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Physical Sciences Standards Preview Standard Set 1. Physical Sciences 1. Electricity and magnetism are related effects that have many useful applications in everyday life. As a basis for understanding this concept: 1.b. Students know how to build a simple compass and use it to detect magnetic effects, including Earth’s magnetic field. 1.c. Students know electric currents produce magnetic fields and know how to build a simple electromagnet. Genre Nonfiction Comprehension Skill Main Idea and Details • • • • 1.d. Students know the role of electromagnets in the construction of electric motors, electric generators, and simple devices, such as doorbells and earphones. 1.e. Students know electrically charged objects attract or repel each other. 1.f. Students know that magnets have two poles (north and south) and that like poles repel each other while unlike poles attract each other. 1.g. Students know electrical energy can be converted to heat, light, and motion. Text Features Captions Diagrams Labels Glossary Science Content Magnetism Scott Foresman Science 4.2 ISBN 0-328-23543-1 ì<(sk$m)=cdfedi< +^-Ä-U-Ä-U by Marcia K. Miller Vocabulary electromagnet generator magnetic field magnetic poles magnetism by Marcia K. Miller Picture Credits Illustrations 5 Peter Bollinger; 23 Tony Randazzo. Photographs Every effort has been made to secure permission and provide appropriate credit for photographic material. The publisher deeply regrets any omission and pledges to correct errors called to its attention in subsequent editions. Unless otherwise acknowledged, all photographs are the copyright of Dorling Kindersley, a division of Pearson. Photo locators denoted as follows: Top (T), Center (C), Bottom (B), Left (L), Right (R), Background (Bkgd). Opener: ©Johnny Johnson/Getty Images; 2 (B) ©Martin Bond/Photo Researchers, Inc.; 16 ©Sheila Terry/Photo Researchers, Inc.; 17 ©Jeremy Walker/Photo Researchers, Inc.; 19 (C) ©Clive Streeter/Courtesy of The Science Museum, London/DK Images; 22 (B) ©Philip James Corwin/Corbis. ISBN: 0-328-23543-1 Copyright © Pearson Education, Inc. All Rights Reserved. Printed in the United States of America. This publication is protected by Copyright, and permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to Permissions Department, Scott Foresman, 1900 East Lake Avenue, Glenview, Illinois 60025. 1 2 3 4 5 6 7 8 9 10 V010 13 12 11 10 09 08 07 06 What are magnetic fields? Magnetism Magnets are objects that attract iron, steel, and some other metals. All magnets have magnetism. Magnetism is a force that acts on moving electric charge. It also acts on magnetic materials that are near a magnet. How do magnets make a train seem to float above the tracks? Electric current flows through the tracks. It produces enough magnetic force to lift the train! Pushing and pulling forces also move the train down the track. Magnetic forces lift and move this train. It is called a maglev train. 2 Magnetic Field A magnet is surrounded by a magnetic field. It is the space around a magnet in which magnetic forces operate. A magnetic field reaches in all directions from a magnet. Its shape is related to shape of the magnet. Iron filings can show a magnetic field’s shape. The filings will align with a magnetic field’s lines of force. The lines of force curve out from a magnet’s ends. Magnetic Poles The two ends of a magnet are its magnetic poles. Every magnet has a north pole and a south pole. One pole points north. The other pole points south. A magnetic field is strongest at the magnetic poles. The field gets weaker as you move away from the poles. Iron filings show the shape of the magnetic field surrounding this magnet. 3 Fields, Forces, and Filings Look at the magnetic field of the bar magnet in the picture shown below. There are filings surrounding the magnet. Most of the filings are at the magnets’ poles. The pattern of filings around each kind of magnet is a bit different. But each pattern curves from one pole toward the other pole. How Magnetic Poles Behave The poles of a magnet act in the same ways that electric charges do. Like poles and like charges push each other apart. They repel. Unlike poles and unlike charges pull each other together. They attract. Place two magnets near each other. Try to put the two north poles together. You will feel their magnetic fields pushing apart. Now try to put a north pole near a south pole. The opposite poles attract each other. You can experience repelling and attracting with two magnetic sheets. These sheets are used to make magnets that look like cards. Each sheet is made up of narrow magnetic strips. If you slide one sheet over the other, you can feel them push apart and stick together. The like poles repel. The unlike poles attract. Broken Magnets What happens when you break a magnet in two? You get two magnets. Each new magnet will have both a north and a south pole. North and south magnetic poles are always paired. Each piece of a broken magnet has its own north pole and south pole. This bar magnet’s curved lines of force are closest together near its ends. 4 5 What are magnetic effects? Compasses and the World’s Biggest Magnet A compass needle always moves to point north. Christopher Columbus used a compass when he crossed the Atlantic Ocean more than 500 years ago. About 400 years ago, English scientist William Gilbert developed the idea that Earth is a gigantic magnet, with a magnetic field all around it. Why does Earth act like a magnet? Scientists think that Earth’s outer core is made of iron. The iron is a very hot liquid. As Earth spins, electric currents flow in the liquid iron. The flowing current creates a magnetic field. Earth’s inner core is probably solid iron that is also very hot. It stays solid due to very high pressure. Magnetic Minerals Some rocks and minerals are natural magnets. Magnetite, or lodestone, is highly magnetic. Pieces of magnetite will point north and south. In the past, sailors used lodestones as compasses. Earth’s Magnetic Poles Earth’s magnetic field is strongest at its poles. But Earth’s magnetic poles and its geographic poles are not the same. Earth’s geographic poles are found on its axis. The axis is the invisible line around which our planet rotates. Earth’s magnetic north pole is in Canada. It is about 1,000 kilometers (600 miles) from the geographic North Pole. Its magnetic south pole is in the ocean near Antarctica. Scientists think that Earth’s magnetic field is made by its outer core. A compass needle normally points north and south. The needle lines up with Earth’s magnetic field. 