Electricity and Magnetism

Transcription

1 Electricity and Magnetism A Science AZ Physical Series Word Count: 1,668 Electricity and Magnetism Written by David Dreier Visit

2 Electricity and Magnetism Key elements Used in This Book The 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, conductor, direct current, electric current, electricity, electromagnet, electrons, generator, hydroelectric plant, insulator, ion, lines of force, magnetic field, magnetism, north pole, nucleus, power plant, protons, repel, resistance, shock, south pole, static electricity, transformer, turbine, volts, watt Key comprehension skills: Identify facts Other 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 questions Other suitable reading strategies: Connect to prior knowledge; summarize; visualize; retell 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 8: istockphoto.com/clint Spencer; page 9: Image Source/ Corbis; page 10 (top left): istockphoto.com/vinicius Ramalho Tupinamba; page 10 (top right): istockphoto.com/john Scott; page 10 (bottom): 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: 3desc/ Dreamstime.com; page 16 (top): istockphoto.com/matthew Cole; page 16 (bottom): 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 AZ/Doug Tepper; page 21 (left): istockphoto.com/ekspansio; page 21 (right): istockphoto.com/robert Dupuis; page 22: Eric Brow/Dreamstime.com Illustration Credit: pages 57,14: Learning AZ Written by David Dreier Electricity and Magnetism Learning AZ, Inc. Written by David Dreier All rights reserved.

3 Table of Contents Introduction... 4 What Is Electricity?... 5 The Two Kinds of Electricity... 8 Static Electricity... 8 Electric Current Measuring Electricity What Produces Magnetism? Magnetism and Electric Currents Introduction Producing Electricity Steam Water Wind Delivering Electricity Electricity and Magnetism in Today s World Glossary Index Have you ever watched lightning during a big storm at night? Did you know that a lightning bolt is a natural form of electricity? But most electricity is made by people. When you turn on a light, you are using electricity made by people. Electricity has become a very important part of our lives. It s hard to imagine the world without it. Magnetism is also a familiar part of our world. Perhaps you have played with magnets. Magnetism is closely related to electricity. This book will teach you about electricity and magnetism. And it will explain how they are connected.

4 What Is Electricity? Electrons Circle the Nucleus You just learned that electricity can be produced naturally. Or it can be made by people. But what is electricity? It is a form of energy. Energy is what makes things happen. For example, the energy in sunlight makes plants grow. electron proton / 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. / / / / / neutron Charges of the same kind repel each other. That means they push each other away. Charges that are opposite attract each other. So two protons or two electrons repel each other. But a proton and an electron are attracted to each other. The number of protons and 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. / / Electrons quickly orbit, or move in a circle around, an atom s nucleus. / / / Electricity comes from atoms. Atoms are very tiny particles. Everything in the world is made of atoms. Inside each atom are even smaller particles. They include ones called protons and electrons. Those particles have a feature called electric charge. Protons have a charge that scientists call positive. Electrons have an opposite Particles Have Charges charge, called electron negative. These charges are the proton source of electricity. / neutron 5 6

5 An atom can gain an electrical charge. It can do that by getting extra electrons. Or it can do it by losing electrons.atoms can gain or lose electrons when they contact other atoms. If an atom gets extra electrons, it has a negative charge. If it loses electrons, the protons will outnumber the electrons, so the atom will have a positive charge. An atom that has a charge is called an ion. An atom with more electrons than protons is a negative ion. An atom with more protons than electrons is a positive ion. positive and negative ions Positive Ion electron The Two Kinds of Electricity There are two kinds of electricity. They are static electricity and electric currents. Static Electricity Static electricity is caused when particles of opposite charge get separated. The two kinds of particles are attracted to each other. When the attraction becomes strong enough, the particles quickly stream back together. Lightning is caused in a storm cloud when particles of opposite charge build up in different areas. When the groups of charges get large enough, they flow rapidly back together. That causes a lightning flash. proton neutron Negative Ion Positive ions have more protons than electrons. Negative ions have more electrons than protons. Lightning that strikes the ground actually starts as a flow of charge from the ground up to the cloud. 7 8

6 Lightning heats the air around it to about 30,000 C (54,000 F). That s five times hotter than the surface of the Sun! Lightning also causes the air to expand violently. That is what causes thunder. Most lightning occurs up in the clouds. But sometimes the separation of charges happens between the ground and the clouds. The particles then flow down to the ground. This is the kind of lightning that strikes buildings and trees. Electric Current An electric current is the flow of electrons through a material. Materials that carry a current are called conductors. Metals are the best conductors, so electrical wires are made of metal. Most wires that are used to carry an electric current are made of copper. Materials that do not conduct electricity are called insulators. Rubber and plastic are both good insulators. Most electrical wires are wrapped in rubber. The rubber prevents shocks. You can make a spark 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, a spark will jump from your finger. Rubbing a balloon on your hair can create a similar effect as scuffing shoes on the carpet insulators insulators conductors conductors Some tools have plastic or rubber handles to prevent electric shocks.

7 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. closed circuit direct current alternating current open circuit With the circuit open, or broken, electrons can no longer flow, so the lightbulb does not light up. An electric current pushes against a force in a wire. That force is called resistance. Resistance causes a loss of energy. The lost energy is released as heat 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. Both kinds of current create a flow of electrical energy. That flow moves very fast. When electrons start moving in a circuit, energy fills the entire circuit right away. This explains why a lamp lights up as soon as you flip a switch.

