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A Science A–Z Physical SeriesWord Count: 1,878

KEY ELEMENTS USED IN THIS BOOKThe Big Idea: Since the late 1800s, electricity has brightened our homes and streets, powered our appliances, and enabled the development of computers, phones, and many other devices we rely on. Understanding what electricity is and how it becomes ready for our safe use helps us appreciate this energy source. Meanwhile, without magnets, we couldn’t generate electricity. Electricity and magnetism, and the relationship between the two, are fundamental to the workings of the modern world.Key words: alternating current, amperes, atoms, attract, charge, circuit, compass, conductor, direct current, electric current, electricity, electromagnet, electromagnetism, electrons, generator, hydroelectric plant, insulator, ion, lines of force, magnetic field, magnetism, neutrons, north pole, nuclear power plant, nucleus, permanent magnet, power plant, protons, repel, resistance, shock, static electricity, temporary magnet, transformer, turbine, volts, watts

Key comprehension skills: Identify factsOther suitable comprehension skills: Compare and contrast; classify information; cause and effect; elements of a genre; interpret graphs, charts, and diagrams; using a glossary and boldfaced terms; using a table of contents and headings

Key reading strategy: Ask and answer questionsOther suitable reading strategies: Connect to prior knowledge; summarize; visualize; retell

Electricity and Magnetism © Learning A–Z, Inc. Written by David Dreier

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www.sciencea-z.com

Written by David Dreier

www.sciencea-z.com

Photo Credits: Front cover: © iStockphoto.com/Giles Angel; back cover: © Ron Giling/PhotoLibrary; title page: © Dannyphoto80/Dreamstime.com; page 3: © iStockphoto.com/Mark Stay; page 4: © Jupiterimages Corporation; page 9 (top): © iStockphoto.com/Clint Spencer; page 9 (bottom): © Image Source/Corbis; page 10 (top left): © iStockphoto.com/Vinicius Ramalho Tupinamba; page 10 (bottom left): © iStockphoto.com/John Scott; page 10 (right): © iStockphoto.com/Yunus Arakon; page 11: © GIPhotoStock/Photo Researchers, Inc.; page 12 (left): © iStockphoto.com/Viacheslav Krisanov; page 12 (right): © iStockphoto.com/Pixhook; page 13 (left): © iStockphoto.com/DSGpro; page 13 (center left): © iStockphoto.com/Nasen Mann; page 13 (center right): © iStockphoto.com/Hywit Dimyadi; page 13 (right): © iStockphoto.com/Gordon Dixon; page 15 (top): © iStockphoto.com/Matthew Cole; page 15 (bottom): © 3desc/Dreamstime.com; page 16: © Michael Newman/PhotoEdit; page 17: © Mary Evans Picture Library; page 18: © iStockphoto.com/Billy Gadbury; page 19: © iStockphoto.com/Leif Norman; page 20 (top): © Aschwin Prein/Dreamstime.com; page 20 (bottom): © Learning A–Z/Doug Tepper; page 21 (left): © iStockphoto.com/Ekspansio; page 21 (right): © iStockphoto.com/Robert Dupuis; page 22: © Eric Brow/Dreamstime.com

Illustration Credit: pages 5–7,14: Learning A–Z

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Introduction

Have you ever watched flashes of lightning during a big storm at night? You might be surprised to know that lightning is a natural form of electricity. In the last two hundred years, people have learned how to make and use their own electricity. Whenever you turn on a light, you are using electricity. Computers and televisions can’t work without electricity, either. Electricity has become such an important part of our lives that it is hard to imagine the world without it.

Magnetism is also a familiar part of our world. Perhaps you have played with magnets or have magnets on your refrigerator door. Did you know that magnetism is closely related to electricity? This book will teach you about electricity and magnetism, and how they are related.

Table of Contents

Introduction .............................................................. 4

What Is Electricity? .................................................. 5

The Two Kinds of Electricity .................................. 8Static Electricity ...................................................... 8Electric Current .................................................... 10

Measuring Electricity ............................................. 13

What Produces Magnetism? ................................. 14

Magnetism and Electric Currents ........................ 16

Producing Electricity ............................................. 17Steam ..................................................................... 18Water ..................................................................... 19Wind ...................................................................... 20

Delivering Electricity ............................................. 21

Electricity and Magnetism in Today’s World ............................................... 22

Glossary ................................................................... 23

Index ........................................................................ 24

5 6

What Is Electricity?

