this - g-w · of their advantages is that they can be recharged. dry cells supply a comparatively...

The multimeter is one of the mostuseful tools in electrical troubleshoot-ing. It can be used to measure resistance, amperage, and voltage.

11Electricity and

Magnetism

Key Terms

alternating currentamperagebatterycellcircuitconductordirect currentdistribution lineelectric currentelectromagnetelectromotive force

electrongenerating stationgeneratormagnetismprimary cellsecondary celltransformertransmission linevoltagevoltaic cell

ObjectivesAfter reading this chapter you will be able to:

� Describe the different ways electricity can be produced.

� List the advantages and disadvantages of each method of generating electricity.

� Describe the principal components of a network for the transmission and distribution of electricity.

� Explain the nature of electricity by referring to the movement of electrons.

� State the laws of magnetism.

� Explain how the laws of magnetism are used in the generation of electricity.

� Describe how an electric motor operates.

� Explain what type of electrical current a battery produces and from what other form of energy is electricity produced.

� Design and make a product that uses electrical control.

This sample chapter is for review purposes only. Copyright © The Goodheart-Willcox Co., Inc. All rights reserved.

284 Technology: Shaping Our World

Imagine your town or city withoutelectricity. It can happen. At seven-teen minutes after five o’clock in

the afternoon of November 10, 1965,the lives of 30 million people weresuddenly interrupted.

� 800,000 riders were trapped in the New York subway.

� All nine television channels in themetropolitan area of New York were forced to go off the air.

� Kennedy International and LaGuardia airports were shut down and airplanes found themselves circling, unable to land.

� 5,000 off-duty police officers were summoned to duty.

� 10,000 National Guardsmen were called up to help protect the city.

� Militiamen were alerted in RhodeIsland and Massachusetts.

� Broadway theaters and movie houses were closed.

� Thousands of people hiked across the Brooklyn and Queensboro bridges.

� Highways were jammed with traffic for more than five hours.

� Thousands of New Yorkers were trapped in elevators in the city’s skyscrapers.

In less than 15 minutes the powerfailure spread across more than 49,000square miles (about 128,000 km2). NewYork state, New England, and parts ofNew Jersey, Pennsylvania, Ontario,and Quebec had no electricity.

It was the largest power failure inhistory. The first signs of troubleappeared at the power company’s con-trol center. An operator noticed prob-lems on the center’s interconnectingsystem with upstate power companies.By then, the blackout was only secondsaway. It was too late for action. Thedemands for reserve power went sohigh that automatic switches shut thesystem down to protect it.

In our daily lives, we take elec-tricity for granted. To most people, itis merely something that alwaysarrives at the home. Only when it isgone do we realize how we dependon it, Figure 11-1.

Generating ElectricityThe electrical energy supplied to

your home comes from a generatingstation. A generating station usesenergy from a source of power to turnturbines, which produce electricity.Themaking of electricity is called generating.

Figure 11-1 How would a blackout thisevening affect you?

There are several types of gen-erating stations. They are named after the power source used. Remem-ber the names: hydroelectric andthermal-electric.

Hydroelectric Generating Stations

“Hydro” is another word for water.Hydroelectric generating stations usethe energy of flowing or falling water.The station is located at a waterfall orat a dam, Figure 11-2. As the waterdrops to a lower level, its mass spins aturbine, Figure 11-3. A turbine is afinned wheel. When the falling waterstrikes the fins, the turbine turns rap-idly.The turbines are connected to gen-erators. A generator is a device thatproduces an electric current as it turns.

Thermal-ElectricGenerating Stations

Thermal-electric generating sta-tions use steam to drive turbines. Aheat source produces the steam. Thesteam is directed onto the blades of aturbine. The turbine spins rapidly. Asin hydroelectric systems, the turbinesdrive generators. The spinning gener-ators produce electricity.

Heat for powering thermal-electricturbines comes from one of twosources. The first is by burning fossilfuels, Figure 11-4. Fossil fuels comefrom once-living animals and plants.They include coal, oil, and natural gas.The second source is nuclear fission.Fission is the splitting of uranium

Chapter 11 Electricity and Magnetism 285

Figure 11-2 Dams are built to storewater. (TEC)

Figure 11-3 This is the turbine room of ahydroelectric generating station. (TEC)


atoms. The process releases enor-mous amounts of heat. In a nuclearstation, Figure 11-5, the nuclearreactor does the same job as the fur-nace in fossil-fuel stations.

Any device that changes one formof energy to another is called a con-verter, Figure 11-6. Hydroelectric gen-erating stations change the potentialenergy of water behind a dam. As itfalls into the turbine, it becomes kineticenergy. Thermal-electric generating


stations convert the heat energystored in fossil fuels and uranium intokinetic energy. In both cases, the kineticenergy is converted to electrical energyby generators.

