Section 20.1 20.1 Electric Charge and Static...

20.1 Electric Charge andStatic Electricity

Reading StrategyIdentifying Main Ideas Copy the tablebelow. As you read, write the main idea foreach topic.

Key ConceptsWhat produces a netelectric charge?

What determines whetheran electric force isattractive or repulsive?

What determinesthe strength of anelectric field?

What are three waysin which chargeis transferred?

How does a staticdischarge occur?

Vocabulary◆ electric charge◆ electric force◆ electric field◆ static electricity◆ law of conservation

of charge◆ induction

Think back to the last time a thunderstorm swept through your area.A bolt of lightning streaked across the sky, followed moments later by thecrash of thunder. Have you ever wondered what causes lightning?Perhaps you’ve observed something similar on a smaller scale closer tohome. When you take clothes out of a dryer, some of them can sticktogether, like the socks and towel in Figure 1. If you pull the clothes apartin a darkened room, you can see sparks that are like tiny bolts of light-ning. This shouldn’t be surprising once you realize that lightning and“static cling” have a similar cause—the movement of electric charges.

Electric Charge Recall that electrical energy is the energy associated with elec-tric charges. But what exactly is electric charge? Electric chargeis a property that causes subatomic particles such as protonsand electrons to attract or repel each other. There are two typesof electric charge, positive and negative. Protons have a posi-tive charge and electrons have a negative charge. Electriccharges move in a flash through a lightning bolt. Electriccharges attract one another in clothes taken from the dryer.Although charged particles are too small to see, just abouteverything in your daily life is affected by charge in one wayor another.

Electric Charge

Electric Forces

Electric Fields

Static Electricity

Topic Main Idea

a.

b.

c.

An excess or shortageof electrons producesa net electric charge.

Figure 1 Electric charge isresponsible for clothes that sticktogether when they are removedfrom a dryer.

600 Chapter 20

600 Chapter 20

FOCUS

Objectives20.1.1 Analyze factors that affect

the strength and direction of electric forces and fields.

20.1.2 Describe how electric forcesand fields affect electric charges.

20.1.3 Describe how electric chargesare transferred and explainwhy electric discharges occur.

Build VocabularyWord-Part Analysis Ask students,What word appears in most of thedefinitions? (Electric, electrical, electricity)Explain that the root word for electricitycomes from the Greek word for “amber,”a substance that is easily charged. Ask,What can be inferred from this?(Electricity deals with charges.) Explain thatthe word static comes most recently fromthe Greek word statikos, which means“causing to stand.” Point out to studentsthat in static electricity, or static dis-charge, charges jump from one locationto another. Thus, charges do move, sothe term static is unfortunate.

Reading Strategya. The attraction or repulsion betweenelectrically charged objectsb. Field strength depends on the netcharge and distance from the charge.c. Charge can be transferred by friction,contact, or induction.

INSTRUCT

Electric ChargeBuild Reading LiteracyOutline Refer to page 156D inChapter 6, which provides theguidelines for an outline.

Have students create an outline of thesection (pp. 600–603). Outlines shouldfollow the head structure used in thesection. Major headings are shown ingreen, and subheadings are shown inblue. Ask, Based on your outline, whatare the three ways static electriccharges are transferred? (Friction,contact, and induction) Verbal, Logical

L1

2

L2

L2

Reading Focus

1

Section 20.1

Print• Laboratory Manual, Investigation 20B• Reading and Study Workbook With

Math Support, Section 20.1• Transparencies, Chapter Pretest and

Section 20.1

Technology• Interactive Textbook, Section 20.1• Presentation Pro CD-ROM, Chapter Pretest

and Section 20.1

Section Resources

Electricity 601

Figure 2 shows how charges are arranged in an atom. A cloudof negatively charged electrons surrounds the positivelycharged nucleus. The atom is neutral because it has anequal number of positive and negative charges. If an atomgains one or more electrons, it becomes a negativelycharged ion. If an atom loses electrons, it becomes apositively charged ion. An excess or shortage ofelectrons produces a net electric charge.

The SI unit of electric charge is the coulomb (C). Ittakes about 6.24 � 1018 electrons to produce a singlecoulomb. A lightning bolt is about 10 to 20 coulombs ofcharge. In comparison, a flash camera uses the energyfrom 0.025 coulombs of charge to produce each flash.

Electric ForcesRub an inflated rubber balloon on your clean, dry hair. If it’s a dry day,you can use the balloon to pick up bits of paper. The balloon attracts thepaper because the balloon is negatively charged and the paper is posi-tively charged. Now rub a second balloon on your hair and bring thetwo balloons close together. You can feel the balloons repel. The twoballoons repel because they are negatively charged. Like chargesrepel, and opposite charges attract. The force of attraction or repulsionbetween electrically charged objects is electric force.

The French scientist Charles-Augustin deCoulomb (1736–1806) discovered that electricforces obey a law similar to the law of universalgravitation. The electric force between twoobjects is directly proportional to the netcharge on each object and inversely propor-tional to the square of the distance betweenthem. As you can see in Figure 3, doubling thenet charge on one object doubles the electricforce. If instead you double the distancebetween the objects, the electric force is onefourth as strong.

Inside an atom, electric forces are muchstronger than gravitational forces. Electricforces form chemical bonds, which must beovercome in chemical changes. Electric forcesalso cause friction and other contact forces. Buton a large scale, matter is mostly neutral and inthat case, electric forces are close to zero.

What is electric force?

Figure 2 A neutral atom hasequal numbers of protonsand electrons. Drawing Conclusions What is the overall charge if the atomloses an electron?

e- e-

e-e-

e-

e-

e- e-

8 protons

Proton

Neutron

Electric force exerted on object Y by object X

Electric force exerted on object X by object Y

X Y

XY

X

A

B

C

+ _

+2 _

+ +

Y

Figure 3 Electric force depends on charge and distance.A Opposite charges attract each other. B Doubling onecharge doubles the force on both charges. C Like chargesrepel. Doubling the distance makes the force one fourthas great.

Build Science SkillsUsing Models Have students applywhat they know about force and massto explain why negative charges aremore mobile than positive charges.(Positive charges are provided by protons,which are much more massive thanelectrons. Electrons, which carry negativecharge, are less massive and thereforemore easily moved.) Logical

Electric Forces

Electric Attraction and RepulsionPurpose To show that attractive forcesexist between unlike charges, andrepelling forces exist between likecharges.

Materials pith ball, thread, rubber orebonite rod, fur

Procedure Attach the thread to thepith ball. Rub the fur against the rubberrod, and then touch this rod briefly tothe pith ball. Bring the rod close to thepith ball. Next, move the fur near thepith ball.

Expected Outcome Friction causes thefur to become positively charged and therod to become negatively charged. Therod then transfers negative charge to thepith ball. When the rod is brought closeto the pith ball, the like charges exert arepelling force on each other, and theball moves away from the rod. When thefur, which has an opposite charge to therod, is brought close to the ball, the ballis attracted to the fur. Visual

L2

L2

Electricity 601

Customize for English Language Learners

Reinforce VocabularyReinforce the vocabulary for this section byhaving English language learners construct aconcept map. This will not only increase their

familiarity with the terms, but will also helpthem understand how the various words,especially those that include the word electricor electrical, are related to each other.

Answer to . . .

Figure 2 Net charge would be �1.

An electric force is a forceof repulsion or attraction

between electrically charged objects.

0598_hsps09te_Ch20.qxp 8/1/07 3:39 PM Page 601

602 Chapter 20

Figure 4 The strength of an electricfield depends on the amount of chargethat produces the field and on thedistance from the charge. A The electricfield around a positive charge pointsoutward. B The electric field around anegative charge points inward.

Electric FieldsThe effect an electric charge has on other charges in the space aroundit is the charge’s electric field. Figure 4 shows the fields of positive andnegative charges. The strength of an electric field depends on theamount of charge that produces the field and on the distance fromthe charge. The lines representing the field are closer together near thecharge, where the field is stronger.

An electric field exerts forces on any charged object placed in thefield. The force depends on the net charge in the object and on thestrength and direction of the field at the object’s position. The more netcharge an object has, the greater is the force on it. The direction of eachfield line shows the direction of the force on a positive charge.

Static Electricity and ChargingStatic electricity is the study of the behavior of electric charges, includ-ing how charge is transferred between objects. There are several waysthat a net charge can build up on an object or move from one objectto another. Charge can be transferred by friction, by contact, andby induction. Keep in mind that whenever there is a charge transfer,the total charge is the same before and after the transfer occurs. Thisis the law of conservation of charge—the total charge in an isolatedsystem is constant.

Charging by Friction The balloon in Figure 5A attracts hairbecause opposite charges attract. But how do balloons and hair pick upa net charge? Rubbing a balloon on your hair is an example of charg-ing by friction. Electrons move from your hair to the balloon becauseatoms in rubber have a greater attraction for electrons than atoms inhair. The balloon picks up a net negative charge. Because your hairloses electrons, it becomes positively charged. Even simple everydayactivities like walking across a carpet can build up charge this way.

Charging by Contact Why do the girl’s hairs repel each otherin Figure 5B? In this case, charge is transferred by contact. A Van deGraaff generator has charged the metal sphere. When the girl touchesthe sphere, she acquires a charge large enough to make her hairs standon end. The sphere is still charged, but its net charge is reduced.

A

B

Figure 5 Charge can betransferred by friction and bycontact. A Friction transferredelectrons from the hair to theballoon. The balloon thenattracts the hair because oppositecharges attract. B A Van deGraaff generator has charged themetal sphere. Touching thesphere transfers charge. The hairsrepel each other because likecharges repel.