You just learned that electricity can be produced naturally or by people. But what is electricity? It is a form of energy. It comes from very tiny particles called atoms. Atoms are the smallest bits of elements, the basic substances that everything is made of. Inside each atom are even smaller particles called protons, neutrons, and electrons.

Protons and neutrons make up an atom’s center, called the nucleus. Electrons whirl around the nucleus. Protons and electrons have a feature called electrical charge. This feature is the source of electricity. Protons have a charge that scientists call positive. Electrons have an opposite charge, called negative. Neutrons are neutral, meaning they have no charge.

Charges of the same kind repel each other. So two protons repel each other, and so do two electrons. Charges that are opposite attract each other. So a proton and an electron are attracted to each other. However, electrons do not get pulled into the nucleus, even though they are attracted by the protons. That is because they are moving so fast.

The number of positive protons and negative electrons in an atom is usually equal. Because the charges are equal in number, they cancel each other out. The atom is neutral, meaning it has no charge.

++

+

+

+ +

–

–

–

–

–

–

+/–

+/–+/–

+/–

+/–+/–

Particles Have Charges

In diagrams, protons have + signs on them, and electrons have – signs on them to show their charges. Neutrons have +/– because they have no charge. This carbon atom has six electrons, six protons, and six neutrons.

Electrons quickly orbit, or move in a circle around, an atom’s nucleus.

+

+

+++

+

–

–

– –

–

–

+/–

+/–

+/–+/–

+/–

+/–

electron

electron

proton

proton

neutron

neutron

Electrons Circle the Nucleus

87

Atoms can gain or lose electrons by contacting other atoms. If an atom gains electrons, it is no longer neutral. It now has a negative charge. If an atom loses electrons, the protons outnumber the electrons. The atom now has a positive charge. An atom that has a charge is called an ion. An atom that has more electrons than protons is a negative ion. An atom that has more protons than electrons is a positive ion.

The Two Kinds of Electricity

There are two kinds of electricity. They are static electricity and electric currents. Both kinds of electricity involve the production and movement of charges. But they act in very different ways.

Static Electricity

Static electricity is caused by a buildup of negative charges in one place and positive charges in another. When the attraction between the separated charges becomes strong enough, the particles quickly stream back together.

Lightning is caused by static electricity. In storm clouds, strong winds cause a buildup of electrons and positive ions. The charges collect in different parts of the cloud. When the collections of charges get big enough, they rapidly flow back together. This heats the surrounding air and causes a lightning flash. Most lightning occurs inside clouds or between clouds.

–

++

+

++

+

–

–

–

–

–

–

Negative Ion

Positive ions have more protons than electrons.

Negative ions have more electrons than protons.

+ ++

+

++

+

–

– –

–

–

Positive Ion electron

proton

neutron

POSITIVE AND NEGATIVE IONS

A lightning bolt heats the air around it to about 30,000° C (54,000° F). That’s five times as hot as the surface of the Sun! The burst of energy creates a bright flash of light. At the same time, the lightning bolt causes the air to expand violently. That is what produces thunder.

9 10

Sometimes the ground and the clouds have strong, opposite charges. When this happens, a charge flows down to the ground. This charge creates the lightning bolts that strike buildings and trees.

You can make a tiny lightning bolt by scuffing your shoes on a carpet when the air is dry. Doing that causes your body to pick up electrons from the carpet. Your body now has a negative charge. If you put your finger near a metal object—or even another person—that has a neutral or positive charge, a spark will jump from your finger.

Electric Current

An electric current is the flow of electrons through a material. Materials that carry a current are called conductors. The atoms of a conductor have electrons that are only loosely attached. Those electrons can easily move to other atoms. Their movement causes an electric current. Metals are the best conductors.

Materials that do not conduct electricity are called insulators. The electrons of an insulator are tightly bound to the atoms, so they do not help electric currents flow. Rubber and plastic are both good insulators. For this reason, most electrical wires are encased in rubber. The rubber prevents shocks.

Rubbing a balloon on your hair can create a similar effect as the carpet.