Each method of generating electricityhas advantages and disadvantages.Look at Figure 11-7. If a generatingstation had to be built near your home,which would you choose? Why?

Most of the electricity used inhomes and factories is produced either

Figure 11-4 This is a simple diagram of a system for generating electricity with fossil fuels. (AEC)

Figure 11-5 Here is a diagram of a nuclear power station. Splitting atoms, rather than burn-ing fuels, creates the heat. (AEC)


in hydroelectric or thermal-electric generating stations. There are, how-ever, other methods. Friction, chemicalaction, light, heat, and pressure alsogenerate electricity, Figure 11-8.

Transmission andDistribution ofElectricity

Generating stations are rarelyfound close to where the electricalenergy is used. The electricity thatcomes to your home may have trav-eled a great distance.

After leaving the generating sta-tion, the electricity is fed into a network of transmission lines and distribution lines. These lines

transport the electricity to wherever itis needed, Figure 11-9.

The transmission lines, Figure 11-10,resist the flow of electrical energy.Thus, some of the energy is lost alongthe way. Increasing voltage and reduc-ing amperage greatly reduces thisloss. Voltage is a measure of electricalpressure. Amperage measures theamount of current.

A transformer changes voltageand amperage. It consists of a pair ofcoils and a core.

Neither your home nor a factorycan use electricity at the high voltagecarried by transmission lines.It would destroy the wiring, appli-ances, and machines. The voltagemust be reduced before currententers distribution lines. Once again, a

Figure 11-6 All generating stations are energy converters.

The electricity is transported byhigh voltage transmission lines

Turbine turnsa generator

andelectricity

is produced

Steam or waterspins a turbine

Heat

Potential energy(water behind dam)

Heat energy(heat from burning fossil

fuels or from nuclearfission)

Kineticenergy

Electrical energy

Th

erm

al-e

lect

ric

Hyd

roel

ectr

ic

Water

Dam

Water is turnedto steam


transformer is used. The first drop involtage occurs when transferringelectrical energy to distribution lines,Figure 11-11. Another reductionoccurs when transferring electrical

energy from distribution lines to ser-vice lines. This transformer may belocated on a pole or on the ground,Figure 11-12. Electricity enters yourhome through a service line. It also


Advantages Disadvantages

Hydroelectric Cheapest method overall.Station Most environmentally safe method.

Cheap to operate — raw material is free.Low maintenance and operational costs.No harmful combustion products.No harmful wastes of any kind.Water used is not polluted.Flow of water and, therefore, the amount

of electricity generated is adjustable.

Fossil-Fuel Uses fuel that is often available locally or Station can be easily transported.

Small generators can be built to supply local needs.

Nuclear Large amounts of electricity can beStation generated using a small amount of

material.Can be built wherever there is a supply of

water for cooling.No acid rain is created.

Sites are normally a long way from cities so long wires are needed to bring the electricity to the consumers.

Transmission towers are unsightly.Large amounts of land are taken up by

transmission corridors.Transmission lines emit electromagnetic

radiation that may be a health hazard.Dams disrupt rivers and, therefore, the

marine life.The reservoir behind the dam covers a

large expanse of land, thereby displacing people and animals.

Uses a nonrenewable resource.Oil-fired thermal plants are becoming too

costly and must be converted to natural gas or coal.

Coal is bulky, heavy, costly to move, and dirty.

Burning coal produces a large amount of ash. Rain that penetrates ash heaps or buried ashes will pollute streams or groundwater.

Harmful particles and gases are released into the air and combine with water vapor in the air to form acid rain that damages trees, lakes, and buildings.

Mining coal is a dangerous occupation.Oil extracted from under the sea may

sometimes leak into the seawater.

Uses a nonrenewable resource.Radioactive nuclear wastes must be

disposed of.Deep burial sites must be found for waste

that will remain radioactive for thousands of years.

Mining the uranium fuel is expensive; it is also hazardous to miners who are exposed to cancer-causing gas.

Reactors are expensive to build and maintain.

A reactor could overheat and release radioactive substances into the environment.

Figure 11-7 There are three types of electricity generating stations.Which type would you select?

Friction causes static electricity.After walking across a carpet on a dry day, you become electrically charged. If you touch a grounded object, the static electricity will discharge (create a spark).

An acid or salt solution, called an electrolyte, removes electrons by chemical action from one piece of material and deposits them in another.Wet cells are used in cars and other vehicles. One of their advantages is that they can be recharged.Dry cells supply a comparatively small amount of electrical power and are used in a variety of portable electrical devices.

