnets 3Formation of Permanent Magnets 7Aiding and Opposing Magnetic Fields 9Electromagnetism 11Volts, Amperes, and Ohms 17Electric and Magnetic Circuits 19Understanding Electric Circuits 25Kirchhoff’s Voltage Law 30Electrical Power 30Quick-Check Instrumentsfor Troubleshooting 32Summary 33Test Questions 34

Chapter 2 Conductors and Insulators 35Classes of Conductors 35Conducting Wire 36Circular Mil-Foot 37American Wire Gauge 39Stranded Wires 43Aluminum Wire 43Line Drop 44Temperature Coefficient of Resistance 44Earth (Ground) Conduction 45Conduction of Electricity by Air 45Conduction of Electricity by Liquids 50Insulators for Support of Wires 52Classes of Insulation 53Insulation Resistance 53Plastic-Insulated Sheathed Cables 55Summary 55Test Questions 56

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vi Contents

Chapter 3 Electric Circuits 59Picture Diagrams and Schematic Diagrams 59Voltage Polarities in Series Circuits 60Voltage Measurements with Respect toGround 62Resistance of a Battery 63Efficiency and Load Power 65Circuit Voltages in Opposition 66Principles of Parallel Circuits 69Shortcuts for Parallel Circuits 72Conductance Values 73Kirchhoff’s Current Law forParallel Circuits 78Practical Problems in Parallel Circuits 80Line Drop in Parallel Circuits 81Parallel Connection of Cells 84Summary 86Test Questions 87

Chapter 4 Series-Parallel Circuits 89Current Flow in a Series-Parallel Circuit 89Kirchhoff’s Current Law 90Series-Parallel Connection of Cells 92Line Drop in Series-Parallel Circuits 93Use of a Wattmeter 94Circuit Reduction 94Power in a Series-Parallel Circuit 98Horsepower 99Three-Wire Distribution Circuit 100Summary 105Test Questions 106

Chapter 5 Electromagnetic Induction 107Principle of Electromagnetic Induction 107Laws of Electromagnetic Induction 109Self-Induction of a Coil 110Transformers 113



Contents vii

Advantage of an Iron Core 119Choke Coils 119Reversal of Induced Secondary Voltage 122Switching Surges 122The Generator Principle 124Summary 126Test Questions 126

Chapter 6 Principles of Alternating Currents 129Frequency 129Instantaneous and Effective Voltages 129Ohm’s Law in AC Circuits 133Power Laws in Resistive AC Circuits 136Combining AC Voltages 137Transformer Action in AC Circuits 142Core Loss and Core Lamination 148DC versus AC Resistance 150Summary 150Test Questions 151

Chapter 7 Inductive and Capacitive AC Circuits 153Inductive Circuit Action 153Power in an Inductive Circuit 157Resistance in an AC Circuit 159Impedance in AC Circuits 161Power in an Impedance 162Capacitive Reactance 163Capacitive Reactance and Resistancein Series 166Inductance, Capacitance,and Resistance in Series 172Inductance and Resistance in Parallel 175Capacitance and Resistance in Parallel 177Inductance, Capacitance, andResistance in Parallel 179Summary 182Test Questions 183



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Chapter 8 Electric Lighting 185Sources of Light 185Measurement of Amount of Illumination 186Tungsten Filaments 187Starting Surge of Current 188Aging Characteristics 190Lamp Bases and Bulbs 191Vapor-Discharge Lamps 194Metal Halide Lamps 201High-Pressure Sodium Lamps 202Low-Pressure Sodium Lamps 203Sun Lamps 204Ozone-Producing Lamps 205Flash Tubes 205Infrared Lamps 208Electroluminescent Panels 208Summary 209Test Questions 210

Chapter 9 Lighting Calculations 213Point-by-Point Method 213Lumen Method 217Lighting Equipment 224Level of Illumination 226Lamp Wattage 226Wire Capacity 227Load and Length of Run 229Panelboards 229Service and Feeders 229Watts per Square Foot 231Summary 232Test Questions 232

Chapter 10 Basic House Wiring 235Service Connections 236Residential Underground Service 240