+

+

–

–

Lightning that strikes the ground actually starts as a flow of charge from the ground up to the cloud.

insulators

insulators

conductors

conductors

Some tools have plastic or rubber handles to prevent electric shocks.

closed circuit

open circuit

–

–

–

–

–

–

11 12

An electric current needs a complete path in which to move. This path is called a circuit. If a circuit is broken, the current stops flowing. A circuit needs to be made of a good conductor, such as metal wire.

You have learned that there are two kinds of electricity. There are also two kinds of electric current. They are called direct current (DC) and alternating current (AC). Batteries use DC. The wall outlets in your home use AC. The electrons in a DC circuit always move in the same direction. The electrons in an AC circuit move rapidly back and forth.

In both kinds of current, the movement of electrons creates a flow of electrical energy. That flow moves very quickly. When an electric current begins to move in a circuit, energy fills the entire circuit right away. This explains why a lamp lights up the instant you flip a switch.

An electric current pushes against a force called resistance. The more resistance a circuit has, the more electrical energy is lost by changing into heat energy. Some electronics get hot when they’re on. That’s mostly due to resistance in the circuits inside.

alternating currentdirect current

13 14

Measuring Electricity

Electrons don’t just move through a circuit by themselves. To get moving, they need a push. This push is called voltage. Voltage is measured in units called volts. In most cases, the greater the number of volts an energy source has, the greater the push and flow of energy. Most flashlight batteries have a voltage of just 1.5 volts. Car batteries produce 12 volts. House wiring is about 120 volts.

The movement of electricity through a circuit is measured in units called amperes, or amps for short. The more electrons that are in motion in a wire, the higher the amperage is. Another common measure used in electricity is the watt. It is a measure of the rate at which electrical energy is being used.

Batteries and outlets have different volt levels that reflect how much power they can deliver.

What Produces Magnetism?

As you learned earlier, electricity and magnetism are related. They are the two parts of a force called electromagnetism. Electrons spin like little tops, and they have an invisible area of magnetism around them. This area is called a magnetic field. In some metals, like iron, the spinning electrons turn each atom into a tiny magnet. Those atoms can be made to line up in the same direction. When that happens, the entire piece of iron becomes magnetic. The more atoms that line up inside a piece of iron, the stronger the magnet will be.

Some metals, such as iron, can be magnetized. A magnetic field forms around the magnet.

non-magnetized

magnetized

+

+

++

++

+

+– –

– –

––

–

–

+ + + ++ +

++

––

N

S–

––––

–

1.5 volts

9 volts

12 volts

220 volts / 110 volts

15 16

Opposite poles attract. Like poles repel.

You can see a magnet’s field. Place the magnet under a piece of paper and sprinkle iron filings around it. The iron filings arrange themselves along the field’s lines of force. lines of force

N N N

NS S S

S

Magnets are strongest at their ends. These ends are the north and south poles. If you cut a bar magnet in half, each piece is a complete magnet with a north and south pole.

A magnet’s poles behave like positive and negative charges. Remember that two positive or two negative charges repel each other. And a negative charge is attracted to a positive charge. In the same way, two south or two north poles repel each other. But the north pole of a magnet is attracted to the south pole of another magnet.

What would happen to the paper clips if the wire weren’t touching the battery?

Magnetism and Electric Currents

A magnet can be made with an iron bar, some wire, and an electric current. This type of magnet is called an electromagnet. A wire carrying current has a small magnetic field around it. When a wire carrying current is wound into a coil, the magnetic field gets concentrated. If the coiled wire surrounds an iron bar, the bar becomes a magnet.

An electromagnet is magnetic only when current is moving through the wire. For that reason, it is also called a temporary magnet. People use electromagnets when they want to be able to turn a magnet on and off. The magnets you use in school are probably permanent magnets. Their magnetism is always on.

Producing Electricity

Now you know how electricity can be used to make a magnet. But did you know that magnets can also be used to make electricity? Scientists learned how to do this in the 1800s. They discovered that if a coil of wire is moved through a magnetic field, an electric current flows through the wire. The same thing happens in reverse when a magnet is moved through a coil of wire.

Scientists used that discovery to build machines called electrical generators. Cities, once mostly dark at night, became bright with electric lighting. Today, generators with large, powerful magnets are used to produce electricity. This important form of energy provides light and power for homes, offices, and factories.

17 18

These generators inside a dam use water to spin giant magnets.

Nikola Tesla supported using AC current to deliver electricity over long distances. He also helped invent radios.