A small electric charge will be generated if the ends of two wires are twisted together and heated. This is the principle of a thermocouple.Commercial thermocouples use unlike metals welded together. They do not supply a large amount of current and cannot be used to produce electric power. They are used as heat indicated devices.

A small electric charge will be generated if quartz is placed between two metal plates while pressure is applied.One application is an electronic lighter of the type used for lighting gas grills.

A generator uses magnetism to produce electricity.In an electric power generating station, generators are run by turbines. Turbines receive power from moving water or from a powerful jet of steam.

The photovoltaic cell is a sandwich of three layers: the outside layers are translucent, the inside layer is iron with a disk of selenium alloy between the two. When light is focused on the selenium, an electric charge develops between the selenium and the iron. Examples of use are automatic headlight dimmers and portable solar-powered calculators.A second way of using light to produce electricity is called photoconduction. A common application of this principle is the control of street lights that come on automatically when daylight fades. Light energy applied to a material that is normally a poor conductor, causes free electrons to be released in the material so it becomes a better conductor.

Method Application Discussion

Solar powered calculator

Light

ThermocoupleHeat

Barbecue lighter

Pressure

Generator

Magnetism

Wet cell battery

Dry cellsChemical

Person pulling off a sweaterFriction

Figure 11-8 There are many ways to produce electricity.




passes through a meter and a mainswitch, Figure 11-13.

How Transformers WorkThere are two types of transformers.

Step-up transformers increase voltage,and step-down transformers reduce it.

Basically, a transformer consistsof two coils of wire around a core.One coil has more turns than the

Figure 11-10 Transmission lines are carriedby triangular towers. Insulators support thelines where they attach to the tower. (TEC)

Figure 11-11 Distribution substationsstep down the voltage. (TEC)

Generating station Step-uptransformer

120 and 240 V12,000 V

Pole transformer

Distribution lines

Step-downtransformer

at substation

13,800 V

Transmission lines 230,000 V

480 or 600 V

Figure 11-9 Shown here is an electrical power transmission and distribution system.Voltage is greatly increased before the electricity is transmitted over long distances.

other. Figure 11-14 shows theconstruction of step-up and step-down transformers.

What Is Electricity?When you flip a light switch, elec-

tricity flows through wires and lights abulb. What exactly is it that flowsthrough the wire when the switch isturned on? There is no perfectanswer. A scientist would say thatelectric current consists of a flow ofelectrons. What does this mean?

One explanation is called the elec-tron theory. Everything around us ismade from very small particles calledatoms. Atoms are made of even smallerparticles called protons, electrons, andneutrons. The protons have a positivecharge, and the electrons have a neg-ative charge. Neutrons have no chargeand play no role in electricity.

An atom normally has the samenumber of electrons as protons. Thenegative and positive charges canceleach other. Such atoms are electricallyneutral, Figure 11-15.

In metals, electrons can detachthemselves from the nucleus.Therefore,metals can conduct electricity becausethe electrons that are detached arefree to move. When a metal wire isconnected in a circuit, the free outerelectrons can all be pushed in the samedirection.This flow of electrons is calledan electric current, Figure 11-16.

Suppose that electrons are made toflow from one end of a wire to the other.The end that loses electrons becomespositively charged—the positive ter-minal. The end that gains electronsbecomes negatively charged—the

Figure 11-12 A pole transformer furtherreduces the voltage in the distribution lines.(TEC)

Figure 11-13 Electricity enters your homethrough a service line, a meter, and a mainswitch. (TEC)




Why are electrons able to movethrough a wire? There are two reasons.

� A force pushes them along a path.

� There is a closed path, called acircuit, in which they can move.

Think of the force as like theforce created by a pump pushingwater through pipes. In a circuit, anelectromotive force (EMF) pusheselectrons through a conductor. Wealso call this force voltage.

Just as we cannot say what anelectron really is, we cannot describean electromotive force. However, it ispossible to describe some of thedevices that are capable of producingelectromotive force.

The two most common sources ofEMF are generators and chemical reac-tions. Generators employ magnetismand mechanical energy to produceelectricity, and dry cells and batteriesuse chemical reactions to producedirect current electricity.

negative terminal. See Figure 11-17.Electrons can move through the wire,which acts as a conductor.A conductoris a material that will allow an electriccurrent to flow easily. For example,when free electrons pass into the fila-ment (thin wire) in a lightbulb, colli-sions between electrons and atomsare more frequent. This increases thetemperature of the wire, causing thewire to become white-hot and emitlight.

Highvoltage

lowamperage

Highvoltage

lowamperage

Lowvoltage

highamperage

Lowvoltage

highamperage

1000 V200 A

500 V400 A

1000 V200 A

500 V400 A

A step-up transformer. When the input voltage is connected to the coil with the least number of turns, the output voltage is increased.