Field of a positive charge

+

A Field of a negative charge

–

B

602 Chapter 20

Electric FieldsUse VisualsFigure 4 Stress that the directions of theelectric fields for positive and negativecharges are a matter of convention,much in the same way that the protonhas charge that is called “positive.”Emphasize that the field lines show thedirection of the force on a positive “test”charge placed in the field. Ask, In whatdirection is the force on a proton thatis placed in each of these fields? (Theforce is outward [repulsive] in A and inward[attractive] in B.)Visual

Static Electricity and Charging

Students may think that most or all ofthe atoms in an object charged byfriction contribute to the object’s netcharge. Emphasize that only a smallfraction of the atoms or molecules in a substance give up electrons. Forinstance, an acrylic rod that has beenrubbed with fur may only have a netcharge of 10�9 C. Explain that, whilesuch a charge is produced by 10 billionelectrons, there are about 1022 to 1024 atoms from which charges couldbe taken. In other words, only about onein a trillion atoms or molecules donatecharges during charging by friction.Logical

Use VisualsFigure 5 Stress that friction usuallyseparates charges on a pair of electricallyneutral objects, whereas charging bycontact moves some of the net chargeon a charged object to another object.Ask, What would happen if electronswere conveyed by contact to anobject with a positive charge? (Theelectrons would reduce the overall positivecharge of the object.) Ask, What wouldhappen if the positive and negativecharges were equal? (The object wouldbecome electrically neutral.)Visual

L1

L2

L1

Section 20.1 (continued)

Electrical Force and Gravitational ForceElectrical forces are much stronger than thegravitational forces that operate among particlesin an atom. For instance, an electron and aproton in a hydrogen atom are 5.3 � 10�11 mapart, on average. The mass of the electron is9.11 � 10�31 kg and the mass of the proton is

1.67 � 10�27 kg, while both have a chargewith magnitude 1.60 � 10�19 C. Thegravitational force of attraction between the two particles is 3.6 � 10�47 N, while theelectrical force of attraction is 8.2 � 10�8 N, or about 2 � 1039 times greater than thegravitational attraction between them.

Facts and Figures

Section 20.1 Assessment

Reviewing Concepts1. How is a net electric charge produced?

2. What determines whether charges attractor repel?

3. Name two factors that affect the strengthof an electric field.

4. List three methods of charge transfer.

5. Explain how static discharge occurs.

6. How does electric force depend on theamount of charge and the distancebetween charges?

7. What is the law of conservation of charge?

Critical Thinking8. Forming Hypotheses Why does plastic

food wrap cling better to some materialsthan to others?

9. Inferring When a glass rod is rubbed withneutral silk, the glass becomes positively charged.What charge does the silk now have? Explain.

10. Relating Cause and Effect Many lightningstrikes occur within a cloud, rather thanbetween clouds and the ground. Explain why.(Hint: Assume a cloud has no net charge.)

Electricity 603

Explanatory Paragraph Write a paragraphexplaining the series of events that may causeyou to receive a shock from a metal door-knob on a dry winter day. (Hint: Use a flow-chart to organize your ideas before writingyour paragraph.)

Figure 6 Induction occurs whencharge is transferred withoutcontact between materials.Negative charges in the handinduce charges to move withinthe metal doorknob. Predicting What wouldhappen if the hand had a netpositive charge?

Charging by Induction Suppose you reach for a doorknobafter walking across a carpet. You have picked up extra electrons fromthe carpet, so your hand is negatively charged. The net negative chargein your hand repels electrons in the metal doorknob. Figure 6 showsthat electrons move to the base of the doorknob, leaving a net posi-tive charge in the part of the doorknob closest to the hand. Overall,the doorknob is still neutral, but charge has moved within it. This isinduction, a transfer of charge without contact between materials.

Static DischargeWhy do you get a shock from a doorknob? The spark you feel is a staticdischarge. Static discharge occurs when a pathway throughwhich charges can move forms suddenly. Charges will not travelthrough air from your hand to the doorknob. But air becomes chargedsuddenly when the gap between your finger and the doorknob is small.This air provides a path for electrons to flow from your hand to thedoorknob. If the room is dark, you can even see this spark.

Lightning is a more dramatic discharge. Charge can build up in astorm cloud from friction between moving air masses. Negative chargein the lower part of the cloud induces a positive charge in the groundbelow the cloud. As the amount of charge in the cloud increases, theforce of attraction between charges in the cloud and charges in theground increases. Eventually the air becomes charged, forming a path-way for electrons to travel from the cloud to the ground.

For: Activity on thunder and lightning

Visit: PHSchool.com

Web Code: ccc-2201

Static DischargeBuild Science SkillsRelating Cause and Effect Havestudents write a short paragraphdescribing lightning. Be sure studentssuggest a reason why excess electrons in a cloud induce a positive charge in the ground. Have students discuss theelectric forces in static discharge. (Frictionbetween water droplets separates charges.Tell students that convection currents withina cloud move positive charges to the top ofthe cloud and negative charges to thebottom. Through induction, the electricforce from the negatively charged region ofthe cloud repels nearby negative charges onEarth’s surface. This leaves a positive chargeon the ground. Lightning is the dischargebetween the oppositely charged regions—either cloud-to-ground or cloud-to-cloud.)Logical

ASSESSEvaluate UnderstandingAsk students to write a paragraphdescribing interactions between charges.The paragraph should indicate the twokinds of charges, the forces between the charges, and the ways (friction,contact, and induction) that charges are transferred to and from surfaces.

ReteachHave students use Figure 3 to summarizethe way electric forces act betweencharges.

Paragraphs should start by describing abuildup of charge on the person, forexample by friction with the carpet.When the hand nears the doorknob,charge moves within the metal doorknobby induction. When the hand gets veryclose, the air suddenly becomes charged.Electrons then flow rapidly either to orfrom the doorknob, depending onwhether the hand has a net negative or positive charge.

If your class subscribes tothe Interactive Textbook, use it toreview key concepts in Section 20.1.

L1

L2

3

L2

Electricity 603

7. The total charge in a system is constant, so that the total charge before charges aretransferred is the same as after.8. The plastic attracts electrons more stronglythan some materials and less strongly than others.9. The charge on the silk must be negativebecause charge is conserved.10. If the cloud is neutral, some areas will have a net positive charge, and others a netnegative charge. Within the clouds, an areawith negative charge buildup discharges to an area with positive charge buildup.


1. An excess or shortage of electrons producesa net electric charge.2. If charges are alike, they will repel. Oppositecharges will attract.3. Net charge and distance from the charge4. Friction, contact, induction5. A pathway through which charges canmove forms suddenly.6. Electric force increases proportionally withcharge, while the force is inversely proportionalto the square of the distance.

Answer to . . .

Figure 6 Electrons in the doorknobwould move close to the hand. The baseof the doorknob would be positivelycharged.

PPLS

0598_hsps09te_Ch20.qxp 4/19/07 9:50 AM Page 603

20.2 Electric Current andOhm’s Law

Reading StrategyPredicting Copy the table below and write aprediction of what electric current is. After youread the section, if your prediction wasincorrect or incomplete, write what electriccurrent actually is.

Key ConceptsWhat are the two typesof current?

What are some examplesof conductors andinsulators?

What factors affectelectrical resistance?

What causes an electriccurrent?

How are voltage, current,and resistance related?

Vocabulary◆ electric current◆ direct current◆ alternating current◆ electrical conductor ◆ electrical insulator ◆ resistance◆ superconductor◆ potential difference◆ voltage◆ battery◆ Ohm’s law

a. ? b. ?

Electric CurrentProbably Means

Electric CurrentActually Means

If you’ve ever tried to fix a flashlight, you know there are severalparts to check. The batteries may be dead, or the bulb may haveburned out. The switch could be broken, or the spring might be cor-roded. If even one part isn’t functioning, the flashlight won’t light.

Electric CurrentAs you can see in Figure 7, the parts of a flashlight form a continuouspath through which charge can flow. This continuous flow of electriccharge is an electric current. The SI unit of electric current is theampere (A), or amp, which equals 1 coulomb per second.

The two types of current are direct current and alternatingcurrent. Charge flows only in one direction in direct current (DC). Aflashlight and most other battery-operated devices use direct current.Electric current in your home and school is mostly alternating current.

Alternating current (AC) is a flow of electriccharge that regularly reverses its direction.

In a flashlight, electrons flow from the neg-ative terminal of one battery to the positiveterminal of the other battery. But notice thatthe current is in the opposite direction. This isbecause scientists define current as the direc-tion in which positive charges would flow.

Negative terminals

Positive terminalsSpring

+ +– –

Flow of current Switch

Figure 7 A complete path isrequired for charge to flow in aflashlight. Batteries must beplaced so that charge can flowfrom negative to positive, passingthrough the bulb. Interpreting Diagrams Whatpurpose does the spring at thebase of a flashlight have?

604 Chapter 20

604 Chapter 20

FOCUS

Objectives20.2.1 Describe electric current and

identify the two types ofcurrent.

20.2.2 Describe conduction andclassify materials as goodelectrical conductors or goodelectrical insulators.

20.2.3 Describe the factors that affectresistance.

20.2.4 Explain how voltage produceselectric current.

20.2.5 Calculate voltage, current, andresistance using Ohm’s law.

Build VocabularyLINCS Have students: List the parts ofthe vocabulary that they know, such aspotential and difference. Imagine what apotential difference might look like andhow the terms might fit together. Notea reminding sound-alike word, such aspotential energy. Connect the terms,perhaps in a long sentence or a shortstory. Self-test (quiz themselves).

Reading Strategya. Electric current is the flow of electric charge.b. Electric current is the continuous flow of electric charge.

INSTRUCT

Electric CurrentUse VisualsFigure 7 Emphasize that electriccurrent needs a continuous paththrough the flashlight. Ask, How is thedirection of current related to thedirection of the flow of electrons?(Electrons flow from the negative terminalof one battery to the positive terminal ofthe other; the direction of current is in theopposite direction.) Then ask, What isthe function of the switch? (Flippingthe switch to the “on” position completesthe path for the flow of charge)Visual, Logical

L1

2

L2

L2

Reading Focus

1

Section 20.2

Print• Laboratory Manual, Investigation 20A• Reading and Study Workbook With

Math Support, Section 20.2• Math Skills and Problem Solving

Workbook, Section 20.2• Transparencies, Section 20.2

Technology• Interactive Textbook, Section 20.2• Presentation Pro CD-ROM, Section 20.2• Go Online, NSTA SciLinks, Conductors

and insulators

Section Resources

Electricity 605

Conductors and InsulatorsWhy is a metal wire usually coated with plastic or rubber? The metalwire is an electrical conductor. The rubber and plastic are electricalinsulators. An electrical conductor is a material through which chargecan flow easily. A material through which charge cannot flow easily iscalled an electrical insulator. The coating around a wire helps to con-trol the current and keep it where it is needed.