8 Measuring Electricity Electrons need a push to get moving in a wire. This push is called voltage. Voltage is measured in units called volts. In most cases, the more volts an energy source has, the greater the push and flow of energy. Most flashlight batteries have just 1.5 volts. House wiring is about 120 volts. The flow of current in a circuit is measured in units called amperes. These units are usually called amps for short. The more electrons that are in motion in a wire, the higher the number of amps. Another measure used in electricity is the watt. It measures how fast electrical energy is being used. 1.5 volts 12 volts What Produces Magnetism? As you have learned, electricity and magnetism are related. They are the two parts of a force called electromagnetism. Magnetism comes from the electrons in atoms. Electrons spin like little tops. And every electron has an invisible area of magnetism around it. This area is called a magnetic field. In some metals, the spinning electrons turn each atom into a tiny magnet. Those atoms can be made to line up in the same direction. Then the entire piece of metal becomes magnetic. Most magnets are made of iron. non-magnetized magnetized N 9 volts Batteries and outlets have different volt levels that reflect how much power they can deliver. 220 volts / 110 volts Some metals, such as iron, can be magnetized. A magnetic field forms around the magnet. S 13 14

9 S N N S Opposite poles attract. S S You can see a magnet s field. Put the magnet under a piece of paper and sprinkle flecks of iron on the paper. The iron bits will arrange themselves in lines of magnetic force. lines of force N N Magnetism and Electric Currents Like poles repel. Magnets are strongest at their ends. These ends are called poles. Like Earth, a magnet has a north pole and a south pole. If you cut a bar magnet into two halves, each piece is a new magnet. Both of the new magnets have a north and south pole. A magnet s poles behave like positive and negative electrical charges. 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. A magnet is also strongly attracted to iron and steel. That is because its magnetic field creates a magnetic pull on the atoms in the metal. A magnet can be made with wire, an electric current, and an iron bar. A wire carrying current has a small magnetic field around it. When a wire with current is coiled up, the magnetic field gets stronger. If the wire is wrapped around an iron bar, the bar becomes a magnet. This type of magnet is called an electromagnet. An electromagnet only works when current is moving through the wire. If the current is turned off, the magnetism stops. What would happen to the paper clips if the wire weren t touching the battery? 15 16

10 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 is used to provide light and power for homes, offices, and factories. These generators inside a dam use water to spin giant magnets. The electricity we use every day is produced at power plants. These places use huge generators to make AC current. Electromagnets in the generators spin inside coils of wire. Three main energy sources spin the magnets. They are steam, rushing water, and wind. Steam Most power plants produce electricity by boiling water to make steam. There are several ways they do this: Nikola Tesla supported using AC current to deliver electricity over long distances. He also helped invent radios. by burning coal or oil by releasing the energy in atoms by using focused sunlight

11 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. 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. Water Some power plants are part of large dams. A dam stops the flow of a river. Then a big lake forms behind the dam. Water from the lake rushes through openings at the bottom of the dam. The water spins the blades of turbines. The turbines are connected to magnets in a generator. The spinning turbine blades make the magnets spin. Wind The power of wind can also be used to make electricity. This is done with machines called wind turbines. They look like huge airplane propellers. The turbines transfer their energy to a generator. The blades of some wind turbines are as long as a truck.

12 Delivering Electricity Current leaving a power plant has to go a long way before it gets used. It is passed through a large device called a transformer. The transformer greatly increases the voltage of the current. The extra volts push the electricity long distances through cables. The electricity in the cables is too strong to be used in homes and factories. The voltage must be reduced first. This is done by using another kind of transformer, far from the power station. This transformer lowers the voltage to about 120 volts. Electricity and Magnetism in Today s World For thousands of years, people had no electricity. Work was done with the muscles of people and animals. Candles and oil lamps provided light. Magnetism was something in unusual rocks called lodestones. Lightning was a mystery. No one understood electricity or magnetism. And they had no idea that the two things are related. But today, electricity and magnetism are well understood and widely used. Magnets are used in computer hard drives, stereo speakers, credit cards, and many other devices. And as you just learned, large magnets are used at power plants to generate electricity. Electricity powers most of the things we use in our everyday lives. 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.

13 alternating current (AC) amperes (amps) atoms charge circuit conductors direct current (DC) electric currents electricity electromagnet Glossary an electric current in which electrons move rapidly back and forth (p. 12) a measure of the amount of current in a wire (p. 13) the smallest parts of an element (p. 5) the property of matter that causes it to be electrically positive or negative, caused by losing or gaining electrons (p. 5) a closed path along which an electric current travels (p. 11) materials, usually metals, that transmit electricity (p. 10) an electric current in which electrons move in one direction (p. 12) the movement of electrons through matter (p. 8) a form of energy made when tiny parts move around in an atom; energy that can power many devices (p. 4) 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 ion magnetic field magnetism neutrons protons static electricity volts watt atoms, 57, 14, 15, 18 electrons, 57, 914 neutrons, 57 protons, 57 generators, 1720 materials, such as rubber and plastic, that do not transmit electricity (p. 10) an atom that has gained or lost electrons and has an electrical charge (p. 7) an area around a magnet where magnetic force can be felt (p. 14) a force that pushes and pulls certain metals (p. 4) particles in the nucleus of an atom that have no electrical charge (p. 5) particles in the nucleus of an atom that have a positive electrical charge (p. 5) electricity caused by a buildup of negative charges in one place and positive charges in another (p. 8) a measure of the amount of push that gets an electric current moving (p. 13) a measure of the rate at which electrical energy is being used (p. 13) Index lightning, 4, 8, 9, 22 poles, 15 power plants, 18, 19, 21, 22 turbines, 19, 20