A step-down transformer. When the input voltage is connected to the coil with the greatest number of turns, the output voltage is decreased.

Figure 11-14 The “inside” of step-up andstep-down transformers helps explain howthey work.

Electrons

NucleusNeutrons

Protons

Figure 11-15 Protons and neutrons makeup the nucleus of the atom. Electrons orbitaround the nucleus.


These two sources provide mostof the electricity that we use. Othersources of EMF are friction, light,heat, and pressure. (Look back atFigure 11-8.)

Magnets and MagnetismThe production of electricity depends

upon magnets and magnetism.Magnetism is the ability of a materialto attract pieces of iron or steel.

Materials that are attracted bymagnets are called magnetic materials.Among them are iron, steel, and nickel.Magnets do not attract nonmagneticmaterials, such as aluminum, copper,glass, paper, and wood, Figure 11-18.Magnets fall into three differentgroups: natural magnets, artificialmagnets, and electromagnets.

Natural MagnetsNatural magnets, such as lode-

stone, occur in nature. Lodestone is ablackish iron ore (magnetite). Its weakmagnetic force varies greatly fromstone to stone.

Artificial MagnetsArtificial magnets, also called per-

manent magnets, are made of hard

Figure 11-16 The movement of electrons occurs when a force is applied to one end of awire.The free electrons move from one atom to the next.This process is repeated along thewhole length of the wire.This causes what we call an electric current.

Figure 11-17 The terminal with a surplus ofelectrons is called negative.The terminal witha scarcity of electrons is called positive.

Electricity and Magnetism

Magnet

Steel nailsattracted

Coppernailsnot

attracted

Figure 11-18 Which materials are mag-netic and which are nonmagnetic?



and brittle alloys. Iron, nickel, cobalt,and other metals make up the alloys.The alloys are strongly magnetizedduring the manufacturing process.

Artificial magnets come in manyshapes and sizes. The most com-mon are horseshoe magnets, bar mag-nets, and those used in compasses,Figure 11-19. Natural and artificialmagnets can retain their magnetismindefinitely.

ElectromagnetsElectromagnets are so named

because they are magnetized by anelectric current.They consist of two mainparts. One is a core of special steel,while the other is a copper wire coilwound on this core, Figure 11-20. Unlikepermanent magnets, electromagnets

can be turned on or off. Their magneticforce can be completely controlled.

How Magnets ActFigure 11-21 shows a bar magnet

suspended from a loop of thread.Held this way, the magnet twists untilit is lined up in a north-south direc-tion. The end that points toward thenorth is called the north-seeking pole.The end that points toward the southis called the south-seeking pole.

Suppose that two magnets aresuspended so that the north pole ofone is brought close to the south poleof the other. The magnets attract oneanother, Figure 11-22. This is the firstlaw of magnetism. It states that unlikemagnetic poles attract each other.

Suppose that the north poles comeclose to one another. The magnetspush or move away from each other.

Figure 11-20 An electromagnet can beturned on and off.

N

N

N

Compass NeedleHorseshoeBar

Figure 11-19 There are three commonartificial magnets.

South-seeking

pole

North-seeking

pole

N

E S

W N

S

Figure 11-21 The north pole of a barmagnet that is free to swing will alwayspoint north.

This is the second law of magnetism.It states that like magnetic poles repelone another, Figure 11-23.

Lines of ForceUnlike poles attract, and like poles

repel. This suggests that around amagnet there are invisible lines offorce. Although you cannot see themthey can be shown to exist. Place asheet of paper over a magnet andsprinkle iron filings on the paper.When the paper is tapped gently, thesmall iron particles form a distinct pat-tern, Figure 11-24. The lines of forceshown by the iron filings take theshape shown in Figure 11-25.

Now suppose that two magnetsare laid end to end, and the experi-ment with iron filings is repeated. Thelines of force demonstrate the twolaws of magnetism, Figure 11-26.

Magnetism and Electric Current

An electric current passed througha wire also creates a magnetic field

NS

Figure 11-22 Unlike magnetic poles attract.

SS

Figure 11-23 Like magnetic poles repel.


Figure 11-24 Iron filings show the lines ofmagnetic force around a magnet.

N S

Figure 11-25 Note the pattern and direc-tion of magnetic lines of force.


around the wire.Magnetism produced bythis means is called electromagnetism.This principle is used to make theelectromagnet in Figure 11-20.

WARNING: The demonstrationshown in Figure 11-27 should only bedone by your teacher. A carbon-zinccell should be used. NEVER use analkaline cell. It may explode.

If the wire in Figure 11-27 iswound to form a coil, it becomes amagnet with poles, Figure 11-28. Themagnetic strength of this coil can becontrolled. It depends on the strength


of the current and the number of loopsin the coil.