A metal is made up of ions in a lattice. The ions are not free tomove. But each ion has one or more electrons that are not tightlybound to it. These free electrons can conduct charge. Most materialsdo not easily conduct charge because they don’t have free electrons.

Metals such as copper and silver are good electrical conductors.Wood, plastic, rubber, and air are good electrical insulators.

ResistanceAs electrons move through a conducting wire, they collide with elec-trons and ions. These collisions convert some kinetic energy intothermal energy. Because less energy is available to move electronsthrough the wire, the current is reduced. Resistance is opposition tothe flow of charges in a material. The SI unit of resistance is the ohm.

When you drink a milkshake as shown in Figure 8, it is easier ifyou use a thicker straw. In the same way, resistance is lowered if youmake a wire thicker because more electrons can flow through a thickerwire. A material’s thickness, length, and temperature affect itsresistance. Resistance is greater in a longer wire because the chargestravel farther. As temperature increases, a metal’s resistance increasesbecause electrons collide more often.

If resistance increases as temperature increases, what happens asyou cool a conductor? Could you reduce the resistance to zero? This isthe idea behind superconductors. A superconductor is a material thathas almost zero resistance when it is cooled to low temperatures. Thebest superconductor found thus far must be cooled to about 138 K.

t metal ions

electron

thin wire thick wire

Figure 8 Using a thick straw todrink a milkshake is easier thanusing a thin straw. Similarly,electrons flow more easilythrough a thick wire than theyflow through a thin wire,assuming the wires are made ofthe same material. Applying Concepts Why shouldthe wire in light bulb filamentsbe very thin?

For: Links on conductors andinsulators

Visit: www.SciLinks.org

Web Code: ccn-2202

Conductors andInsulators

Students may think that only metalsmake good conductors. Point out thatmany solutions containing ions are good conductors. The acid inside abattery is one example. Charges alsoflow freely in ionized gases, such as thegases inside a fluorescent light or a neon light. In contrast to metals, where electrons are the only movingcharges, both positive and negativecharges move freely in ionized gases and in solutions containing ions. Logical

ResistanceIntegrate Earth ScienceAlthough early research in superconduc-tivity concentrated on metals, the bestsuperconducting materials known to datebelong to the broad class of mineralscalled perovskites. Perovskites are com-posed of barium, lanthanum, copper, and oxygen. Certain ceramics made fromclays containing perovskites were found in 1986 to become superconducting atthe relatively high temperature of 35 K.This was 12 K higher than the highesttemperature at which any metal alloy was superconducting. By altering some of the components in perovskites, highersuperconducting temperatures have beenreached. At present, the highest temper-ature for a superconducting material is138 K. Have students research the historyof superconducting ceramics and reporton their findings. Verbal

L2

L2

Electricity 605

Customize for Inclusion Students

Visually ImpairedAnalogies, such as drinking a milkshake througha straw, are useful tools for improving thecomprehension of visually impaired students.Develop similar analogies for other electricalprocesses. For instance, have students considerthe insulating qualities of a winter ski jacket. Justas an electrical insulator coating a wire helps to

control electric current, the jacket containsthermal insulation that reduces the flow ofthermal energy to help a person stay warm. Be sure that students understand that suchanalogies are not exactly the same as theprocesses that they help to illustrate, but haveenough similarities in certain physical propertiesto make them useful.

Answer to . . .

Figure 7 It provides firm contact tothe negative terminal of one batteryand pushes the other battery intocontact with the light bulb.

Figure 8 Thin wire will heat up andbecome hot enough to glow.

Download a worksheet on conduc-tors and insulators for students tocomplete, and find additionalteacher support from NSTA SciLinks.

0598_hsps09te_Ch20.qxp 4/19/07 9:50 AM 05

606 Chapter 20

Voltage If you remove the batteries from a flashlight, the light will not shine.Why? Because there is resistance in the wires and the bulb, charges donot flow on their own without a source of energy. In order forcharge to flow in a conducting wire, the wire must be connected in acomplete loop that includes a source of electrical energy.

Potential Difference Recall that potential energy is related toposition. In Figure 9, water at the top of the fountain has more gravi-tational potential energy than water at the bottom. That is why waterfalls spontaneously from a higher to a lower height. In the same way,charges flow spontaneously from a higher to a lower potential energy.

The potential energy of a charge depends on its position in an elec-tric field. Potential difference is the difference in electrical potentialenergy between two places in an electric field. Potential difference ismeasured in joules per coulomb, or volts. Because it is measured involts, potential difference is also called voltage.

Voltage Sources How does water get to the top of the fountain?A pump inside the fountain does work on the water to increase itspotential energy. In the same way, a source of voltage such as a batterydoes work to increase the potential energy of electric charges.

Three common voltage sources are batteries, solar cells, and genera-tors. A battery is a device that converts chemical energy to electricalenergy. Batteries, like other voltage sources, have terminals that can con-nect to wires in a circuit. One terminal is positive and the other is negative.A voltage drop, or potential difference, is maintained across the termi-nals. In a 9-volt battery, for example, the voltage drop is about 9 volts.

Modeling Resistance in a Wire

Materialswhite paper, metric ruler, number 2 pencil,multimeter

Procedure1. Draw a narrow rectangle 1 cm wide by 5 cm

long on the paper. Draw a wide rectangle,3 cm by 5 cm. Use the pencil to completelyfill in both rectangles with graphite.

2. Place the multimeter electrodes at oppositeends of the narrow rectangle. Record theresistance. Keep one electrode in place and

slowly drag the other one toward it. Recordyour observations.

3. Repeat Step 2 using the wide rectangle.

Analyze and Conclude1. Observing In which rectangle is the

resistance greater? How does resistancechange as the electrodes move together?

2. Using Models Explain why a thick wire haslower resistance than a thin wire if all else isequal. Why does resistance decrease as awire’s length decreases?

Figure 9 A water fountain has apump inside that lifts water tothe top, increasing the gravita-tional potential energy of thewater. In the same way, avoltage source increases theelectrical potential energy ofelectric charges.

606 Chapter 20

Modeling Resistance in a WireObjectiveAfter completing this activity, studentswill be able to• describe how the thickness of a

conductor affects its resistance.

Skills Focus Observing, Measuring,Controlling Variables, Using Models

Prep Time 15 minutes

Materials white paper, metric ruler,number 2 pencil, multimeter

Class Time 25 minutes

Safety Students should wear safetygoggles and aprons during the lab. Besure students wash their hands aftercompleting the lab.

Expected Outcome Students willlearn that electrical resistance decreasesas the width of the resisting material isincreased or the length of the resistingmaterial is decreased.

Analyze and Conclude1. Resistance is greater in the narrowerrectangle; it decreases.2. There are more available electrons forcurrent within the cross-sectional area ofa thick wire than of a thin wire. As wirelength decreases, the distance electronstravel decreases, so the number ofcollisions that cause resistance decreases. Logical, Group

For EnrichmentHave students repeat the experimentwith different metal strips of identicaldimensions and make a table listing the resistances. Kinesthetic

VoltageBuild Reading LiteracyRelate Text and Visuals Refer to page 190D in Chapter 7, which providesthe guidelines for relating text and visuals.

Have students read the paragraphsabout potential difference while referringto Figure 9. Emphasize that the figureprovides an analogy, so that the difficultconcept in the text will be easier tovisualize. Ask, How are gravity and anattractive electric field similar? (Workmust be done against each to increase thepotential energy of an object.) Logical

L1

L3

L2


Diamond As a rule, good conductors of heatare also good conductors of electricity. This isbecause most conductors are metals, and theelectrons in metals can move with ease,transferring energy through the metal,regardless of whether the energy is thermal orelectrical. However, one substance that is agood thermal conductor (in fact, the bestthermal conductor) is an electrical insulator:

diamond. Diamond is a form of carbon inwhich all carbon atoms are bound to othercarbon atoms in a tetrahedral crystal. Thesestrong bonds give diamond its great hardness,and make it possible for kinetic energy to passeasily through the crystal by heat. However,because there are no free electrons to movethroughout the crystal, diamond cannotconduct electricity.

Facts and Figures


Reviewing Concepts1. List the two types current.

2. Name two good electrical conductors andtwo good electrical insulators.

3. What variables affect the resistance ofa material?

4. What causes charge to flow?

5. According to Ohm’s law, how is voltagerelated to resistance and current?

6. What is a superconductor?

Critical Thinking7. Problem Solving Suppose you have two

wires of equal length made from the samematerial. How is it possible for the wires tohave different resistances?

8. Applying Concepts Use Ohm’s law toexplain how two circuits could have the samecurrent but different resistances.

Electricity 607

Ohm’s LawThe unit of resistance, the ohm, is named after the German scientistGeorg Ohm (1789–1854). It was Ohm who first determined how resist-ance and current affect voltage. He discovered that voltage is not the sameeverywhere in a circuit. Ohm hypothesized that resistance reduces thevoltage. He published his research in 1826, but his findings were so con-troversial that he lost his job. Eventually his work became widely accepted.

Ohm found a mathematical relationship between voltage, current,and resistance. This relationship became known as Ohm’s law.According to Ohm’s law, the voltage (V) in a circuit equals the productof the current (I) and the resistance (R).

Ohm’s LawV � I � R or I �

When the current is in amperes and the resistance is in ohms, thevoltage is in volts. What is the voltage if the resistance is 3 ohms and thecurrent is 3 amps?

V � I � R � 3 amps � 3 ohms � 9 volts

Increasing the voltage increases the current. Keeping thesame voltage and increasing the resistance decreases the current. Amultimeter, shown in Figure 10, is a device used to measure current,voltage, and resistance.

VR

Figure 10 A multimeter can beused to measure current, voltage,or resistance. Here the voltage ofa 9-volt battery is measured.

Compare-Contrast Paragraph Write aparagraph comparing and contrasting con-ductors and insulators and the ways in whichthey might be used. (Hint: Identify materialsthat are good conductors and materials thatare good insulators.)