The electricity we use every day is produced at power plants. The plants use huge generators to make AC electricity. In most of these generators, large electromagnets spin inside coils of wire. Three main energy sources spin the magnets: steam, rushing water, and wind.

Steam

Most power plants produce electricity by boiling water to make steam. Three of the most widely used methods are:

• burning coal, oil, or natural gas

• splitting uranium atoms to release the energy in the atoms

• focusing sunlight.

Wind

The power of wind can also be used to make electricity. Wind turbines, which look like huge airplane propellers, transfer their energy to a generator.

19 20

The steam produced in power plants spins large blades in a machine called a turbine. The turbine is connected to the magnets in a generator. As the turbine blades spin, they make the magnets spin. The moving magnets produce an electric current.

Water

Some power plants are part of large dams. They are called hydroelectric plants. Hydro- comes from the Latin language and means “water.”

A dam creates a big lake behind it. In a hydroelectric plant, water rushes through openings at the bottom of the dam. The water spins the blades of turbines. The turbines make magnets spin inside wire coils in a generator. This movement produces electricity.

The largest dam in the world is the Three Gorges Dam in China. This hydroelectric dam is 185 meters (606 feet) high and 1,983 meters (6,500 feet) wide.

The blades of some wind turbines are as long as a truck.

21 22

Delivering Electricity

When the current leaves the power plant, it passes through a large device called a transformer. The transformer greatly increases the voltage of the current. All these volts are used to “push” electricity long distances through large cables. The cables are strung between tall metal towers.

Before the current can be used in homes and factories, the voltage must be reduced. This is done by another kind of transformer, far from the power station. This transformer lowers the voltage to 120 volts before electricity enters homes, factories, and offices.

Electricity and Magnetism in Today’s World

For thousands of years, people got along without using electricity. Work was done with muscle power. Candles and oil lamps provided light. Magnetism was just a feature of unusual rocks that people called lodestones. Lightning was a mystery. No one understood electricity or magnetism. And they had no idea that electricity and magnetism are related.

But today, electricity and magnetism are well understood and widely used. As you just learned, power plants use large magnets to generate electricity. Computer hard drives, stereo speakers, credit cards, and many other devices also use magnets. Our understanding of electricity and magnetism has allowed us to create huge amounts of electric current. That electricity powers most of the things we use in our everyday lives. Even many automobiles now use electric motors. More than ever, we depend on electrical energy.

Large cables called power lines can carry electricity hundreds of miles to transformers that reduce the voltage for use.

All cars use magnets and electricity. Some use electricity instead of gas.

23 24

Glossary

alternating an electric current in which current (AC) electrons move rapidly back

and forth (p. 12)

amperes (amps) a measure of the amount of current in a wire (p. 13)

atoms the smallest parts of an element (p. 5)

charge the property of matter that causes it to be electrically positive or negative, caused by losing or gaining electrons (p. 5)

circuit a closed path along which an electric current travels (p. 11)

conductors materials, usually metals, that transmit electricity (p. 10)

direct an electric current in which current (DC) electrons move in one direction

(p. 12)

electric currents the movement of electrons through matter (p. 8)

electricity a form of energy made when tiny parts move around in an atom; energy that can power many devices (p. 4)

electromagnet a magnet that can be turned on or off and is made by sending electricity through metal (p. 16)

electromagnetism a combined force of electricity and magnetism (p. 14)

electrons particles in an atom that orbit the nucleus and have a negative electrical charge (p. 5)

insulators materials, such as rubber and plastic, that do not transmit electricity (p. 10)

ion an atom that has gained or lost electrons and has an electrical charge (p. 7)

magnetic field an area around a magnet where magnetic force can be felt (p. 14)

magnetism a force that pushes and pulls certain metals (p. 4)

neutrons particles in the nucleus of an atom that have no electrical charge (p. 5)

protons particles in the nucleus of an atom that have a positive electrical charge (p. 5)

static electricity electricity caused by a buildup of negative charges in one place and positive charges in another (p. 8)

volts a measure of the amount of push that gets an electric current moving (p. 13)

watt a measure of the rate at which electrical energy is being used (p. 13)

Index

atoms, 5–7, 10, 14, 18 electrons, 5–10, 12–14 neutrons, 5–7 protons, 5–7generators, 17–20

lightning, 4, 8, 9, 22poles, 15power plants, 18, 19, 21, 22turbines, 19, 20