If a wire is coiled around a core ofmagnetic material, such as a soft ironnail, the nail becomes an electromag-net. It remains strongly magnetic onlyas long as there is current in the wire,Figure 11-29.

The Generation ofElectricity UsingMagnetism

So far, you have learned that electric current is the flow of electrons

Ironfilings

Touchfor

only asecond1.5 V

Figure 11-27 To produce a magnetic field,connect a wire to a dry cell.This will makethe wire a magnet without poles.

Insulated wireIron

filings

Iron core

1.5 V

Figure 11-29 Here is a simple electro-magnet.The nail remains magnetized as longas there is current in the coil.

N S

Figure 11-28 Coiling the wire shown inFigure 11-27 creates a magnet with poles.

Like poles repel

Unlike poles attract

N

S S

S

Figure 11-26 Try this experiment withiron filings and two magnets.


in a circuit. Causing electrons to flow is called “generating electricity.”Electrical energy is not created. It isconverted from other forms of energy.

A generator is the most practicaland economical method today of pro-ducing electricity on a large scale. Ituses magnetism to cause electrons to flow.

To see this in action, connect alength of copper wire to a milliammeter.As shown in Figure 11-30, move partof the wire loop through a magneticfield. A small current flows while thewire is cutting across the magneticfield.

The strength of the currentdepends on two things. One is thestrength of the magnetic field. Theother is the rate at which the lines offorce are cut. The stronger the mag-netic field or the faster the rate atwhich the lines of force are cut, thegreater the current.

The direction of electron flowdepends on the direction in which thelines of force are cut. Look atFigure 11-30 again. When the wiremoves down through the lines of forceof the magnet, electrons flow in onedirection. When the wire moves up,electrons flow in the opposite direction.

The end that loses electronsbecomes positively charged. The endthat gains electrons becomes nega-tively charged.

Alternating CurrentAlternating current (AC) is elec-

tron flow that reverses direction on aregular basis. It is the type of currentyou use in your home. It is the type ofcurrent produced by power stations.

How is it produced? This willbecome clear as the basic operationof a generator is explained.

Figure 11-31 shows a simple gen-erator. It is no more than a loop ofwire turning clockwise between thepoles of a magnet. Remember whatwas explained earlier. Current is pro-duced only when a wire cuts throughlines of magnetic force.

Now refer once more toFigure 11-31. With the loop (wire) inposition A, no lines of force are cut.Thegenerator produces no current. As theloop continues turning, it reaches posi-tion B. At this point, one side of the loopmoves downward through the lines offorce. At the same time, the other sideof the loop is moving up through thelines of force. Because the wire is aclosed loop, current travels through it inone direction.

As the loop reaches position C,half a revolution is completed. As in A,there is no current. Why? No lines offorce are being cut.

The loop continues to turn. Itreaches position D.The two sides oncemore cut lines of force. There is a dif-ference, however.The side that moveddownward before is now movingupward. Likewise, the side that moved

Movement of wire

Movementof wire

N

NS

S

Figure 11-30 Moving a wire loop through amagnet creates a small current in the wire.



upward before is now moving down-ward. What happens? The electronflow reverses. Because the directionof flow alternates as the loop turns,the current produced is called alter-nating current.

Electricity produced by the gener-ator must have a path along which itcan flow. You know the path as a cir-cuit. Therefore, the terminals (ends)of the loop must always be in contact(in touch) with an outside wire. Thisoutside wire is stationary. The contactis made with slip rings and brushes.

A separate slip ring is perma-nently fastened to each terminal ofthe wire loop. Each slip ring turns withthe loop. A brush is placed against

each slip ring. As the slip rings turn,the brushes maintain rubbing contactwith them. The wire forming the sta-tionary part of the circuit is attachedto the brushes. Electrical devices,such as a lightbulb, are connected to the external part of the circuit,Figure 11-32.

Current produced in the loop of thegenerator flows from the generatorthrough a slip ring and brush into theexternal circuit. It travels in the externalcircuit through the electrical device.Then it returns to the generatorthrough the other brush and slip ring.

As already noted, the direction ofelectron flow keeps changing or alter-nating. About 90% of the electricity

N S

S

N S

N SN

A 0A

B 10A

C 0A

D -10A

Figure 11-31 This is a basic AC generator. Note that an ammeter is connected across theterminals of the wire loop.At positions B and C, the loop or wire is cutting across lines offorce to create current in the wire.The current moves through the wire into the ammeter.At B, it is going one direction.At D, current reverses. Notice that when the current isreversed the current is read as negative current. In reality, AC current is changing so quicklythat a negative voltage or current cannot be read.


produced in the world today is alter-nating current. It is easier to generatein large quantities than direct current.Even more importantly, it is easier totransmit from one place to another.