Contents ix

Making Connections to theEntrance Panel 241Circuitry 244Individual Appliance Circuits 246Regular Appliance Circuits 246General-Purpose Circuits 246New Work in Old Houses 250Improving Circuits 251Placing Outlets 251Placing Switches 254Roughing-In 254Mounting Boxes 255

Old Work 256Special Mounting Devices 258Running Sheathed Cable 259Metal Plates 261Running Boards 261

Connecting Cable to Boxes 262Fishing Cable in Old Work 263

Two-Person Technique 265Bypassing One Floor of a Two-StoryHome 266

Completing House Wiring: Splicing 268Solderless Connectors 270Installing Outlet Receptacles 271Single-Pole Switches 273Ceiling Box and Fixture Mounting 274Summary 274Test Questions 276

Chapter 11 Wiring with Armored Cableand Conduit 279Cutting Armored Cable 279Installing Armored Cable 280Conduit Fittings 284Wiring in Flexible Conduit 293



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Connectors 296Summary 297Test Questions 299

Chapter 12 Home Circuiting, Multiple Switching,and Wiring Requirements 301Lamp Control from Electrolier Switch 303Control of Lamps from More ThanOne Location with Three-Way andFour-Way Switches 307Stairway Lamp Control Lighting 307Color-Coding of Switch Wiring 307Functional Planning 310Planning of Equipment 311Lighting the Halls 312Lighting the Living Room 314Lighting the Dining Room 316Lighting the Kitchen 318Lighting the Bedroom 320Lighting the Bathroom 323Lighting the Attic 324Lighting the Basement 324Lighting the Garage 326Fluorescent Lights 327Ground Circuit Fault Interrupters 330Arc-Fault Circuit Interrupters 331Summary 331Test Questions 334

Chapter 13 Electric Heating 337General Installation Considerations 337Circuiting 341AC-Operated Relays 344Hot-Water Electric Heat 346Radiant Heater 346Electric Water Heaters 347Ground Circuit 352



Contents xi

Heating Cables 354Summary 356Test Questions 357

Chapter 14 Alarms and Intercoms 359Alarm Devices 359Intercoms 365System Planning 365Planning Details 370Wireless Intercom Systems 371Music Distribution Systems 374Industrial Installations 375School Installations 375Bells and Chimes 376Photoelectric Control of Bell Circuits 378Summary 378Test Questions 379

Chapter 15 Generating Stations and Substations 381Generating Stations 381

Single-Bus System 385Circuit Breakers 385Instrument Transformers 386Double-Bus System 387Paralleling Generators 397Distribution 398

Substations 404Automatic Substations 404Semiautomatic Substations 408

Summary 412Test Questions 413

Appendix 415

Glossary 417

Index 439



xii



Introduction

Electricity is, almost unarguably, the most important basic technol-ogy in the world today. Almost every modern device, from cars tokitchen appliances to computers, is dependent upon it. Life, for mostof us, would be almost unimaginable without electricity.

In fact, electricity cuts such a wide path through modern life thatthe teaching of electricity has developed into several different spe-cialties. Typically, one learns electricity for computers, electricity forelectronics, electricity for power wiring, or some other subcategory.And while this is somewhat understandable, it has often eliminateda real and basic coverage of electricity.

Practical Electricity, more than any other book available, coverselectricity in broad terms. The first half of the book is not writ-ten for one particular specialty; it is written for all specialties. Thefundamental forces of electricity are explained in terms that areunderstandable to almost everyone without eliminating anything ofany importance. A computer technician can begin his or her trainingwith the text, as could an electrician or a radio engineer.

Electrical circuits, test procedures, and electromagnetic inductionare fully covered in Chapters 1 through 7.

In the second half of the book, Chapters 8 through 15 cover themost common applications of electricity, including house wiring,lighting, cables, electric heating, and generating. These apply notonly to the electrical construction trade but also to anyone wholives in a wired dwelling.

One of the primary additions to this edition is a completely re-vised set of artwork. The many drawings contained in this text havebeen updated both for ease of use and for better application to mod-ern technology and practices.

No doubt Practical Electricity will find broad use in technicalschools, as well as for self-instruction. It should apply equally wellto either. Complex mathematics is avoided, and when trigonometrymust be used for certain circuit theory applications, it is carefullyexplained. Only a basic knowledge of arithmetic is required of thereader.

