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2 Photo/Illustration Credits: Cover: Photodisc/Getty Images; 2 Dorling Kindersley/ Getty Images; 3 C Squared Studios/Photodisc/Getty Images; 6 HMH; 7 SuperStock RF/SuperStock; 8 Erich Schrempp/Photo Researchers, Inc.; 9 Scott Speakes/Corbis; 10 CreativeAct-Technology Series/Alamy; 11 Stockbyte/Jupiterimages; 12 Tim Graham/ Getty Images; 13 Martin Bond/Photo Researchers, Inc.; 14 Kevin Phillips/Getty Images; 15 William R. Sallaz/Getty Images. If you have received these materials as examination copies free of charge, Harcourt School Publishers retains title to the materials and they may not be resold. Resale of examination copies is strictly prohibited and is illegal. Possession of this publication in print format does not entitle users to convert this publication, or any portion of it, into electronic format.

3 Copyright by Harcourt, Inc. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopy, recording, or any information storage and retrieval system, without permission in writing from the publisher. Requests for permission to make copies of any part of the work should be addressed to School Permissions and Copyrights, Harcourt, Inc., 6277 Sea Harbor Drive, Orlando, Florida Fax: HARCOURT and the Harcourt Logo are trademarks of Harcourt, Inc., registered in the United States of America and/or other jurisdictions. Printed in Mexico ISBN ISBN

4 Every day, without thinking much about it, we use some kind of electric current. An electric current is a steady stream of charges. The current moves through something, usually a wire, and connects to the object we use.

5 We control the electric current around us to make our lives easier. Electric current is used to provide both light and heat and to operate machines. Lamps, toasters, and televisions all need electric current to operate. An electric current follows a path that is called a circuit. A circuit has no beginning and no end. It has to be complete, or closed, for electricity to work. If the circuit is broken, or open, the electric current cannot go through the wire. Think of the things we turn on each day. We turn on computers, CD players, and air conditioners. We turn on machines that light up, give heat, make things move, and make sounds. Every time we push a power button or a switch, we are closing an electrical circuit, allowing current to go through wires.

6 Some closed circuits are series circuits. In a series circuit, the current only follows one path. Another kind of circuit is a parallel circuit. In a parallel circuit, the current can follow more than one path. You have probably seen strings of lights used for decoration. When stringed lights were first made years ago, they were made as series circuits. If one light bulb went out, every light bulb in the string went out. The flow of charges became blocked. Today these kinds of lights are made using parallel circuits. If one light goes out, the other lights stay lighted. In a parallel circuit, there is more than one path for the current to flow through. If a light bulb is broken or burned out, the charges moving along one path can take another. Musical speakers are another example of an electrical system that can be series circuits or parallel circuits. When speakers are on a series circuit, if one stops working, the other also fails. If they are on a parallel circuit, one may work when the other one does not work.

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8 You have learned that some materials, such as metal, carry heat better than others. Another property of materials is how well they carry electricity. Materials that allow electric charges to flow through them easily are called conductors. Conductors are used to carry electricity in appliances and machines. In the same way that metal is a good conductor of heat, it is also a good conductor of electricity. The wires behind your wall that carry electricity to lights and appliances are made of metal, usually copper. The tips of electrical cords that plug into the wall are also metal. When they connect to the metal wire, electricity flows easily.

9 Even though it is important for electricity to be carried from one place to another, electric current is not safe for people to touch. So how do we use electricity and make it safe for people? We use materials called insulators. Insulators do not allow electric current to flow easily through them. A good example of an insulator is rubber. Plastic is also a good insulator. Electrical wires are not bare they have coatings of plastic or rubber, so we can touch them without getting burned. When electric objects break, electric tape may be used. Electrical tape is made of insulating material and is used to keep electrical connections safe.