In North America, with few excep-tions, alternating current makes 60complete cycles each second. A cycleis a flow or pulse in one direction anda pulse in the opposite direction. Inmany European countries, the alter-nating frequency is 50 cycles per sec-ond. Cycles are given in Hertz ratherthan cycles per second. A Hertz isequal to one cycle per second.

Generators at any of the large gen-erating stations are more complex thanthe simple loop generator shown in thischapter. However, their basic principleis the same. Generated current can beincreased in the following two ways:

� Increasing the rate at which the lines of force are cut.

� Strengthening the magnetic field.Therefore, many loops of wire are

used instead of one. Powerful electro-magnets supply the magnetic field.

For practical reasons, the loops aremounted around the inner surface ofthe generator housing. They remainstationary and are called the stator.Theelectromagnets are mounted around arotating shaft.This assembly is called arotor. It is placed inside the stator. Inthis way, current is created by havinglines of force cutting across conductorsinstead of by having a conductor cut-ting across lines of force, Figure 11-33.

Direct CurrentDirect current (DC) is current that

does not change direction in an exter-nal circuit. The direct current generatoruses a single, split ring. It replaces thetwo slip rings of an alternating currentgenerator. Current in the loop still alter-nates. However, the split ring, called a

Slip ring

Brush

N S

Stationaryor outside

circuit

Figure 11-32 Look at this simple genera-tor and its external circuit.What wouldhappen if one end of the wire becomesdetached from its brush?

Figure 11-33 Look at this large commer-cial generator.Which part is the rotor andwhich the stator? (Transalta)


commutator, sends current only oneway through the circuit. The brushesand commutator of a DC generator areshown in Figure 11-34.

Each half of the commutator isattached to one of the wire loop’s terminals. As the current changesdirection, the rotating commutatorswitches the terminals from one

brush to the other every half revolu-tion. Figure 11-35 shows a small DC generator.

Direct current is used in portableand mobile equipment, such as flash-lights and car accessories. It is alsoused in electronic and sound reproduc-tion equipment. A disadvantage of DCcurrent is that it is difficult to transmitover long distances.

Electric MotorsIn many ways, an electric motor is

like a generator. However, while thetwo have similar parts, their purposesare different. A generator convertskinetic energy to electrical energy. Anelectric motor changes electricalenergy to kinetic energy.

Both a generator and an electricmotor apply the laws of magnetism, andboth contain magnets and a rotatingcoil of wire. The coil of wire of an elec-tric motor is placed in a magnetic field,Figure 11-36. The motor spins when acurrent is applied to the coil of wire.

What makes an electric motor run?Electrons flowing through the coil of

Terminal 2

N N SS

Terminal 1Brush

A B

BrushCommutator Commutator

Terminal 2

Figure 11-34 This figure explains how a commutator produces direct current.A—At thispoint of rotation, terminal 1 is contacting the brush connected to the negative side. B—Halfa turn later, as current changes direction, terminal 1 is in contact with the brush connectedto the positive side. Current through the external circuit continues in the same direction.


Figure 11-35 A bicycle’s light uses directcurrent supplied by the small generator.What advantage does this generator haveover a light operated by a battery? (Ecritek)

The split copper ring is called acommutator, Figure 11-37. Currentpasses into and out of the coil throughbrushes that press against the com-mutator. In this way, current alwayspasses down on the right side andback on the left side of the coil. Theeffect of this is to switch the poles inthe coil’s magnetic field. The rotationthen continues in one direction.

The brushes also serve a secondpurpose. Since they do not rotate,they prevent the wires from twisting.

Brushes are usually made fromcarbon. It is a good conductor andproduces less friction than metal. Thebrushes are spring-loaded. Pressurefrom the spring ensures continuouscontact with the commutator.

Cells and BatteriesWhat most of us call a battery is

not a battery at all. It is really a cell.The energy source for a car, however,is rightly called a battery. It is made upof several cells. How they differ andhow they work are explained in the fol-lowing sections.

Voltaic CellA cell has a single positive elec-

trode, a single negative electrode,and an electrolyte. A battery is a pack-age containing several cells together.Cells are classified as either primaryor secondary.

The simplest of cells is the voltaiccell. Two rods, one copper and one zinc,are immersed in a container filled with asolution of water and sulfuric acid. (Themixture is known as an electrolyte.) Theacid attacks and corrodes both of themetals, Figure 11-38.

wire of an electric motor cause a mag-netic field around the coil. Rememberthe laws of magnetism? Unlike polesattract, and like poles repel. When cur-rent is introduced in the coil, the coil’smagnetic field reacts with the magnetsin the motor. The coil spins as it iseither attracted or repelled by themotor’s permanent magnets.

