1、What Youll Learn You will explain energytransfer in circuits.You will solve problemsinvolving current,potential difference,and resistance.You will diagram simpleelectric circuits.Why Its ImportantThe electric tools andappliances that you use are based upon the abilityof electric circuits totransfer
2、energy resultingfrom potential difference,and thus,perform work.Power TransmissionLines Transmission linescrisscross our country to transfer energy to where it is needed.Thistransfer is accomplished at high potentialdifferences,often as high as 500,000 V.Think About This?Transmission line voltages a
3、re too high to use safely in homes and businesses.Why aresuch high voltages used in transmission lines?Lester Lefkowitz/CORBIS 590-600 CH22-S1-845813 3/22/04 3:31 AM Page 590Section 22.1 Current and Circuits591Can you get a lightbulb to light?QuestionGiven a wire,a battery,and a lightbulb,can you ge
4、t the bulb to light?Procedure1.Obtain a lightbulb,a wire,and a battery.Try to find as many ways as possible to get thelightbulb to light.Caution:Wire is sharp and can cut skin.Wire can also get hot ifconnected across the battery.2.Diagram two ways in which you are able toget the lightbulb to work.Be
5、 sure to label the battery,the wire,and the bulb.3.Diagram at least three ways in which you arenot able to get the bulb to light.AnalysisHow did you know if electric current wasflowing?What do your diagrams of the lit bulbhave in common?What do your diagrams of the unlit bulb have in common?From you
6、robservations,what conditions seem to benecessary in order for the bulb to light?Critical Thinking What causes electricity toflow through the bulb?22.1Current and Circuits?Objectives Describe conditions thatcreate current in an electric circuit.Explain Ohms law.Design closed circuits.Differentiate b
7、etweenpower and energy in anelectric circuit.?Vocabularyelectric currentconventional currentbatteryelectric circuitampereresistanceresistorparallel connectionseries connectionAs you learned in Chapter 11,flowing water at the top of a waterfall has both potential and kinetic energy.However,the large
8、amount ofnatural potential and kinetic energy available from resources such asNiagara Falls are of little use to people or manufacturers who are 100 kmaway,unless that energy can be transported efficiently.Electric energy pro-vides the means to transfer large quantities of energy great distances wit
9、hlittle loss.This transfer usually is done at high potential differences throughpower lines,such as those shown in the photo on the left.Once this energyreaches the consumer,it can easily be converted into another form or com-bination of forms,including sound,light,thermal energy,and motion.Because
10、electric energy can so easily be changed into other forms,it hasbecome indispensable in our daily lives.Even quick glances around youwill likely generate ample examples of the conversion of electric energy.Inside,lights to help you read at night,microwaves and electric ranges tocook food,computers,a
11、nd stereos all rely on electricity for power.Outside,street lamps,store signs,advertisements,and the starters in cars all useflowing electric charges.In this chapter,you will learn how potential differences,resistance,and current are related.You also will learn aboutelectric power and energy transfe
12、r.Horizons Companies 590-600 CH22-S1-845813 6/10/04 8:59 PM Page 591592Chapter 22 Current ElectricityProducing Electric CurrentIn Chapter 21,you learned that when two conducting spheres touch,charges flow from the sphere at a higher potential to the one at a lowerpotential.The flow continues until t
13、here is no potential difference betweenthe two spheres.A flow of charged particles is an electric current.In Figure 22-1a,twoconductors,A and B,are connected by a wire conductor,C.Charges flowfrom the higher potential difference of B to A through C.This flow of pos-itive charge is called conventiona
14、l current.The flow stops when thepotential difference between A,B,and C is zero.You could maintain theelectric potential difference between B and A by pumping charged particlesfrom A back to B,as illustrated in Figure 22-1b.Since the pump increasesthe electric potential energy of the charges,it requ
15、ires an external energysource to run.This energy could come from a variety of sources.One famil-iar source,a voltaic or galvanic cell(a common dry cell),converts chemicalenergy to electric energy.Several galvanic cells connected together arecalled a battery.A second source of electric energya photov
16、oltaic cell,orsolar cellchanges light energy into electric energy.Electric CircuitsThe charges in Figure 22-1b move around a closed loop,cycling from thepump to B,through C,to A and back to the pump.Any closed loop or conducting path allowing electric charges to flow is called an electric circuit.A
17、circuit includes a charge pump,which increases the potential energy of the charges flowing from A to B,and a device that reduces the potentialenergy of the charges flowing from B to A.The potential energy lost by thecharges,qV,moving through the device is usually converted into some otherform of ene
18、rgy.For example,electric energy is converted to kinetic energy bya motor,to light energy by a lamp,and to thermal energy by a heater.A charge pump creates the flow of charged particles that make up a cur-rent.Consider a generator driven by a waterwheel,such as the one picturedin Figure 22-2a.The wat
19、er falls and rotates the waterwheel and generator.Thus,the kinetic energy of the water is converted to electric energy by thegenerator.The generator,like the charge pump,increases the electricpotential difference,V.Energy in the amount qV is needed to increase thepotential difference of the charges.