6 7 Compasses Earth’s Magnetic Field A compass is a tool people use to find directions. A compass needle is a tiny magnet. It can turn freely inside the compass. One end of a compass needle always points to Earth’s magnetic north pole. But why? Earth’s magnetic field attracts the needle. The lines of the field run north and south between the poles. A compass needle lines up with the magnetic field. Suppose you head east with a compass. You keep the N on the compass under the end of the needle that points north. Suddenly the needle swings away. Why? Any magnet can detect Earth’s magnetic field. Just hang the magnet so it can swing freely. The magnet will soon line up with Earth’s magnetic lines of force. Its ends will point north and south. But how do you know which pole points north and which points south? Compare your hanging magnet with a compass. Keep the compass far enough away from the magnet so it does not pull the compass needle. The north end of the magnet points the same way as the north end of the compass. Label that end of the magnet N. Now you can use it as a compass. A Magnetic Rock You backtrack. Soon you find a large rock that causes the compass needle to move. You found magnetite! The magnetite’s magnetic field was stronger than Earth’s. So the compass needle swung around. A donut magnet does not have ends. But its poles still line up with Earth’s magnetic field. Hang a magnet so it turns freely. Its poles will line up with Earth’s magnetic field. The bar magnet’s magnetic field causes the compass needle to swing. 8 9 How to Make a Compass Finding Your Way A compass can be made with a needle, a magnet, a bowl of water, and a sponge. First, rub the needle on the magnet. Rub it quickly and always in the same direction. Rubbing the needle on the magnet gives it a magnetic field. Then, put the needle on the sponge. Place the needle parallel to the water’s surface. Let the sponge float in a bowl of water. This lets the needle turn freely. Magnetized needles line up with Earth’s magnetic field. They point north and south. A real compass will show the direction of magnetic north. Magnetic north is the north end of the needle in a floating compass. How does a floating compass show directions? Label north, south, east, and west on the edges of the bowl. Turn the bowl so that the north end of the needle, or magnet, points to the “north” mark on the bowl. It is hard to use this kind of compass on a hike. But it works just like a real compass does. A piece of cork works just as well as sponge does for making a floating compass. A magnetized needle floats on a piece of sponge. It will point toward Earth’s magnetic north pole. 10 11 How are electric currents and magnetic fields related? In 1820, the Danish scientist Hans Christian Oersted was running electric current through a wire. A nearby compass’s needle moved each time he turned on the current. Oersted wondered why. He realized that the flow of electric current made a magnetic field. These pictures show how an electric current affects compasses. When the current flows, the compass needles line up along the lines of a magnetic field. When the current is not flowing, the compass needles point north. Moving Magnets Produce Electricity In 1831, British scientist Michael Faraday found that you can make an electric current by moving a magnet inside a wire coil. He had invented the dynamo. It uses a moving magnet to make electricity. The meter shown here measures electric current. A magnet sitting in the coil of wire does not seem to do much. The meter reading is zero. No current is flowing. What if the magnet is moved back and forth? The meter’s needle swings away from zero. The moving magnet makes electricity that flows through the meter. Electric charges in motion cause magnetism. They also cause electric current. So you can conclude that magnetism and electricity are related. When current is flowing, the compass needles line up with the magnetic field. The meter’s needle swings away from zero as the magnet is moved back and forth. 12 13 Electromagnets An electromagnet is a coil of wire with many loops. Electric current passes through the loops. The moving current creates a magnetic field. You can change the strength of an electromagnet. You can make its magnetic field stronger. One way is to put an iron or steel bar inside the coil of wire. You can also add more coils of wire. You can wrap the coils more tightly to make them closer together. You can connect more batteries or use a more powerful battery. The pictures on these pages show two electromagnets of different strengths. A more powerful battery makes more electric current. So its magnetic field is stronger. This battery makes a small amount of current, which makes a weak magnetic field. 14 Current flows through the coils that are attached to the batteries. It changes some energy to heat. The heat made by the currents changes the mat’s color. 