Unfortunately, the coil spins foronly part of a turn from the effects ofmagnetism. The rotation would stopexcept for the effects of a split copperring that rotates with the coil. For aninstant, the current stops and the coilcoasts. When current starts up again,magnetic force keeps the coil turning.


Currentsuppliedby circuit

N S

Figure 11-36 A simple electric motor ismuch like a generator.

Commutator

Carbon brush

Spring

Figure 11-37 Brushes of an electric motorare always rubbing against the spinning com-mutator.This allows the electricity to flowinto the commutator and into the coil.



Some of the atoms from the met-als pass into the solution. Each atomleaves behind a pair of electrons.However, the zinc rod tends to loseatoms to the solution faster than thecopper rod. Since the zinc rod buildsup more electrons than the copperrod, it becomes negative. If the twoelectrodes are connected by a con-ductor, excess electrons tend to flowalong the conductor from the zinc tothe copper. This flow of electrons pro-duces an electric current, which willilluminate a lightbulb. Since this flowof electrons is in one direction only,cells and batteries produce DC voltage.

Primary CellA primary cell is one whose elec-

trode is gradually consumed duringnormal use. It cannot be recharged.Primary cells are used in flashlights,digital watches, and cameras.

The primary cells used todayemploy the same principles as thevoltaic cell in Figure 11-38. There aremany different types of primary cells.They all have three main parts: theelectrolyte and two electrodes. Theelectrolyte is usually a very activechemical such as an acid or an alkali.(Acids are compounds that react with abase, such as metal. An alkali is a sub-stance capable of neutralizing acids.)Inside the battery, Figure 11-39, twochemical reactions take place. One isbetween the electrolyte and the nega-tive electrode (cathode). The other isbetween the electrolyte and the posi-tive electrode (anode).These reactionschange the chemical energy stored inthe cell into electrical energy. When thechemicals have all reacted, the cell has

+ Terminal

Carbon electrode(positive)

Zinc electrode(negative)

Electrolyte paste

- Terminal

Figure 11-39 This cross section of a drycell shows how it provides chemical storageof electricity.

Zincelectrode

Copperelectrode

Sulphuric acid and water

Glasscontainer

Figure 11-38 Notice on this voltaic cellthat the zinc electrode is negative and thecopper electrode is positive.


no chemical energy left, and it can giveno more electricity.

Carbon-Zinc CellsThe carbon-zinc cell is the most

common primary cell. It is also the leastexpensive, but it is short-lived. Carbon-zinc cells are produced in a range ofstandard sizes, Figure 11-40. Theseinclude 1.5 V AA, C, and D cells, aswell as 9 V rectangular batteries. Six1.5 V cells are put together to form a9 V battery, Figure 11-41.

Alkaline CellsAlkaline cells are produced in the

same sizes as carbon-zinc cells.However, they can supply current longer.

Mercury CellsBoth the carbon-zinc and alkaline

cells are much too large for some uses,such as digital watches, hearing aids,calculators, and miniature electronicequipment. For these applications,mercury cells are used, Figure 11-42.Mercury cells develop 1.34 V.

Secondary CellsA secondary cell, Figure 11-43, is

one that can store electrical energy fedinto it. Then, as needed, the electricitycan be drawn from the cell in the formof an electric current. In other words, itcan be recharged time and time againwhen the electrical energy is used.Lead plate electrodes are placed in asolution of sulfuric acid. A current pass-ing through the lead plates produceschemical changes. The sulfuric acidsolution gets stronger and the cellbecomes capable of producing an

electric current. This is called charginga cell. When charged, the cell can pro-duce a current in a circuit.

As electricity is drawn from thecell, the chemical change that tookplace during charging reverses.However, the materials in the cell arenot used up; they are only changed.Therefore, the entire process can be repeated.

Each pair of electrodes in a sec-ondary cell can produce about 2 V.

Figure 11-40 These are typical primarycells. Shown from left to right are AA, C, D,and 9 V sizes. (TEC)

Figure 11-41 This cutaway of a 9 V batteryshows that six 1.5 V cells are connected.What other sizes (voltages) of batterieshave you used? (TEC)


Most motor vehicles require 12 V tooperate the starter motor. Therefore,six pairs of electrodes, or cells, mustbe connected together. Figure 11-44shows a number of cells connectedtogether to form a battery commonlycalled a lead-acid battery. These


Negative terminal

Positive terminal

Figure 11-42 Mercury cells are smallerand shaped differently from carbon-zinc andalkaline cells. (TEC)

12 V

2 V 2 V 2 V 2 V 2 V 2 V

Figure 11-44 A modern automobile battery is made by connecting six cells toproduce 12 V.