Ohm’s LawBuild Science SkillsCalculating Help students use Ohm’slaw to calculate the voltage when thecurrent is 4.0 amps and the resistance is3.0 ohms.

V � I � R �4.0 amps � 3.0 ohms � 12 volts

Then, help students understand thatcurrent is indirectly proportional toresistance by doubling the resistanceand calculating the new current whilethe voltage remains constant:

I � V/R � 12 volts/6.0 ohms � 2.0 amps

Point out that when the resistance isdoubled, the current decreases by one half.Logical

ASSESSEvaluate UnderstandingAsk students to write two questions eachabout current, resistance, and voltage.Review the questions for accuracy. Then,have students form groups and ask eachother their approved questions.

ReteachHave students use Figure 9 to explainhow voltage increases the electricalpotential of a charge.

Paragraphs should describe conductorsas materials, such as copper and silver,through which charges move easily.Insulators should be described asmaterials, such as wood, plastic, orrubber, through which charges do notmove easily. Conductors are used inelectrical wiring for buildings, and inelectrical appliances, while insulators are used to isolate conductors fromunwanted contact.


L1

L2

3

L2

Electricity 607

6. A superconductor is a material that hasalmost zero resistance when cooled to lowtemperatures.7. The two wires may have differentthicknesses or different temperatures, whichwould give them different resistances.8. If the voltages in the two circuits were suchthat the ratio of voltage to resistance was thesame, both circuits would have the sameamount of current.


1. Direct current, alternating current2. Good conductors: silver, copper; goodinsulators: air, plastic3. Length, thickness, temperature4. A source of electrical energy causes chargeto flow. 5. Voltage equals the product of current andresistance. An increase in voltage increasescurrent. Keeping the same voltage andincreasing resistance decreases current.


608 Chapter 20

Should Car Companies BeRequired to Make Electric Cars?

1. Defining the Issue Describe the major issuesinvolved in requiring car companies to produceelectric cars.

2. Analyzing the Viewpoints What are somereasons people think the government should orshould not regulate production of electric cars?

3. Forming Your Opinion Should car companiesbe required to produce electric cars? Explain whyor why not.

4. Writing in Science Write a letter to the editorof a local newspaper stating your opinion.

Battery-powered electric cars were first introduced in the late 1880s. Theycould only be used for short distances at low speeds, but they were quiet andhad low maintenance costs. By the 1920s, electric cars were mostly replacedby gasoline-powered cars. Since the 1970s, growing concerns about fuelshortages and pollution from car exhaust has renewed interest in electric cars.

When an electric car is turned on, current flows from the battery to a controller. The controller converts the battery’s direct current intoalternating current that can be used by the motor. The controller alsodetermines how much power the motor needs.

Electric cars rely indirectly on fossil fuels used at electric power plants.However, these cars don’t have engines that burn fossil fuels, so they haveno harmful emissions. For this reason, the U.S. government has begunrequiring vehicle manufacturers to meet development requirements forelectric cars. Should car manufacturers be required to make electric cars?

The Viewpoints

Research and Decide

For: More on this issueVisit: PHSchool.comWeb Code: cch-2203

Car Companies Should BeRequired to Make Electric CarsThe issue seems clear to those in favor of electriccars. If everyone agrees that electric cars have thelowest emissions of any automobile, shouldn’t every-one use them? These people feel that the best way to encourage drivers to use electric cars is to requirecar manufacturers to produce them. It may takepeople time to get used to driving electric cars, but now is the time to start changing attitudes.

Electric cars are efficient and quiet, and theyhave low upkeep costs. Today’s electric cars areexpensive because they are a relatively newtechnology and aren’t widely used. When the cars can be mass produced, prices will drop.

Car Companies Should Not BeRequired to Make Electric CarsOther people argue that production of electric carsshould not be encouraged by governmentregulations. This type of technology shouldprogress as people need and want it.

At present electric cars are expensive andunpopular. Recent sales of these cars have been low, so manufacturers don’t want to keepmaking them.

Opponents of required production also pointout that electric cars have some drawbacks. Withgasoline-powered cars, you can fill the tank anddrive hundreds of miles before refueling. Electriccars run on batteries that need frequent recharging.

608 Chapter 20

Should Car Companies Be Required to Make Electric Cars?BackgroundIn the 1990s, attempts were made by anumber of auto manufacturers to developan electric automobile. Difficulties withthe early models include the long amountof time needed to recharge the batteriesand the limited distance that such vehicles can travel before they need to be recharged. Such problems may beovercome in the future. Nevertheless,electric vehicles work well enough fordriving within a city, and they can berecharged overnight. For this reason, theUnited States Postal Service is planning toacquire fleets of electric vehicles for localmail delivery. Additionally, the research on the electric vehicle has led to thedevelopment of another new type of car:the hybrid automobile (see p. 488).

Answers1. Answers may include the costs ofproduction regardless of demand, legalquestions about how much governmentcan or should be involved with business,and the disadvantage of having tomanufacture electric cars even if thetechnology of the cars does not improve. 2. Answers for regulation may include:Auto companies are not likely to developor market the cars on their own; electriccars are efficient and quiet with lowupkeep costs. Answers against regulationmay include: Technology should progressas people need and want it; electric carsare expensive and unpopular, and theyneed frequent recharging.3. Students should state their opinionsand provide reasons based on facts.4. Students’ letters should be written ina persuasive style.

L2

Have students further research theissues related to this topic.

Electricity 609

20.3 Electric Circuits

Reading StrategyRelating Text and Visuals Copy the tablebelow. As you read, look at Figure 13 on page610. List three things that the diagram helpsyou understand about circuits.

Key ConceptsWhat is included in acircuit diagram?

How do series and parallelcircuits differ?

How do you calculateelectric power andelectrical energy use?

What devices makeelectricity safe to use?

Vocabulary◆ electric circuit◆ series circuit◆ parallel circuit◆ electric power◆ fuse◆ circuit breaker◆ grounding

If you’ve ever seen a house being built, you know that wires hiddeninside the walls connect to every electrical outlet and to every lightswitch. If you were responsible for wiring a house, like the electricianin Figure 11, how would you do it? A good place to start would be thecircuit diagrams supplied by the builder or contractor.

Circuit DiagramsAn electric circuit is a complete path through which charge can flow.Wires in a house form a complex network of circuits. It may look likea maze of wires, but each connection has a purpose. An electrician usescircuit diagrams to keep track of how elements in a circuit are con-nected. Circuit diagrams use symbols to represent parts of acircuit, including a source of electrical energy and devices that arerun by the electrical energy. In a simple circuit, for example, a batteryprovides the energy to operate a device such as a bell or a light bulb.

A circuit diagram shows one or more complete paths in whichcharge can flow. Switches show places where the circuit can be opened.If a switch is open, the circuit is not a complete loop, and current stops.This is called an open circuit. When the switch is closed, the circuit iscomplete and charge can flow. This is called a closed circuit.

What is an open circuit?

What Can Be Seen in the Circuit Diagram?

Wire bringing current from outside

a. ?

b. ?

c. ?

Figure 11 To bring electriccurrent into a building, anelectrician installs wiring. In ahouse, all of the wires usuallycome from one main box.

FOCUS

Objectives20.3.1 Analyze circuit diagrams for

series circuits and parallelcircuits.

20.3.2 Solve equations that relateelectric power to current,voltage, and electrical energy.

20.3.3 Describe devices andprocedures for maintainingelectrical safety.

Build VocabularyVocabulary Knowledge Rating ChartHave students construct a chart with fourcolumns labeled Term, Can Define or UseIt, Have Heard or Seen It, and Don’t Knowto rate their knowledge of each term. Askstudents to share what they know aboutelectric circuit and the other terms for thissection. Help students establish a purposefor reading by using the terms to predictthe text content. After students have readthe section, have them re-rate themselves.

Reading Strategya. Circuits are wired in parallel so devicescan operate independently. b. The fuse box or circuit breaker is located whereelectrical energy enters the house. c. Allof the circuits connect to the ground wire.

INSTRUCT

Circuit DiagramsBuild Reading Literacy

Summarize Refer to page 598D in thischapter, which provides the guidelines for summarizing.

Summarizing the information presentedin the text will help students to focus onmain ideas and remember what theyread. Have students read the paragraphsabout circuit diagrams and summarizethem by restating the main idea in theirown words. Verbal

L1

2

L2

L2

Reading Focus

1

Electricity 609

Print• Reading and Study Workbook With

Math Support, Section 20.3 and Math Skill: Power, Voltage, and Current

• Math Skills and Problem SolvingWorkbook, Section 20.3

• Transparencies, Section 20.3

Technology• Probeware Lab Manual, Lab 8• Interactive Textbook, Section 20.3• Presentation Pro CD-ROM, Section 20.3• Go Online, NSTA SciLinks, Electric circuits

Section Resources

Section 20.3

Answer to . . .

An open circuit is not acomplete loop, so charge

cannot flow through it.

PPLS

0598_hsps09te_Ch20.qxp 3/6/07 2:44 PM Page 609

Figure 12 shows two circuit diagrams. The � and � on the batterysymbol indicate the positive and negative terminals. Arrows show thedirection of current, from positive to negative. Recall that the directionof current is defined as the direction in which positive charges wouldflow. Electrons in a wire flow in the opposite direction.

Series CircuitsIn a series circuit, charge has only one path through which it can flow.Look at the series circuit in Figure 12A. If one light bulb burns out ina series circuit, it becomes an open circuit. If one element stopsfunctioning in a series circuit, none of the elements can operate. Thebulbs in a circuit are a source of resistance. Adding bulbs to a seriescircuit increases the resistance. As a result, the current decreases, andeach bulb shines less brightly.

Parallel CircuitsImagine what would happen if circuits in your home were wired inseries. If a light bulb burned out, the television would turn off. To avoid

this problem, circuits in the home are mostly wiredin parallel. A parallel circuit is an electric circuitwith two or more paths through which charges canflow. If one bulb in Figure 12B burns out, chargestill flows along the other path, and the other bulbstays lit. If one element stops functioning in aparallel circuit, the rest of the elements still canoperate.

Figure 13 shows a network of circuits connect-ing electrical devices in a home. These circuits arewired in parallel so they can operate independently.