15 Applications of Electromagnets If you have used a computer recently, then you have used an electromagnet. If you looked inside a computer, you would not see an iron bar with wire wrapped around it. But the computer’s hard drive uses electromagnets just the same. A bar with a coil of wire is the simplest type of electromagnet. Electromagnets can come in different sizes and shapes. They may be made of high-tech materials. But they can only make a magnetic field when electric current passes through them. This computer hard drive uses electromagnetic fields to save and call up files. Scrap Metal Lifter A scrap metal lifter is a giant electromagnet. It weighs far less than the metal it picks up. The lifter’s face is one pole of a bar magnet. This strong magnet needs special parts. The parts stop the magnetic field after the current stops flowing. 16 17 How are electromagnets used? Electromagnets in Sound Devices Electromagnets are found in electronic devices that make sound. Electromagnets in speakers form a magnetic field. The field changes as the amount of current changes. The changes cause motion, or vibrations. Vibrations make the sound waves you hear. Earphones You can also hear sound in earphones. With earphones, electric current comes through the wires. A metal disc and an electromagnet are inside each earphone. Changes in the current make the magnetism weaker or stronger. These changes make each metal disc vibrate. The vibrations are the sounds that you can hear. This speaker works in a similar way. magnet speaker cone How a Doorbell Works Pressing the button closes the circuit. This makes current flow to the transformer, which controls the amount of current. Current in the wire coil makes an electromagnet. The electromagnet pulls up a contact arm. The contact arm moves a clapper to hit the bell. electromagnet contact arm bell How a Speaker Works Electromagnets quickly turn on and off inside the speaker. The magnet pushes and pulls on the speaker cone. This makes the cone vibrate. The vibrations make sound waves. 18 19 Electric Energy Changing to Motion Going Around in Circles A motor changes electrical energy into mechanical energy. Mechanical energy is the energy that makes things move. Computers, CD and DVD players, and some toy cars have motors. The rotor is stopped from changing directions after it turns halfway. Why? Because one of the wires connected to one side of the rotor has had its insulation removed. But the wire connected to the other side of the rotor still has one side of its insulation. The insulation stops the flow of charges in the spinning coil. But before the coil stops spinning, the bare wire closes the circuit. This allows charges to flow again. So how does the motion of the motor turn into the spin of a fan or blender? Find the axle in the picture. The axle gets turned when the rotor spins. In some objects, the axle connects to something that can spin. These might be the blades of a fan or a blender. Can you picture where the blades might attach? Simple Electric Motor Look at the simple motor on page 21. It has a battery, a magnet, and a coil with seven loops. There are also parts that connect or hold the other parts together. The rotor is the coil of wire at the center. Its spins. The battery’s electric current makes a magnetic field in the rotor. The permanent magnet’s poles attract and repel the poles of the rotor’s magnetic field. This is what makes the rotor itself spin. The fan’s electric motor turns a rod called an axle. The axle moves the fan blades. The fan blades and motor spin at the same speed. Battery Rotor Permanent magnet Axle 20 21 How Generators Make Electricity A generator is a machine that turns coils of wire around very strong magnets. It changes motion into electrical energy. Huge generators make the electric power for most homes, buildings, and factories. They change the motion of moving water flowing through a dam into electrical energy. Or they change the wind that blows past blades into electrical energy. These forces of motion turn the coiled wire fields around each other. A wind farm is made up of rows and rows of wind turbines. A turbine is a machine that turns by the power of wind or water. A wind turbine has long blades. The turbine’s axle connects to an electric generator. California has over 13,000 wind turbines. The power they make can light San Francisco. Wind spins the turbine blades. An axle connects them to the generator. As the blades spin, gears make the generator spin much faster. More electricity is made as the generator spins faster. Wind farms are often located in narrow mountain passes with strong winds. The wind turns a rotor. The rotor is made of coils of wire around an iron core. The rotor spins inside the magnetic field of another magnet in the generator. As the rotor turns, an electric current is produced. 22 The generator is located on top of a pole. The pole can be 20 stories high. 23 Glossary What did you learn? 1. Where is a magnetic field strongest? electromagnet a coil of wire through which electric current passes, creating a magnetic field generator a machine that uses moving magnets to produce electrical energy magnetic field the space around a magnet in which magnetic forces operate magnetic poles the two ends of a magnet, called the north magnetic pole and the south magnetic pole magnetism 24 a force that acts on either a moving electric charge or a magnetic material that is near a magnet 2. What happens if you place one magnet’s south pole near another magnet’s north pole? 3. Who was William Gilbert? What idea did he develop? 4. Write a paragraph about electromagnets, based on what you read on pages 14 and 15. Then edit and revise the paragraph by adding information about electromagnets from pages 16 and 17. 5. Main Idea and Details What is the main idea on page 8? What details support this idea?