Generator

Charging Discharging

Sulphuricacid more

concentrated

Figure 11-43 Here is a typical secondarycell. Charging—The cell can store energyfed into it during charging. Discharging—Asneeded, the stored energy discharges andproduces an electric current.

batteries are used to power lights,radios, and all accessories in mostcars. Rechargeable power packs arepopular as emergency power andlight sources. They can jump-startcars, inflate tires, run power tools, beused on campsites, and power back-seat televisions or video games. Apower pack can also provide a stablepower source in emergency medicalsituations and in locations such asthe International Space Station (ISS).

Recently, there has been a lot ofinterest in electrically powered vehi-cles, which help to reduce air pollutionin urban centers. Lead-acid batteriescan be used, but they only provideenough energy for a short travel distance of 50 miles (80 km) beforethey need to be recharged. A nickel-cadmium battery gives slightly betterperformance and an increased range,up to 60 miles (100 km). Nickel-metalhydride is currently the highest per-forming battery with more than twicethe power and range of a lead-acidbattery. Other batteries in experimen-tal phase include lithium-ion, lithiummetal-polymer and ZEBRA batteries.

Chapter 11 ReviewElectricity and

Magnetism

305

SummaryMost of the electricity we use is produced at hydroelectric or

thermal-electric generating stations. Other ways to produce elec-tricity include friction, chemical action, light, heat, and pressure.

Electric current consists of a flow of electrons. Electrons movebecause an electromotive force pushes them. This force is pro-vided by a generator or by chemical change in a dry cell or battery.

Most electricity is produced by generators using the principlesof magnetism. An electric current flows in a wire if it is movedthrough a magnetic field. The current generated may be alternat-ing or direct.

An electric motor is similar to a generator. However, a gener-ator converts kinetic energy to electrical energy; an electric motorchanges electrical energy to kinetic energy.

Small amounts of electricity may be stored in cells or batteries.Primary cells are consumed gradually and cannot be recharged.Secondary cells can be recharged.

The information in this chapter provides the required founda-tion for the following types of modular activities:

� Technological Systems

� Electrical Systems

� Electronic Systems

Modular Connections



Write your answers to these review questions on a separatesheet of paper.

1. If the electricity supply were cut off at 6:00 p.m. tonight, how would your community be affected?

2. What is the meaning of the word “hydro” in the phrase“hydroelectric generating station?”

3. Compare hydroelectric generating stations and thermal-electric generating stations by stating:

A. In what way they are similar.

B. In what way they are different.

4. After each entry below, describe where in a transmission and distribution system you would find the voltages listed.

A. 230,000 V.

B. 13,800 V.

C. 240 V.

D. 120 V.

5. A transformer can increase or decrease _____ and _____.

6. Electric current may be described as _____.

7. Name the three different groups of magnets.

8. What is the main difference between a permanent magnetand an electromagnet?

9. State the two laws of magnetism.

10. The most practical and economical method of producingelectricity is _____.

11. If a wire is moved through a magnetic field created by ahorseshoe magnet, _____ are caused to flow.

12. The alternating frequency in North America is _____ cycles per second (Hertz).

13. Describe the difference between alternating current anddirect current.

14. How is the purpose of an electric motor different from that ofa generator?

15. What are the disadvantages of most dry cells?

Test Your Knowledge


16. How does a dry cell produce electrical energy?

17. Over a period of a year a portable stereo system uses a large number of dry cells (often referred to as batteries). In order to reduce the amount of money you spend on power, what kind of cells could be used and why?

18. Describe the difference between a dry cell and a battery.

19. To produce 24 V, a lead acid battery needs _____ cells.

Apply Your Knowledge1 Where and how is the electricity used in your home

produced?

2. Make a model to illustrate one method of generating electricity.

3. Describe the components of a network for the transmission and distribution of electricity. How many of these components can you see in your neighborhood?

4. Make sketches with notes to illustrate (a) an atom and (b)electron flow.

5. Describe how the laws of magnetism are used to generateelectricity.

6. Repeat the experiment illustrated in Figure 11-24. Iron filingscan be made by cutting steel wool into tiny pieces using anold pair of scissors. You can fix the pattern of iron filings inplace using hair spray.

7. Make a sketch to show how you would make an electromagnet.

8. List five objects in your home that use an electric motor.

9. Research one career related to the information you havestudied in this chapter and state the following:

A. The occupation you selected.B. The education requirements to enter this occupation.C. The possibilities of promotion to a higher level at a later date.D. What someone with this career does on a day-to-day basis.You might find this information on the Internet or in your library. If pos-

sible, interview a person who already works in this field to answerthe four points. Finally, state why you might or might not be inter-ested in pursuing this occupation when you finish school.


this - g-w · of their advantages is that they can be recharged. dry cells supply a comparatively...

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