Finally, review questions are included with each chapter. Thisallows students to test themselves and gives instructors an extralearning tool.

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Chapter 1Magnetism and ElectricityEarly experimenters dating back to the dawn of history discoveredthat certain hard black stones attracted small pieces of iron. Later, itwas discovered that a lodestone, or leading stone, pointed north andsouth when freely suspended on a string, as shown in Figure 1-1.Lodestone is a magnetic ore that becomes magnetized if lightninghappens to strike nearby. Today we use magnetized steel needlesinstead of lodestones in magnetic compasses. Figure 1-2 illustratesa typical pocket compass.

THREAD WIRE STIRRUP

LODESTONE

Figure 1-1 Lodestone is a magnetic ore.

N

EN EW NE

SES

WW E

340 0 20

40

6080

100120

140160180200

220

240

260

280

300

320

W E

Figure 1-2 A magneticcompass.

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Magnetic PolesAny magnet has a north and a south pole. We know that the earth is ahuge, although weak, magnet. In Figure 1-1, the end of the lodestonethat points toward the North Star is called its north-seeking pole;the opposite end of the lodestone is called its south-seeking pole.It is a basic law of magnetism that like poles repel each other andunlike poles attract each other. For example, a pair of north polesrepel each other and a pair of south poles repel each other, but anorth pole attracts a south pole.

Magnetic forces are invisible, but it is helpful to represent mag-netic forces as imaginary lines. For example, we represent the earth’smagnetism as shown in Figure 1-3. There are several important factsto be observed in this diagram. Since the north pole of a compassneedle points toward the earth’s geographical North Pole, we recog-nize that the earth’s geographical North Pole has a magnetic southpolarity. In other words, the north pole of a compass needle is at-tracted by magnetic south polarity.

SOUTH MAGNETIC POLE NORTH GEOGRAPHIC POLE

COMPASSNEEDLE

COMPASSNEEDLE

SS

N N

SOUTH GEOGRAPHIC POLE NORTH MAGNETIC POLE S

N

Figure 1-3 Earth’s magnetic poles.

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Magnetism and Electricity 3

Another important fact shown in Figure 1-3 is the location ofthe earth’s magnetic poles with respect to its geographic poles. Theearth’s magnetic poles are located some distance away from its geo-graphic poles. Still another fact to be observed is that magnetic forcelines have a direction, which can be indicated by arrows. Magneticforce lines are always directed out of the north pole of a magnetand directed into the south pole. Moreover, magnetic force lines arecontinuous; the lines always form closed paths. Thus, the earth’smagnetic force lines in Figure 1-3 are continuous through the earthand around the outside of the earth.

The actual source of the earth’s magnetism is still being debatedby physicists. However, insofar as compass action is concerned, wemay imagine that the earth contains a long lodestone along its axis.In turn, this imaginary lodestone will have its south pole near theearth’s north geographic pole; the imaginary lodestone will have itsnorth pole near the earth’s south geographic pole.

Experiments with MagnetsIf we bring the south pole of a magnet near the south pole of asuspended magnet, as shown in Figure 1-4, we know that the poleswill repel each other. It can also be shown that magnetic attractiveor repulsive forces vary inversely as the square of the distance be-tween the poles. For example, if we double the distance between apair of magnetic poles, the force between them will be decreased toone-fourth. It can also be shown that if the strength of the magnetin Figure 1-4 is doubled (as by holding a pair of similar magnetstogether with their south poles in the same direction), the force ofrepulsion is thereby doubled.

S

N

S

N

Figure 1-4 Showing repulsionbetween like magnetic poles.

The strength of a magnetic field is measured in gauss (G). Forexample, the strength of the earth’s magnetic field is approximately

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4 Chapter 1

0.5 G. The gauss unit is a measurement of flux density—that is, it isa measure of the number of magnetic force lines that pass througha unit area. One gauss is defined as one line of force per squarecentimeter. In turn, one gauss is equal to 6.452 lines of force persquare inch. For example, the strength of the earth’s magnetic fieldis approximately 3.2 lines of force per square inch.