20、This energy comes from the change inenergy of the water.Not all of the waters kinetic energy,however,is con-verted to electric energy,as shown in Figure 22-2b.If the generator attached to the waterwheel is connected to amotor,the charges in the wire flow into the motor.The flow ofcharges continues t
21、hrough the circuit back to the generator.Themotor converts electric energy to kinetic energy.Conservation of charge Charges cannot be created or destroyed,but they can be separated.Thus,the total amount of chargethenumber of negative electrons and positive ionsin the circuitdoes not change.If one co
22、ulomb flows through the generator in1 s,then one coulomb also will flow through the motor in 1 s.Thus,charge is a conserved quantity.Energy also is conserved.Thechange in electric energy,?E,equals qV.Because q is conserved,BCACurrent maintainedCharge pumpBCACurrent soon ceasesPositive chargesabFigur
23、e 22-1 Conventionalcurrent is defined as positivecharges flowing from the positiveplate to the negative plate(a).A generator pumps the positivecharges back to the positive plateand maintains the current(b).Inmost metals,negatively-chargedelectrons actually flow from thenegative to the positive plate
24、,creating the appearance ofpositive charges that are movingin the opposite direction.590-600 CH22-S1-845813 7/22/04 3:38 PM Page 592Section 22.1 Current and Circuits593Figure 22-2 The potentialenergy of the waterfall iseventually converted into workdone on the bucket(a).Theproduction and use of elec
25、triccurrent is not 100 percent efficient.Some thermal energy is producedby the splashing water,friction,and electric resistance(b).WaterwheelWaterfallGeneratorMotorPositivechargeflowPositivechargeflowbThermal energyGeneratorMotorPotentialenergyof waterWorkdone bymotor Electricenergythe net change in
26、 potential energy of the charges going completely aroundthe circuit must be zero.The increase in potential difference produced bythe generator equals the decrease in potential difference across the motor.If the potential difference between two wires is 120 V,the waterwheeland the generator must do 1
27、20 J of work on each coulomb of charge thatis delivered.Every coulomb of charge moving through the motor delivers120 J of energy to the motor.Rates of Charge Flow and Energy TransferPower,which is defined in watts,W,measures the rate at which energyis transferred.If a generator transfers 1 J of kine
28、tic energy to electric energyeach second,it is transferring energy at the rate of 1 J/s,or 1 W.The energycarried by an electric current depends on the charge transferred,q,and thepotential difference across which it moves,V.Thus,E?qV.Recall fromChapter 20 that the unit for the quantity of electric c
29、harge is the coulomb.The rate of flow of electric charge,q/t,called electric current,is measuredin coulombs per second.Electric current is represented by I,so I?q/t.Aflow of 1 C/s is called an ampere,A.The energy carried by an electric current is related to the voltage,E?qV.Since current,I?q/t,is th
30、e rate of charge flow,the power,P?E/t,of anelectric device can be determined by multiplying voltage and current.Toderive the familiar form of the equation for the power delivered to an elec-tric device,you can use P?E/t and substitute E?qV and q?It.If the current through the motor in Figure 22-2a is
31、 3.0 A and the potentialdifference is 120 V,the power in the motor is calculated using the expres-sion P?(3.0 C/s)(120 J/C)?360 J/s,which is 360 W.PowerP?IVPower is equal to the current times the potential difference.a590-600 CH22-S1-845813 3/22/04 3:35 AM Page 593594Chapter 22 Current ElectricityEl
32、ectric Power and EnergyA 6.0-V battery delivers a 0.50-A current to an electric motor connected across its terminals.a.What power is delivered to the motor?b.If the motor runs for 5.0 min,how much electric energy is delivered?Analyze and Sketch the Problem Draw a circuit showing the positive termina
33、l of a batteryconnected to a motor and the return wire from the motorconnected to the negative terminal of the battery.Show the direction of conventional current.Known:Unknown:V?6.0 VP?l?0.50 AE?t?5.0 minSolve for the Unknowna.Use P?IV to find the power.P?IVP?(0.50 A)(6.0 V)Substitue I?0.50 A,V?6.0
34、V?3.0 Wb.In Chapter 10,you learned that P=E/t.Solve for E to find the energy.E?Pt?(3.0 W)(5.0 min)Substitute P?3.0 W,t?5.0 min?(3.0 J/s)(5.0 min)(?16m0isn?)?9.0?102JEvaluate the Answer Are the units correct?Power is measured in watts,and energy is measured in joules.Is the magnitude realistic?With r