How is the direction of current defined?

+

–

Direction of current

Battery

Bulb

+

–

Figure 12 Circuits can berepresented with circuit diagrams.Symbols correspond to eachelement. A A series circuit hasone path that each charge canfollow. B A parallel circuit hasmore than one path each chargecan follow.Interpreting Diagrams Whichsymbol represents a light bulb?

TV

Lightswitch

Light circuit

Dryer

Dryercircuit

Fuse box or circuit breaker

Meter

GroundDesk with computer

WallsocketLamp

Figure 13 Most circuits in ahouse are parallel. This way, evenif one device stops working, theothers will still work.

610 Chapter 20

A B� � � �

Series Circuit Parallel Circuit

610 Chapter 20

Series Circuits

Series and Parallel CircuitsPurpose To show how current movesin each kind of circuit.

Materials a 6-volt battery, 3 small lightbulbs with sockets, 2 long strands of wire(12 cm), 4 short strands of wire (6 cm)

Procedure Strip the insulation from theends of the wires and connect the batteryand light bulbs in series, similar to thesetup shown in Figure 12A. Unscrew onebulb and have students note the results.Rewire the circuit in parallel, similar toFigure 12B. Unscrew one bulb, and havestudents note the results.

Expected Outcome When one bulbin series is unscrewed, the circuit isbroken and all bulbs go out. When thebulb in the parallel circuit is unscrewed,the other lamps are still lit up.Visual

Parallel CircuitsUse VisualsFigure 12 Have students trace thepossible paths for current to movethrough each circuit, starting at thepositive terminal of the battery andending at the negative terminal. Ask, If there is a break in the part of thecircuit containing the outer light bulbin Figure 12B, will there still be acomplete circuit? (Yes) Where willthere be a current? (In the part of thecircuit with the inner bulb) Why will thecurrent in the inner bulb be the sameas before the outer circuit was bro-ken, but the current in the battery will be less than before? (The voltageacross and resistance in the inner bulb areunchanged, so the current in the bulb isunchanged. There are fewer bulbs thatneed current, so the overall current in thebattery is less.)Visual

L1

L2


Customize for English Language Learners

Clarifying Key ConceptsThe details of electrical circuits may be confusingto English language learners. To be sure thatthese students understand circuits, have themkeep a Reading/Learning Log. Encourage

students to write what they understand in the left column, and what they still havequestions about in the right column. Checkstudent logs to determine which, if any,concepts remain unclear.

Electricity 611

Calculating Electric PowerAn electric oven is connected to a 240-volt line, and it uses 34 ampsof current. What is the power used by the oven?

Read and UnderstandWhat information are you given?

Current = I = 34 amps

Voltage = V = 240 volts

Plan and SolveWhat unknown are you trying to calculate?

Power = P � ?

What formula contains the given quantities and the unknown?

P = I � V

Replace each variable with its known value.

P � 34 amps � 240 volts

� 8200 watts

Look Back and CheckIs your answer reasonable?

The answer is reasonable because an electric oven shoulduse much more power than a 1875-watt hair dryer.

1. A clothes dryer uses about27 amps of current from a 240-volt line. How muchpower does it use?

2. A camcorder has a power ratingof 2.3 watts. If the outputvoltage from its battery is7.2 volts, what current doesit use?

3. A power tool uses about 12 ampsof current and has a power ratingof 1440 watts. What voltagedoes the tool require?

Power and Energy CalculationsRecall that power is the rate of doing work. The rate at which electricalenergy is converted to another form of energy is electric power. The unitof electric power is the joule per second, or watt (W). Power often ismeasured in thousands of watts, or kilowatts (kW). Electric powercan be calculated by multiplying voltage by current.

Electric Power P (watts) � I (amps) � V (volts)

Every time you use a 1875-watt hair dryer or turn on a 75-wattlight bulb you use electric power. Appliances vary in the amount ofpower they use.

For: Links on electric circuits

Visit: www.SciLinks.org

Web Code: ccn-2203

Power and EnergyCalculations

Students may think that electrical energyis produced or consumed, rather thanconverted. Stress that electrical energy isconserved in electric circuits. Power isthe rate at which electrical energy is usedor converted to other forms of energy.Examples include electromagneticenergy emitted by a light bulb, thermalenergy provided by the heating elementon a stove, and mechanical energyprovided by an electric motor. Logical

Solutions1. P � I � V � (240 V)(27 A) � 6500 W2. I � P/V � (2.3 W)/(7.2 V) � 0.32 A 3. V � P/I � (1440 W)/(12 A) � 120 VLogical

For Extra HelpStudents can write and solve forms of theequation for electric power. Logical

Direct students to the Math Skills in the Skills and Reference Handbookat the end of the student text foradditional help.

Additional Problems1. A lamp has a voltage of 120 V acrossit and uses a current of 0.5 A. What isthe power used by the lamp? (60 W)2. A 1.5 V battery provides 0.067 A ofcurrent for a calculator. What is thecalculator’s power rating? (0.10 W)Logical, Portfolio

L1

L2

L2

Electricity 611

Download a worksheet on electriccircuits for students to complete,and find additional teacher supportfrom NSTA SciLinks.

Answer to . . .

Figure 12 A zigzag line in a circle

The direction in whichpositive charges

would flow


An appliance’s power rating lets you know how much power it usesunder normal conditions. An electric stove uses about 6000 watts, anda microwave oven uses about 1000 watts. To find the electrical energyused by an appliance, multiply power by time.

Electrical EnergyE � P � t

For example, the power rating of a typical clothes dryer is 5400 watts,or 5.4 kilowatts. If you use the clothes dryer for 2 hours, the energyuse is 5.4 kilowatts multiplied by 2 hours, or 10.8 kilowatt-hours.Electric power companies usually determine charges on your electricbill using kilowatt-hours as a unit of energy. A kilowatt-hour equals3,600,000 joules.

Electrical SafetyInspectors check all new houses to make sure electrical wiring isinstalled safely. All wires must be able to carry the maximum expectedcurrent. But correct wiring is not enough to prevent electrical acci-dents. Correct wiring, fuses, circuit breakers, insulation, andgrounded plugs help make electrical energy safe to use.

In the United States, most household circuits usually have an aver-age voltage of 120 volts. The amount of current in a circuit can vary,depending on the number of devices that are in the circuit. Each devicethat is turned on increases the current. If the current exceeds the cir-cuit’s safety limit, the wire may overheat and start a fire.

Home Safety A fuse prevents current overload in a circuit. A wirein the center of the fuse melts if too much current passes through it.This melting is known as “blowing a fuse.”After a fuse like one of thoseshown in Figure 14 blows, it must be replaced with a new fuse beforethe circuit can carry a current again.

Most houses today use circuit breakers instead of fuses to preventoverloads. A circuit breaker is a switch that opens when current in acircuit is too high. The circuit breaker must be reset before the circuitcan be used again.

Personal Safety Imagine what could happen if your body becamepart of an electric circuit. Figure 15 shows some effects that current mayhave on a person. You might not notice a current of 1 milliamp, buthigher currents can be quite dangerous.

Electrical wiring in a home is insulated to protect people. If theinsulation is damaged, you may accidentally touch the bare wire andget a shock. Avoid touching electrical devices with wet hands becauseyour hands conduct current more readily when they are wet.

Modeling a Fuse

Materials6-volt battery, two wires withstripped ends, aluminum foil,scissors, wooden block,unpainted metal thumbtacks

Procedure1. Connect the two wires to

the two battery terminals.

2. Cut a strip of aluminum foilabout 0.5 cm by 3 cm. Inthe center, cut the widthdown to 1 mm. Attach theends of the foil strip to thewooden block with thethumbtacks.

3. Touch one of the wires toeach end of the foil strip toform a circuit.

4. Observe what happens tothe foil strip. CAUTION Ifthe wire and the batterybecome very hot, removethe wires from the foil.

Analyze and Conclude1. Observing What

happened to the foilstrip when the wireswere attached to it?

2. Using Models How is thefoil like a fuse? Explain.

Figure 14 Fuses have aninternal wire that burns out if a current is too great.

612 Chapter 20

612 Chapter 20

Electrical Safety

Modeling a FuseObjectiveAfter completing this activity, studentswill be able to• describe the principle of an electric fuse.

Skills Focus Observing, Inferring,Using Models

Prep Time 20 minutes

Materials 6-volt battery, 2 wires withstripped ends, strip of aluminum foil,scissors, wooden block, unpaintedthumbtacks

Advance Prep Strip the insulationfrom the ends of the two pieces of wireand the battery leads.


Safety Students should wear safetygoggles. Caution students to be carefulto avoid injury from splinters in thewood or from the thumbtacks. The wirecan become very hot. Caution them toavoid touching it when the ends areconnected to the battery.

Expected Outcome The foil fuse willmelt, interrupting the circuit. Studentswill learn that a fuse works by conductinga small amount of electric current and by blocking the flow of a large amount of electric current.

Analyze and Conclude1. The current caused the aluminum foilto become heated until it melted.2. The foil, like the metal wire in a fuse,is a conductor that, when too muchcurrent passes through it, becomes sohot that it melts. Logical, Group

For EnrichmentHave students repeat the experimentusing a multimeter to measure thecurrent and voltage across the foil stripbefore and after it melts. Ask them toexplain their observations. When the foilmelts, the current will stop and thevoltage will increase to the voltage ofthe battery.Kinesthetic, Logical

L3

L2


Early Fuses Electrical fuses date back almost tothe time when electricity itself was first used.Thomas Edison is credited with inventing thefirst fuse. It is said that platinum wires were used

as fuses to protect the submarine telegraphcables that established electrical communicationbetween Europe and North America.

Facts and Figures


Reviewing Concepts1. Name two elements included in a

circuit diagram.

2. What is the difference between a seriescircuit and a parallel circuit?

3. Write the equations for calculating electricpower and electrical energy.

4. Name five safety devices used withelectric current.

Critical Thinking5. Problem Solving Two bulbs connected in

parallel shine more brightly than when theyare connected to the same voltage source inseries. Explain why this doesn’t violate the lawof conservation of energy.