Note that there are 2.54 centimeters in 1 inch, or 0.3937 inchesin 1 centimeter. Therefore, there are 6.452 square centimeters in1 square inch, or 0.155 square inch in 1 square centimeter. Since onegauss is defined as one line of force per square centimeter, it followsthat one gauss is also equal to 6.452 lines of force per square inch.

A unit of magnetic pole strength is measured in terms of force.That is, a unit of magnetic force is defined as one that exerts a forceof one dyne on a similar magnetic pole at a distance of 1 centimeter.If we use a pair of like poles, this will be a repulsive force; if weuse a pair of unlike poles, it will be an attractive force. There are444,800 dynes in one pound; in other words, a dyne is equal to1/444,800 of a pound. It is not necessary to remember these basicdefinitions and conversion factors. If you should need them at somefuture time, it is much more practical to look them up than to tryto remember them.

Another important magnet experiment is shown in Figure 1-5. Ifwe break a magnetized needle into two parts, each of the parts willbecome a complete magnet with north and south poles. No matterhow many times we break a magnetized needle, we will not obtain anorth pole by itself or a south pole by itself. This experiment leads usto another basic law of magnetism, which states that magnetic polesmust always occur in opposite pairs. Many attempts have been madeby scientists to find an isolated magnetic pole (called a magneticmonopole). All attempts to date have failed, though scientists arestill trying.

N

N N

NN N N

S

S

S S

S

S S

Figure 1-5 Showing the effects of breaking a magnet into several parts.

It has been found that iron and steel are the only substancesthat can be magnetized to any practical extent. However, certain

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alloys, such as Alnico, can be strongly magnetized. Substances suchas hard steel and Alnico retain their magnetism after they have beenmagnetized and are called permanent magnets. Since a sewing nee-dle is made from steel, it can be magnetized to form a permanentmagnet. On the other hand, soft iron remains magnetized only aslong as it is close to or in contact with a permanent magnet. Thesoft iron loses its magnetism as soon as it is removed from the vicin-ity of a permanent magnet. Therefore, soft iron is said to form atemporary magnet.

Permanent magnets for experimental work are commonly man-ufactured from hard steel or magnetic alloys in the form of horse-shoe magnets and bar magnets, as shown in Figure 1-6. The spacearound the poles of a magnet is described as a magnetic field andis represented by magnetic lines of force. The space around a lode-stone (Figure 1-1), around a compass needle (Figure 1-2), aroundthe earth (Figure 1-3), and around a permanent magnet (Figure 1-4)are examples of magnetic fields. Since a magnetic field is invisible,we can demonstrate its presence only by its force of attraction foriron.

S

S

N

N

Figure 1-6 A barmagnet and a horseshoemagnet.

Consider the patterns formed by magnetic lines of force in variousmagnetic fields. One example has been shown in Figure 1-3. It canalso be easily shown experimentally that when a bar magnet is heldunder a piece of cardboard and then iron filings are sprinkled on thecardboard, the filings will arrange themselves in curved-line patternsas shown in Figure 1-7. The pattern of iron filings formed providesa practical basis for our assumption of imaginary lines of force todescribe a magnetic field. The total number of magnetic force linessurrounding a magnet, as shown in Figure 1-8, is called its magneticflux.

A similar experiment with a horseshoe magnet is shown in Fig-ure 1-9. The iron filings arrange themselves in curved lines that sug-gest the imaginary lines of force that we use to describe a magneticfield. Note that the magnetic field is strongest at the poles of themagnet in Figure 1-7. Since the field strength falls off as the square

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6 Chapter 1

SN

Figure 1-7 Pattern of iron filings around a bar magnet.

N S

Figure 1-8 Field around a bar magnet represented by lines of force.

Figure 1-9 Pattern of iron filings in the space above a horseshoemagnet.

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of the distance from a pole, a magnet exerts practically no force on apiece of iron at an appreciable distance. A magnet exerts its greatestforce on a piece of iron when in direct contact.

Formation of Permanent MagnetsAnother important and practical experiment is the magnetizationof steel to form a permanent magnet. For example, if we wish tomagnetize a steel needle, we may use any of the following methods:

� The needle can be stroked with one pole of a permanent mag-net. The needle can be stroked several times to increase itsmagnetic strength, but each stroke must be made in the samedirection.