35、elatively low voltage and current,a few watts of power is reasonable.321Math HandbookSignificant Digits page 834MotorBatteryIIV1.The current through a lightbulb connected across the terminals of a125-V outlet is 0.50 A.At what rate does the bulb convert electricenergy to light?(Assume 100 percent ef
36、ficiency.)2.A car battery causes a current of 2.0 A through a lamp and produces12 V across it.What is the power used by the lamp?3.What is the current through a 75-W lightbulb that is connected to a125-V outlet?4.The current through the starter motor of a car is 210 A.If the batterymaintains 12 V ac
37、ross the motor,how much electric energy isdelivered to the starter in 10.0 s?5.A flashlight bulb is rated at 0.90 W.If the lightbulb drops 3.0 V,howmuch current goes through it?590-600 CH22-S1-845813 3/22/04 3:36 AM Page 594Table 22-1Changing ResistanceFactorHow resistance changesExampleLengthResist
38、ance increases as length increases.RL1?RL2Cross-sectionalareaResistance increases as cross-sectionalarea decreases.RA1?RA2TemperatureResistance increases as temperatureincreases.RT1?RT2MaterialKeeping length,cross-sectional area,andtemperature constant,resistance varieswith the material used.Platinu
39、mIronAluminumGoldCopperSilverR increasesResistance and Ohms LawSuppose two conductors have a potential difference between them.Ifthey are connected with a copper rod,a large current is created.On theother hand,putting a glass rod between them creates almost no current.The property determining how mu
40、ch current will flow is called resistance.Table 22-1 lists some of the factors that impact resistance.Resistance ismeasured by placing a potential difference across a conductor and divid-ing the voltage by the current.The resistance,R,is defined as the ratio ofelectric potential difference,V,to the
41、current,I.The resistance of the conductor,R,is measured inohms.One ohm(1?)is the resistance permitting anelectric charge of 1 A to flow when a potential differ-ence of 1 V is applied across the resistance.A simplecircuit relating resistance,current,and voltage isshown in Figure 22-3.A 12-V car batte
42、ry is connectedto one of the cars 3-?brake lights.The circuit is com-pleted by a connection to an ammeter,which is adevice that measures current.The current carrying theenergy to the lights will measure 4 A.ResistanceR?VI?Resistance is equal to voltage divided by current.3 12 V4 A VI?R 12 V3 4 A?Fig
43、ure 22-3 One ohm,?,isdefined as 1 V/A.In a circuit with a3-?resistance and a 12-V battery,there is a 4-A current.Section 22.1 Current and Circuits595T1T2A1A2L1L2590-600 CH22-S1-845813 3/22/04 3:37 AM Page 595596Chapter 22 Current ElectricityAAA0.2 A0.1 A0.1 A30?30?60?6 V3 V6 VIII?Figure 22-4 The cur
44、rentthrough a simple circuit(a)canbe regulated by removing some ofthe dry cells(b)or by increasingthe resistance of the circuit(c).12 V12 V12 VSwitchBatteryMotorPotentiometer?SwitchBatteryMotorPotentiometerIIThe unit for resistance is named for German scientist Georg Simon Ohm,who found that the rat
45、io of potential difference to current is constant for agiven conductor.The resistance for most conductors does not vary as themagnitude or direction of the potential applied to it changes.A device hav-ing constant resistance independent of the potential difference obeysOhms law.Most metallic conduct
46、ors obey Ohms law,at least over a limited range ofvoltages.Many important devices,however,do not.A radio and a pocket cal-culator contain many devices,such as transistors and diodes,that do notobey Ohms law.Even a lightbulb has resistance that depends on its tem-perature and does not obey Ohms law.W
47、ires used to connect electric devices have low resistance.A 1-m lengthof a typical wire used in physics labs has a resistance of about 0.03?.Wires used in home wiring offer as little as 0.004?of resistance for eachmeter of length.Because wires have so little resistance,there is almost nopotential dr
48、op across them.To produce greater potential drops,a largeresistance concentrated into a small volume is necessary.A resistor is adevice designed to have a specific resistance.Resistors may be made ofgraphite,semiconductors,or wires that are long and thin.There are two ways to control the current in
49、a circuit.Because I?V/R,I can be changed by varying V,R,or both.Figure 22-4a shows a simplecircuit.When V is 6 V and R is 30?,the current is 0.2 A.How could thecurrent be reduced to 0.1 A?According to Ohms law,the greater the volt-age placed across a resistor,the larger the current passing through i
50、t.If thecurrent through a resistor is cut in half,the potential difference also is cutFigure 22-5 A potentiometercan be used to change current in an electric circuit.abcab590-600 CH22-S1-845813 3/22/04 3:37 AM Page 596Section 22.1 Current and Circuits597VAConductorGroundElectricconnectionSwitchFuseC