6. Applying Concepts You plug in a string ofholiday lights and notice that the entire stringturns off when you remove one bulb. Explainwhy this happens.

Electricity 613

7. A stereo receiver uses a current of2.2 amps from a 120-volt line. What isits power?

8. A television connected to a 120-volt lineuses 102 watts of power. How muchcurrent flows through it?

Current Level Effect

1 mA

5 mA

6–30 mA

50–150 mA

1000–4300 mA

10,000 mA

Slight tingling sensation

Slight shock

Painful shock; loss of muscular control

Extreme pain; severe muscular contractions.Breathing stops; death is possible.

Nerve damage; heart stops, death is likely.

Severe burns; heart stops, death is probable.

Effect of Current on Human Body

Figure 15 Even a small current inyour body can cause a painful shockor injury. Analyzing Data What isthe lowest level of current thatcauses serious injury?

Insulation also prevents short circuits. In a short circuit, current findsa short path through the circuit with less resistance than the full paththrough the circuit. A three-prong plug can prevent shocks caused byshort circuits. In Figure 15 you can see the circular third prong, whichconnects to ground. These plugs are used on devices with metal exteri-ors, such as an electric drill. If a short circuit develops, you might get ashock by holding the drill. But instead of entering your body, the currenttakes an easier path to ground through the grounding wire. The trans-fer of excess charge through a conductor to Earth is called grounding.

A ground-fault circuit interrupter (GFCI) like the one shown inFigure 15 is an electrical safety outlet. It monitors current flowing toand from an outlet or appliance. If these two currents are not equal,it means current is escaping. The GFCI opens the circuit to preventserious electric shocks.

Ground-fault circuitinterrupter (GFCI)

Build Science SkillsAnalyzing Data Have students look atthe information given in the table inFigure 15 after they have read theparagraph about the ground-fault circuitinterrupter. Suggest students imaginethat they are designing a GFCI deviceand have to determine how great acurrent difference must exist betweenthe two currents before the circuit isinterrupted. Tell students to decide what this maximum current difference is based on the information in the table.(Answers should be no greater than 5 mAof current.) Logical

ASSESSEvaluate UnderstandingHave students write three math problems(with solutions) based on the electricpower equation used in this section. Each problem should require solving for a different variable: power, voltage,and current. Have students take turnsanalyzing and solving the problems inclass. Note that even incorrectly wordedproblems are useful, as students can beasked to identify and correct the errors.

ReteachOn six different cards, draw a circuit dia-gram of a series or parallel circuit. Includeseveral components in each circuit,making each diagram more challengingthan the previous one, but be sure thatcomponents are only in series or only inparallel with each other. Hold up onecard at a time and ask students to identifythe type of circuit shown.

Solutions7. P � V � I � 120 V � 2.2 A = 260 W8. I � P/V � (102 W)/(120 V) � 0.85 A

If your class subscribesto the Interactive Textbook, use it toreview key concepts in Section 20.3.

L1

L2

3

L2

Electricity 613

5. In the parallel circuit, the overall resistanceis lower and the voltage across each bulb isthe same as the voltage across the powersource. Therefore, the current through thebulbs is greater in the parallel circuit than in the series circuit. Energy is still conserved,but it is used at a faster rate by the batterywhen the bulbs are in a parallel circuit.6. The bulbs are connected in a series circuit.When one of the holiday lights burns out,current in the entire string is stopped.


1. Sample answers: a source of electricalenergy; one or more devices that use electricalenergy; conducting wires.2. A series circuit has only one path for thecurrent. A parallel circuit has two or morepaths for the current.3. The equation for electric power is P � I � V.The equation for electrical energy is E � P � t.4. Fuses, circuit breakers, insulation, grounded(three-prong) plugs, and ground-fault circuitinterrupters (GFCIs)

Answer to . . .

Figure 15 The lowest level of currentthat causes serious injury is 6–30 mA.


Keyboard

Mouse

Monitorordisplay

Getting Personalwith ComputersMillions of people use personal computers every day foractivities such as writing letters, browsing the Internet, orplaying games.

Inside a ComputerThe CPU and other chips in acomputer contain integratedcircuits. The circuits consist ofmillions of miniaturized electroniccomponents deposited onto a thinslice of silicon. Computers use theelectric signals and circuits withinthese chips to represent andprocess data. A binary numbersystem is used, in which data arestored digitally as strings of 1’s and0’s. The digits of a binary numberare transmitted as electrical pulses.Each digit is called a bit, with 8 bitsmaking up a byte.

Basic ComponentsA computer system is made up of a monitor or display, a keyboard, amouse, and a computer. Input devices, such as the mouse and keyboard,feed information into the computer. Output devices, such as the mon-itor, display information that has been taken in by the input devices andprocessed by the central processing unit (CPU). The computer containsthe CPU, memory chips, hard disk, and motherboard.

Disk drive

Computer

Magnified section ofpart of the surface of asilicon chip throughwhich data aretransmitted

Microprocessor chip

Componentsformed fromlayers of silicon

614 Chapter 20

614 Chapter 20

Getting Personal with ComputersBackgroundThroughout the 1960s and early 1970s a number of steps were taken towardcreating smaller computers. The Altair8800 came on the market in April, 1975.It was the first affordable microcomputerthat was widely available for use. TheAltair had 256 bytes of memory, nokeyboard or video monitor, and sold for$375. The Alto, which was developed in1973, more closely resembled modernpersonal computers. Although the Altowas not marketed commercially, itanticipated some standard features that all personal computers have today,such as a video monitor and a mouse.

In 1969, the first computer network,ARPANET, established a connectionamong four university computer sys-tems. This network ultimately led to thecreation of the Internet. While the earlynetworks did not reach the generalpublic or involve personal computers,the 1970s saw the development of the capability of sending informationbetween computers. This paved the wayfor e-mail and other features which arenow so popular on personal computers.

The personal computer began takingits familiar form in the early 1980s as itbecame faster, more versatile, and easierto use. These changes were largely dueto the higher density of components onmicroprocessors. As personal computerswith increased capability were created,their commercial appeal increased. TheWorld Wide Web was established in1991. The Internet made it possible totransmit text, pictures, and soundinstantly across great distances. Withinfive years, the Internet was being usedcommercially by over 50 million people.

L2

Hard disk

Powersupply

DVD/CDwriter

Fanhousing

Motherboard This largecircuit board houses the CPU,RAM, and ROM (read-onlymemory). ROM is permanentmemory that includes startupinstructions.

CPU The central processingunit carries out programinstructions and controls allthe information flowingaround the computer.

File andprogram

Harddisk

CPU

RAM

■1 Program startup The fileto be worked on and theprogram that allows it to beworked on are stored on thehard disk. Copies of both aremoved, via circuits in themotherboard and cables, fromhard disk storage to RAM.

■1

■2 Work in progress Under the user’s control, the CPUworks through programinstructions. It makes changesto the copy of the data heldin RAM.

■2

■3

Manipulating Bits and BytesComputer programs and data, such as word processingfiles, are stored for the long term on magnetic storagedevices such as the hard disk. It is only during actual use ofa program and work on data files that they are moved toRAM. Both the hard disk and RAM store data in bytes.

Video card houses speciallydesigned graphics chips.

RAM Random-access memory(RAM) is used to store programsthat are being run. Information istemporarily stored in RAM as aseries of 1’s and 0’s. When thecomputer is switched off, anyinformation stored in RAM is lost.

Heat Sink Heat is generatedby the flow of electricity insidethe computer. A heat sink isneeded to dissipate the heat.It draws heat away from thecomponents to protect them.

OPENEDCOMPUTER

Electricity 615

■3 End of session Theupdated data are savedback to the hard disk, andthe program that was beingused is erased from RAM.

Build Science SkillsCommunicating Results Havestudents research the origins of thevarious components of the personalcomputer. Allow time for them topresent their findings. Encourage themto include in their presentations someform of visual aid (for example, picturesof the components at various stages oftheir development, or a time line show-ing advances). You may want to havetwo or three students work together to research and create presentations for more complex components, such as the CPU.

Ask, Was the personal computerinvented at one time? (No. It was theresult of several different inventions beingdeveloped independently at about thesame time.) What feature of the per-sonal computer has contributed to itsbecoming smaller, more efficient, andeasier to use? (Although there may beseveral valid opinions, such as the videomonitor, which has made computers moreaccessible to many nontechnical users, themost important development has beensmaller, higher-capacity CPUs.) Do new inventions become widely usedimmediately? (Not as a rule. Many of thefeatures of the personal computer, like themouse, did not become widely used untilmany years after their invention.)Verbal, Portfolio

L2

Electricity 615


Using ComputersEven before there were personal computers, digitalcomputing was put to use in a variety of differentways. Today, computer technology has beenminiaturized to create mobile phones, andexpanded to allow supercomputers to work at highspeed. Virtual reality, a relatively recentdevelopment, now allows for realistic simulations ina three-dimensional, computer-generated world.

SupercomputersBy cooling the components so that theyconduct electricity more efficiently,supercomputers are able to processinformation at a very fast rate. Thesecomputers gain speed by multi-tasking,performing several processes at once.The computer shown here is used inthe study of particle physics.

Mini ComputersMobile phones and othersmall electronic devices,such as hand-heldcomputers, use smallerand smaller chips toallow for portabilityand convenience.

616 Chapter 20

Instruments and controls

3-D imageprojector

Virtual RealityVirtual reality is a three-dimensional computer-generated world. Programssuch as the NASA flightsimulator shown here trainstudents through interactivesimulations.

616 Chapter 20

(continued)

Build Science SkillsApplying Concepts

Purpose Students become familiar with the inside parts of a personal computer.

Materials several discarded computers,each having its top removed


Procedure Have students separate intogroups to observe the inside parts of acomputer. They should try to locate the computer parts shown on p. 615 of their textbooks.

Expected Outcome Students will beable to point out the different parts of a computer and gain a better under-standing of how the parts work together. Kinesthetic, Visual, Group

L2

� Use the library or the Internet to researchhow making computer circuits smaller allowsa computer to operate at afaster speed. Write aparagraph describing onetechnology that has increasedprocessing speed.

� Take a Discovery Channel VideoField Trip by watching “CurrentComputers.”