� If the needle is held in a magnetic field (such as between thepoles of a horseshoe magnet) and the needle is tapped sharply,it will become magnetized.

� We can heat a needle to dull red heat and then quickly coolthe needle with cold water while holding it in a magnetic field,and the needle will become magnetized.

The formation of permanent magnets is explained in terms ofmolecular magnets. Each molecule in a steel bar is regarded as atiny permanent magnet. As shown in Figure 1-10, the poles of thesemolecular magnets are distributed at random in an unmagnetized

(A) Unmagnetized.

(B) Partially magnetized.

(C) Magnetized.

Figure 1-10 Representation of molecular magnets in a steel bar.

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8 Chapter 1

steel bar. Therefore, the fields of the molecular magnets cancel outon the average, and the steel bar does not act as a magnet. On theother hand, when we stroke an unmagnetized steel bar with the poleof a permanent magnet, some of the molecular magnets respond bylining up end-to-end. In turn, the lined-up molecular magnets havea combined field that makes the steel bar a magnet. If the steel baris stroked a number of times, more of the molecular magnets arelined up end-to-end, and a stronger permanent magnet is formed,as shown in Figure 1-11A.

(A) Relative polarity produced in a steel bar.

(B) One magnet floating in the field of another magnet.

GLASS

WOOD

N S

N

S

N S

N S

Figure 1-11 Magnet characteristics.

Steel is much harder than iron; therefore, it is more difficult toline up the molecular magnets in a steel bar than in a soft-iron bar.To make a strong permanent magnet from a steel bar, we must strokethe bar many times with a strong permanent magnet. A soft-iron bar

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becomes fully magnetized as soon as it is touched by a permanentmagnet but will return to its unmagnetized state as soon as it isremoved from the field of a permanent magnet. Once the molecularmagnets have been lined up in a hard steel bar, however, they willretain their positions and provide a permanent magnet.

Although there are very large numbers of molecules in an ironor steel bar, the number of molecules to be lined up is not infinite.Therefore, there is a limit to which the bar can be magnetized, nomatter how strong a field we use. When all the molecular magnetsare aligned in the same direction, the bar cannot be magnetizedfurther, and the iron or steel is said to be magnetically saturated.The ability of a magnetic substance to retain its magnetism after themagnetizing force has been removed is called its retentivity. Thus,retentivity is very large in hard steel and almost absent in soft iron.Magnetic alloys such as Alnico V have a very high retentivity andare widely used in modern electrical and electronic equipment. TheAlnico alloys contain iron, aluminum, nickel, copper, and cobalt invarious proportions depending on the requirements.

A permanent magnet that weighs 11/2 lbs may have a strength of900 G and will lift approximately 50 lbs of iron. This type of magnetis constructed in a horseshoe form and is less than 3 in. long. A 5-lbmagnet may have strength of 2000 G and will lift approximately100 lbs of iron. A 16-lb magnet 51/2 in. long may have a strength of4800 G and will lift about 250 lbs of iron.

Bar magnets can be magnetized with sufficient strength so thatone of the magnets will float in the field of the other magnet, asshown in Figure 1-11B. A similar demonstration of magnetic forcesis provided by circular ceramic magnets. Each circular magnet isabout 21/2 in. in diameter and has a hole in the center that is 1 in. indiameter. One surface of the disc is a north pole, and the oppositesurface is a south pole. When placed on a nonmagnetic restrainingpole, with like poles adjacent, the circular magnets float in the air,being held in suspension by repelling magnetic forces.

Aiding and Opposing Magnetic FieldsAn experiment that demonstrates the repulsion of like magneticpoles was illustrated in Figure 1-4. The question is often asked howmagnetic force lines act in aiding or opposing magnetic fields. Fig-ure 1-12 shows the answer to this question. Note that when unlikepoles are brought near each other, the lines of force in the air gap arein the same direction. Therefore, these are aiding fields, and the linesconcentrate between the unlike poles. It is a basic law of magnetismthat lines of force tend to shorten as much as possible; lines of force

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10 Chapter 1

AIR GAP

(A) Unlike poles attract.