Going Further

Illuminatedmarkers

Computer GamesIn human-movement tracking technology,computers simulate human movement forrealistic computer games. As shown here,markers are placed on a person and used totrack his movements. A computer analyzes thepath shown by each marker. The computer thenrecreates the paths for the simulated figures ina computer game, as shown above.

Electricity 617

Video Field Trip

Going FurtherStudent research should indicate thatsmaller computer circuits increase com-puter speed by shortening the circuit size,and thus the distance electricity musttravel. Smaller circuits also allow for morecomponents to be placed in a CPU. TheCPU can perform more tasks at once, soless time is required to carry out a seriesof operations. Also, smaller componentsprovide more space for components tostore data. This allows more informationto be processed in a given time, and soshortens the overall processing time of agiven task. Students’ paragraphs ontechnologies affecting computer speedmay include the means of etching smallercircuits on chips, bus design to providequicker access between the processorand memory, and faster hard drives forreading input faster.Verbal

Electricity 617

After students have viewed the Video Field Trip, askthem the following questions: Name a device thatconnects the computer to the outside world inways that allow information to flow in. (Studentanswers may include keyboard, mouse, modem, high-speed broadband connection, or a connection to adigital camera or scanner.) Describe how pressingthe key for the letter p on the keyboard gets

translated into information the computer canuse. (Pressing the key sends a signal to the BIOS, orbasic input/output system, chip. There it is convertedinto a number in binary notation using an internallystored table, and the binary number is transferred tothe central processing unit.) Name one device thatconnects to the computer and allows usefulinformation to flow out from it. (Student answersmay include computer screen, printer, modem,broadband connection, and a set of speakers.) Whichpart of the computer controls its operation? (Thecentral processing unit, or CPU.)

Video Field Trip

Current Computers


20.4 Electronic Devices

Reading StrategySummarizing Copy the table below. As youread, complete the table to summarize whatyou learn about solid-state components.

Key ConceptsHow do electronic signalsconvey information?

How do vacuum tubescontrol electron flow?

What are two types ofsemiconductors?

How are semi-conductors used?

What are the benefits ofusing microchips incommunication devices?

Vocabulary◆ electronics◆ electronic signal◆ analog signal◆ digital signal◆ semiconductor◆ diode◆ transistor◆ integrated circuit◆ computer

How do a toaster and a lamp use electric current differently thana computer or a mobile phone? The toaster and lamp are electricaldevices. They change electrical energy into heat or light. The com-

puter and mobile phone are electronic devices, which useelectric current to process or send information.

Electronic SignalsThe science of using electric current to process or transmitinformation is electronics. The information is carried by anelectronic signal. An electronic signal is information sent aspatterns in the controlled flow of electrons through a circuit.

To understand how this works, think about circuits. If avoltage source is connected to a circuit by a wire, electrons willflow through the wire. Controlling the electron flow—by eitheraltering the voltage or turning the current on and off—produces a coded signal. Electronics conveys informationwith electrical patterns called analog and digital signals.

a. ? b. ?

c. ? d. ?

e. ? f. ?

Solid-State Component Description Uses

Diode

Transistor

Integrated circuit

618 Chapter 20

Figure 16 A computer uses electriccurrent to process information. Atoaster uses electric current to changeelectrical energy into thermal energy.

618 Chapter 20

FOCUS

Objectives20.4.1 Explain how electronics

conveys information withanalog or digital signals.

20.4.2 Describe electronic devicesused to control electron flow.

20.4.3 Illustrate how semiconductorsare used to make three kinds ofsolid-state components.

20.4.4 Describe how solid-statecomponents are used inelectronic devices.

Build VocabularyConcept Map Have students construct a concept map of the vocabulary termsused in this section. Instruct students towrite the vocabulary terms in ovals andconnect the ovals with lines on whichlinking words are placed. Students shouldplace the main concept (Electronics) atthe top or the center. Moving away fromthe main concept, the content shouldbecome more specific.

Reading Strategya. Two semiconductors combined so thatcurrent moves in one direction but notthe other. b. Maintains proper voltagelevel in circuits, controls direction ofcurrent c. Three semiconductors com-bined so that current can be switched on or off, or voltage can be amplified.d. Amplifies telephone signals e. A thinslice of silicon with many tiny solid-statecomponents built up on it f. Processesand stores information in computers

INSTRUCT

Electronic SignalsUse Community ResourcesArrange for your class to visit a localtelephone company. Have studentsobserve the type of equipment used totransmit telephone calls to anywhere inthe world. Encourage students to askabout the relative importance of analogand digital signals in present telephonecommunication, and have them explainit in their own words in a brief writtenreport. Interpersonal, Portfolio

L2

2

L2

L2

Reading Focus

1

Section 20.4

Print• Reading and Study Workbook With

Math Support, Section 20.4• Transparencies, Section 20.4

Technology• Interactive Textbook, Section 20.4• Presentation Pro CD-ROM, Section 20.4

Section Resources

Electricity 619

Analog Signals An analog signal is a smoothlyvarying signal produced by continuously changing thevoltage or current in a circuit. Information is encoded inthe strength or frequency of the analog signal. Figure 17Ashows one familiar example—a signal used by an AMradio station. The music is encoded as a smoothly chang-ing pattern of the voltage.

Digital Signals A digital signal encodes informa-tion as a string of 1’s and 0’s. Figure 17B shows howpulsing a current on and off can produce a digital signal.When the current is off, it represents a “0.” When thecurrent is on, it represents a “1.” You may be familiarwith Morse code, which in a similar way uses two sig-nals. A dot and a dash are all you need to represent the26 letters of the alphabet and the digits 0 through 9.

Digital signals are more reliable than analog signals.For example, a DVD, or digital video disc, encodes digital signals as aseries of pits in the DVD surface. If a pit is damaged, it is often still read-able as a 0 or a 1. The quality of video is not affected unless the damageis severe. In comparison, damage to an analog videotape adds noise tothe signal.

Vacuum TubesTo create an electronic signal, you must be able to control the flow ofelectrons. A vacuum tube was used to control electron flow in earlyelectronic devices. Vacuum tubes can change alternating currentinto direct current, increase the strength of a signal, or turn a currenton or off.

One useful type of vacuum tube is a cathode-ray tube (CRT)shown in Figure 18. Many computer monitors and televisions containCRTs. One side of the CRT has three metal plates that emit electronbeams. The electrons are emitted at one end of an airless tube andstrike a glass surface on the other end. The glass is coated withphosphors that glow red, green, or blue in response to theelectron beams. An electronic signal controls the strengthand position of the beams to produce images with the lightfrom the phosphors.

Although vacuum tubes have many useful features, sometypes burn out frequently and need to be replaced. They arealso much too large for use in small electronic devices.

What is a CRT?

Figure 18 A cathode-ray tube isused in many computer monitorsand television sets.

Vo

ltag

e

Time

Vo

ltag

e

Time

A

B

Figure 17 Electronic signalsconvey information by changingvoltage or current in a circuit. A An analog signal can beproduced by smoothly changingvoltage. B A digital signal can beproduced by pulsing a current onand off. Applying ConceptsCould a portable radio, intheory, receive and process adigital signal?

Use VisualsFigure 17 Emphasize that, because adigital signal requires a computer toencode and decode the signal, its appear-ance differs greatly from the source signal.Ask, How can you tell that the signal inFigure 17A is analog? (There is a wideand continuous range of voltages in thesignal.) Ask, How can you tell that thesignal in Figure 17B is digital? (There areonly two values of voltage in the signal,corresponding to a “1” or a “0.”)Visual

Build Science SkillsComparing and Contrasting Havestudents list in columns under the headings Analog and Digital the ways in which the two signals contrast. Then, have them write below the table a list of the similarities of the signals.(Analog changes continuously, has manyvoltage values, resembles the pattern ofthe original signal, and is easily distorted.Digital changes abruptly, has only twovoltage values, does not resemble theoriginal signal, and is not easily distorted.Both signals accurately reproduce theoriginal signal.)Verbal, Logical

Vacuum TubesBuild Reading LiteracyAnticipation Guide Refer topage 388D in Chapter 13, whichprovides the guidelines for ananticipation guide.

Ask students if they have ever seen an old radio or television set withvacuum tubes. Emphasize that thesetubes performed the same functions as modern diodes and transistors, butwere larger and less efficient. Askstudents to think of problems that might arise from using vacuum tubes.(Possible answers include heat producedby tubes, tended to burn out easily,required a long time to warm up, and made appliances very large.)Verbal, Interpersonal

L1

L2

L1

Electricity 619

Customize for Inclusion Students

Learning DisabledStrengthen students’ understanding ofelectronic components by bringing in samples of the various components for them to examine.Examining a vacuum tube and having the parts explained to them will increase their

understanding of these devices. An old transistorthat has been opened so that the junction insideis visible can be compared to a vacuum tube. Byvisually experiencing what these devices look likeand how they are put together, students canbetter understand how they work.

Answer to . . .

Figure 17 Yes, if it contained theelectronics necessary to decode thedigital signal.

A CRT is a cathode-raytube, in which electrons

emitted at one end strike a glasssurface at the other end.


620 Chapter 20

Digital CameraWhen light enters an analog camera, it strikes a stripof light-sensitive film behind the lens. In a digitalcamera, the pattern of light is sensed electronicallyand turned into digital code. Interpreting Diagrams What controls the sharpnessof a digital image?

Miniature digital cameraDigital cameras can be much smallerthan traditional cameras becausehigh resolution CCDs do not need touse large lenses.

How the CCD worksIn a CCD, a lens focuses light ontoa photosite, which converts the lightintensity into an electric current. Tocreate a full color image, the CCDcontains a color filter. The filterseparates incoming light into oneof the three primary colors at eachphotosite. The microprocessor createsa color image by evaluating the colordata for groups of photosites.

Lens Colorfilter

Photosite

Computer outputport This port allowsthe digital image to be transferred to a computer.

Analog-to-digitalconverter Theanalog-to-digitalconverter turns theoutput of eachphotosite intodigital code.

Lens The lensfocuses lightfrom the sceneonto the CCD.

Digital-to-analogconverter Herethe digital data

are turned into aform that allowsthe image to bedisplayed on the

LCD screen.