LINES OF FORCE

(B) Like poles repel.

NS N S

N S N S

Figure 1-12 Lines of force between unlike and like poles.

have been compared to rubber bands in this respect. In turn, a forceof attraction is exerted between the unlike poles in Figure 1-12A.

On the other hand, a pair of like poles have been brought neareach other in Figure 1-12B. The lines of magnetic force are directedin opposition, and the lines from one pole oppose the lines fromthe other pole. In turn, none of the lines from one magnet entersthe other magnet, and a force of repulsion is exerted between themagnets. We observe that the magnetic fields in Figure 1-12 arechanged in shape, or are distorted with respect to the field shown inFigure 1-8. If the magnets in Figure 1-12 are brought more closelytogether, the fields become more distorted. Hence, we recognize thatforces of attraction or repulsion between magnets are produced bydistortion of their magnetic fields.

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ElectromagnetismElectromagnetism is the production of magnetism by an electric cur-rent. An electric current is a flow of electrons; we can compare theflow of electrons in a wire to the flow of water in a pipe. Todaywe can read about electronics and electrons in newspapers, mag-azines, and many schoolbooks. However, the practical electricianneeds to know more about electrons than a mechanic or machinistdoes. Therefore, let us see how electric current flows in a wire.

An atom is the smallest particle of any substance; thus, the small-est particle of copper is a copper atom. We often hear about splittingthe atom. If a copper atom is split or broken down into smaller parti-cles, we can almost say that it is built from extremely small particlesof electricity. In other words, all substances, such as copper, iron, andwood, have the same building blocks, and these building blocks areparticles of electricity. (This is technically not an entirely true state-ment, but it is close enough for our use here.) Copper and wood aredifferent substances simply because these particles of electricity arearranged differently in their atoms. An atom can be compared to oursolar system in which the planets revolve in orbits around the sun.For example, a copper atom has a nucleus, which consists of posi-tive particles of electricity; electrons (negative particles of electricity)revolve in orbits around the nucleus.

Figure 1-13 shows three atoms in a metal wire. An electron in oneatom can be transferred to the next atom under suitable conditions,and this movement of electrons from one end of the wire to the otherend is called an electric current. Electric current is electron flow. Tomake electrons flow in a wire, an electrical pressure must be appliedto the ends of the wire. This electrical pressure is a force calledelectromotive force, or voltage. For example, an ordinary dry cell is

ELECTRONORBITS

Figure 1-13 Atoms in a metal wire.

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12 Chapter 1

a source of electromotive force. A dry cell produces electromotiveforce by chemical action. If we connect a voltmeter across a drycell, as shown in Figure 1-14, electrons flow through the voltmeter,which indicates the voltage of the cell.

ELECTRONFLOW

VENT

CARBONROD

ZINCCYLINDER

LINED WITHPOROUS

CARDBOARD

GRAPHITE

MANGANESEDIOXIDE

AMMONIUMCHLORIDE

WAX ORASPHALT

Figure 1-14 A dry cell produces electromotive force by chemicalaction.

We measure electromotive force (emf) in volts (V). If a dry cellis in good condition, it will have an emf of about 1.5 V. Note thata dry cell acts as a charge separator. In other words, the chemicalaction in the cell takes electrons away from the carbon rod and addselectrons to the zinc cylinder. Therefore, there is an electron pressureor emf at the zinc cylinder. When a voltmeter is connected across adry cell (see Figure 1-14), this electron pressure forces electrons toflow in the connecting wire, as shown in Figure 1-13. We observethat electrons flow from the negative terminal of the dry cell, aroundthe wire circuit, and back to the positive terminal of the dry cell.

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ELECTRONFLOW

WIRE

LOOKING DOWN UPON NEEDLEWIRE ABOVE NEEDLE

S

NN

WIRE

S ELECTRONFLOW

Figure 1-15 A compass needle is deflected in the vicinity of a current-carrying wire.

Next, we will find that an electric current produces a magneticfield. For example, if a compass needle is brought near a current-carrying wire, the compass needle turns, as shown in Figure 1-15.Since a compass needle is acted upon by a magnetic field, this ex-periment shows that the electric current is producing a magneticfield. This is the principle of electromagnetism. The magnetic linesof force surrounding a current-carrying wire can be demonstratedas shown in Figure 1-16. When iron filings are sprinkled over

DRY CELL

ELECTRON FLOW

HORIZONTAL PIECEOF CARDBOARD

MAGNETIC LINESOF FORCE

HOLE INCARDBOARD

N

N

S

S

+

Figure 1-16 Demonstration of electromagnetism.

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14 Chapter 1

the cardboard, the filings arrange themselves in circles around thewire.

Although a current-carrying wire acts as a magnet, it is a form oftemporary magnet. The magnetic field is not present until the wireis connected to the dry cell. Magnetic force lines are produced onlywhile there is current in the wire. As soon as the circuit is opened(disconnected from the dry cell), the magnetic force lines disappear.

Let us observe the polarities of the compass needles in Fig-ure 1-16. The magnetic lines of force are directed clockwise, lookingdown upon the cardboard. This experiment leads us to a basic ruleof electricity called the left-hand rule. Figure 1-17 illustrates the left-hand rule; if a conductor is grasped with the left hand, with yourthumb pointing in the direction of electron flow, then your fingerswill point in the direction of the magnetic lines of force.

DIRECTION OFCURRENT

DIRECTION OFLINES OF FORCE

+

Figure 1-17 Left-hand rule used to determine direction of magneticforce lines around a current-carrying conductor.

Experiments show that the magnetic field around the wire inFigure 1-16 is weak, and we will now ask how the strength of anelectromagnetic field can be increased. The magnetic field arounda straight wire is comparatively weak because it is produced over alarge volume of space. To reduce the space occupied by the magneticfield, a straight wire can be bent in the form of a loop, as shownin Figure 1-18. Now the magnetic flux lines are concentrated in thearea enclosed by the loop. Therefore, the magnetic field strength iscomparatively great inside the loop. This is an elementary form ofelectromagnet.

Next, to make an electromagnet with a much stronger magneticfield, we can wind a straight wire in the form of a helix with anumber of turns, as shown in Figure 1-19. Since the field of oneloop adds to the field of the next loop, the total field strength of

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ELECTRONFLOW

DRY CELL

+

Figure 1-18 The magnetic field is concentrated by forming a conduc-tor into a loop.

+

S N

–

Figure 1-19 Magnetic field around an air-core solenoid.

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16 Chapter 1

the electromagnet is much greater than if a single turn were used.Note that if we use the same wire shown in Figure 1-16 to formthe electromagnet in Figure 1-19, the current is the same in bothcircuits. That is, we have not changed the amount of current; wehave merely concentrated the magnetic flux by winding the wireinto a spiral. Electricians often call an electromagnet of this type asolenoid.

The name solenoid is applied to electromagnets that have an aircore. For example, we might wind the coil in Figure 1-19 on awooden spool. Since wood is not a magnetic substance, the elec-tromagnet is essentially an air-core magnet. Note the polarity of themagnetic field in Figure 1-19 with respect to the direction of currentflow. The left-hand rule applies to electromagnets, just as to straightwire. Thus, if we grasp an electromagnet as shown in Figure 1-20,with the fingers of the left hand in the direction of electron flow,then the thumb will point to the north pole of the electromagnet.

ELECTROMAGNETN S

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Figure 1-20 Method of finding the magnetic polarity of a coil by meansof the left-hand rule.

Since iron is a magnetic substance, the strength of an electro-magnet can be greatly increased by placing an iron core inside asolenoid. For example, if we place a soft-iron bar inside the woodenspool in Figure 1-19, we will find that the magnetic field strengthbecomes much greater. Let us see why this is so (with reference toFigure 1-10). The molecular magnets in the soft-iron bar, or core,are originally oriented in random directions. However, under theinfluence of the flux lines inside the electromagnet, these molecularmagnets line up in the same direction. Therefore, the magnetic field

Practical Electricity · 2013-07-17 · Electric and Magnetic Circuits 19 Understanding Electric...

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Transcript of Practical Electricity · 2013-07-17 · Electric and Magnetic Circuits 19 Understanding Electric...