CCD (charged-coupled device) TheCCD has millions oflight-sensitive cells,called photosites.The more photositesper unit area, thesharper the image is.

Microprocessor Themicroprocessor calculatescolor values for each partof the image.

Memory chip Dataabout the image arestored on this non-removable chip.

Memory card This removable cardstores the image asa digital file.

Cable tocomputer

Light fromscene

LCD (liquid crystaldisplay) screen The LCDscreen displays an imageof the scene that thecamera is aimed at. Imagesthat have been previouslysaved can also bedisplayed on the screen.

620 Chapter 20

Digital CameraDigital photography was initially devel-oped by NASA to transmit images fromspacecraft back to Earth. Early camerason space vehicles were originally tele-vision camera units called vidicons, butin the 1970s, charged-coupled devices(CCDs) were developed. Unlike photo-graphic film or vidicon tubes, CCDshave the advantage of detecting mostof the light that falls on them. As thetechnology improved and becamemore affordable, the number of photo-sites on a CCD became larger, and the photosites themselves becamesmaller. This improved the resolution,or sharpness, of the image. The firstdigital camera for the general publicwas marketed in 1994.

Interpreting Diagrams The numberof photosites on a CCD controls theimage sharpness.Visual

For EnrichmentInterested students or students familiarwith digital camera and image tech-nology can make a presentationshowing a digital camera and how itsimages are electronically stored andmanipulated. They can obtain addi-tional information on the Internet and in science and engineering periodicals.Verbal, Portfolio

L3

L2


Electricity 621

A

Figure 20 A A diode is twodifferent semiconductors joined inone component. B A transistor isthree semiconductors with themiddle one different from the outer ones. Applying Concepts Inaddition to the arrangement of the transistor in B, how else mightyou arrange the transistor?

SemiconductorsA semiconductor is a crystalline solid that con-ducts current only under certain conditions.Most semiconductors are made with silicon orgermanium. In pure form, these elements arepoor conductors. But when trace amounts ofother elements are added, it becomes possibleto control the current inside of the crystals.Figure 19 shows two types of semiconductors.

In n-type semiconductors, the current isa flow of electrons. In p-type semiconductors,it appears as though positive charge flows.

A p-type semiconductor can be made by adding a trace amountof boron to silicon. In Figure 19A, you can see spaces, called holes, ateach boron atom. The holes are positively charged. Figure 19B showsan n-type semiconductor made by adding phosphorus to silicon.Phosphorus atoms provide weakly bound electrons that can flow.

By themselves, n-type and p-type semiconductors cannot domuch. But when joined together, electrons in the n-type semiconduc-tor are attracted toward the positively charged holes in the p-typesemiconductor. As electrons jump from hole to hole, it looks like a flowof positive charge because the locations of the holes change.

Solid-State ComponentsSemiconductor devices were first used in the late1940s. These devices were named solid-state compo-nents because they used solids rather than vacuumtubes to control current. Most modern elec-tronic devices are controlled by solid-statecomponents. Three of the most useful solid-statecomponents are diodes, transistors, and integratedcircuits.

Diodes A diode is a solid-state component that combines an n-typeand p-type semiconductor. When a voltage is applied across a diode,electrons flow from the n-type to the p-type semiconductor. There isno current if voltage is applied in the opposite direction. Because thecurrent can be in only one direction, a diode can change alternatingcurrent to direct current.

Transistors Figure 20B shows a transistor, a solid-state compo-nent with three layers of semiconductors. A small current flowingthrough its center layer changes its resistance. A transistor can be usedas a switch because the small current can turn another current on oroff. It can also be used as an amplifier. A small voltage applied to oneside of the transistor produces a large voltage on the other side.

P-type silicon,with boron

Hole (+)

Electron (–)

N-type silicon,with phosphorus

A

B

–

+Direction ofelectron flow

Exchangeof electronsand holes

Direction ofhole flow

Figure 19 A semiconductorbecomes a good conductor ofcharge if trace amounts ofelements are added to it.

B

Semiconductors

Semiconductors and CurrentPurpose Students observe how asemiconductor allows and limits currentdepending on how a voltage is appliedacross it.

Materials a light-emitting diode (LED),an ohmmeter, a DC power source, wires,alligator clips

Procedure Attach the terminals of theohmmeter to the LED and have studentsnote the resistance reading. Reverse the positions of the terminals and showstudents the new resistance. Repeat thetest, attaching the wires to the ends ofthe LED and the DC power source, sothat the LED offers minimum resistance.Slowly increase the voltage until the LEDglows. Turn off the power supply, reversethe connections, and increase the voltageslowly to the same level as before. Havestudents note the LED’s appearance.

Expected Outcome When the voltageacross the LED is positioned one way,the resistance in the LED is high, withlittle current. When the voltage isreversed, the resistance is low and thecurrent is larger. Visual

Solid-StateComponents

Students may think that the positivecharges in p-type semiconductors are theresult of positive charges moving. Whilethis is the effective result, this is not whatis literally happening. The only chargesthat are actually moving are electrons.The term “holes” suggests the absence of something (in this case, negativecharges). Have students think of otherways to consider the motion of an objectas the opposite motion of what is leftbehind. For instance, moving checkersacross a board is like moving emptyspaces in the opposite direction.Electrons, like checkers, can only moveinto an empty space. Logical

L2

L2

Electricity 621

Smaller Components Over the last 40 years,electronic components have become smallerand more efficient, allowing more computingpower to be placed in a smaller space. In 1965,the engineer Gordon Moore stated that thenumber of transistors on an integrated circuitwould double about every two years. Thisobservation is called Moore’s law. Over theyears, the rate of technological developmenthas met or surpassed Moore’s prediction.

In 1972, the leading processing chip held 2500 transistors. By 1974, the number hadincreased to 5000. In 2003, the largest chip had 410 million transistors, nearly 10 times the number in 2000. There is considerablequestion as to how small electronic devicescan become. Moore himself said that “Nophysical quantity can continue to changeexponentially forever.”

Facts and Figures

Answer to . . .

Figure 20 Place two p-type semicon-ductors on either side of an n-typesemiconductor


622 Chapter 20


Reviewing Concepts1. How are electronic devices used to

process information?

2. Describe how electron flow is controlled invacuum tubes.

3. What are two types of semiconductors?

4. How are solid-state components used?

5. How are microchips beneficial forcommunication devices?

Critical Thinking6. Comparing and Contrasting How are

solid-state components like vacuum tubes?How are they different?

Integrated Circuits An integrated circuit is a thin slice of sil-icon that contains many solid-state components. The components arecarefully built layer by layer on the silicon base. Integrated circuits aresometimes called chips or microchips. They perform as well as a net-work of vacuum tubes, but they need only a tiny fraction of the space.Mobile phones, pagers, and computers all use microchips.

Electronic devices today are so small because hundreds of mil-lions of components fit on a microchip smaller than your fingertip.Integrated circuits are also blindingly fast compared to vacuumtubes. One reason is that current does not have to travel far to getfrom point to point in the circuit. So it shouldn’t be surprising thatas technology improves, and chips get smaller, the chips operate athigher speeds.

Communications TechnologyA computer is a programmable device that can store and process infor-mation. Today you find microchips in all sorts of devices that youwouldn’t call computers. Communication devices use microchipsto make them more portable, reliable, and affordable.

Figure 21 shows the inside a mobile phone, which contains manysolid-state components. Transistors amplify the phone’s incomingsignal. Electronic devices called capacitors store electric charge. Theyallow a mobile phone to store data such as phone numbers, even if thebattery is removed for a short time. Diodes maintain proper voltagelevels in the circuits. Without solid-state components, none of thiswould be possible.

Figure 21 A mobile phone usesmany solid-state components.Drawing Conclusions Would it be possible to make a mobilephone using vacuum tubes rather than solid-state components?

Conservation of Energy Review Section 15.2. How is energy conservedwhen a heat sink is used to protectelectronic components in a computer?

7. Applying Concepts Explain how a diodecan be used to change alternating current intodirect current.

8. Using Analogies Explain how a diode is likea one-way street.

CommunicationsTechnologyIntegrate Language ArtsThe development of solid-state technol-ogy has introduced many new termsinto the world’s languages. Some areborrowed from other languages. Forinstance, the term cybernetics refers tothe study of communication and controlin machines. The term comes from theGreek for the steersman of a boat, thatis, the one who navigates or controls.Another term, robot, comes from theCzech word meaning “worker.” Otherterms, like Internet, are abbreviations,while some terms are initials (such asWWW or Y2K). Have students researchand report on several expressions thathave appeared because of developmentsin communications technology. Verbal

ASSESSEvaluate UnderstandingRandomly ask students to list the generalproperties of one electronic device, eithera solid-state or vacuum-tube component.Then, have them give examples ofapplications for these devices.

ReteachHave students use Figure 20 to explainhow semiconductors allow electrons toflow in a preferred direction.

All electronic components have someresistance. Their temperature increaseswhen electrons moving through themcollide with the atoms in the component.Thus, electrical energy is converted tothermal energy. Heat sinks withdraw thisthermal energy from the circuits in variousways. In each energy conversion, energyis conserved, but its form is different.


L1

L2

3

L2


6. Both control electron flow in electronicdevices. Both can convert alternating currentto direct current, or amplify a signal. Solid-statecomponents use semiconducting materials toachieve these results instead of the hot wiresand charged plates in vacuums.7. When alternating current enters a diode,only current moving in one direction can pass through. Current moving in the reversedirection is blocked.8. Charges can only move in one directionthrough the diode, just as an automobile canonly go in one direction on a one-way street.


1. Electronic devices process information byusing electric current to form patterns calledanalog and digital signals.2. Vacuum tubes can control electron flow bychanging alternating current into direct currentor by turning a current on or off.3. N-type and p-type4. Solid-state components are used to controlmodern electronic devices.5. Microchips make communications devicesmore portable, reliable, and affordable.

Answer to . . .

Figure 21 No, the vacuum tubeswould be too large.

622 Chapter 20

Section 20.1 20.1 Electric Charge and Static...

Documents

Transcript of Section 20.1 20.1 Electric Charge and Static...