10 A magnet is an object that attracts iron, steel (which has iron in it), and a few other metals. You can test to see if something is a magnet by holding it near a steel paper clip. If the paper clip moves toward the object, it is a magnet. Also, you can test an object to see if it has iron or steel by holding it near a magnet. If the object doesn t move toward the magnet, it is not made of iron or steel. A magnet s pull becomes stronger as it gets closer to metal objects. If you hold it far away from the object, it will not work as well. Also, some magnets are weaker than others. Sometimes you can use a magnet to make other objects magnetic. If you take a nail and rub it against a magnet, it becomes a temporary magnet.

11 Magnets have two ends called magnetic poles at which its force is strongest. The poles are called N (north-seeking) and S (southseeking). Opposite poles (N and S) will move toward each other. They attract. Like poles (N and N or S and S) will move away from each other. They repel. Sometimes you can feel the like poles pushing apart.

12 The space around a magnet where magnetic forces act is called a magnetic field. Electric currents, which you have already read about, also make a magnetic field. A single wire that carries current has a weak magnetic field. The magnetic field of many wires bound together is strong. If you take wire coils and wrap them around a solid piece of iron, the inside piece the core becomes an electromagnet. An electromagnet is a temporary magnet that is made by an electrical current. Regular magnets are always on, but an electromagnet works only when there is electric current. We use electromagnets to lift heavy items like cars. Electromagnets are also used in switches, telephone receivers, and bells.

13 Not only can an electric current make a magnetic field, but a magnetic field can be used to produce electricity. If you put a wire coil near a magnet, electricity moves through the wire coil. Generators are machines that produce electric current in this way. Many different energy sources can move the coil inside generators to produce electricity. Oil, coal, and gas are used as energy sources. People can also be used! A person can turn a crank and produce enough energy to turn the coil of a generator.

14 Some objects have potential energy. Potential energy is energy that exists because of an object s position or condition. Suppose a tree is about to fall. It has potential energy. What happens when the tree falls? Then it has kinetic energy the energy of motion. Whenever an object gets warmer, gets cooler, or moves, energy is being changed from one form to another.

15 Another important source of energy is heat from deep under Earth. This energy is stored in hot water located miles below Earth s surface. It can be brought to the surface and used to generate electricity for use in different situations. Direct sunlight can provide the electricity to run many devices. Some people use sunlight to supply all the electricity in their home. Energy from the sun can be gathered by large panels on roofs and used to provide electricity, heat, and light. Other sources of energy are nuclear power and the wind. The wind s energy can be captured by windmills.

16 Electricity is an important form of energy. We use it to light our homes. It helps keep buildings warm and cool. We use it to heat water for showers or cooking. We also use it to keep food cold and fresh. Businesses, like homes, use electricity for light and heat. We use electricity to run machines such as computers. In factories, we use machines to put together other machines, to freeze ice cream, or to cut wood. In addition to electricity, there are other forms of energy that are very important to us. One of these is chemical energy. Chemical energy is energy stored in the arrangement of particles of matter. We use the chemical energy that comes from gasoline, coal, and natural gas.

17 Chemical energy can be changed into other forms of energy, such as heat and light. Electric generating plants burn chemical energy and change it into heat. The heat is then used to make electricity. When car engines burn gasoline, they turn chemical energy into mechanical energy energy that moves a machine. Other kinds of machines, such as airplanes, work the same way. Electrical current moves through paths called circuits. Conductors help electrical current move easily. Insulators block electrical energy from flowing through objects. Electromagnets use electricity and magnetic force to perform many tasks and to produce more energy. Two types of energy are potential energy and kinetic energy. Electrical energy can come from many sources, including from the sun and from underground. It can also come from gasoline, coal, natural gas, nuclear power, and wind. Chemical energy and mechanical energy are other important forms of energy.

18 chemical energy (KEM ih kuhl EN er jee) Energy stored in the arrangement of particles of matter (14, 15) circuit (SER kuht) A path that electricity follows (3, 4, 5, 15) conductors (kuhn DUK ter) Materials that allow electric charges to flow through them easily (6, 7, 15) electric current (ee LEK trik KUR ) A steady movement of charges through certain materials (2, 3, 7, 10, 11, 15) electromagnet (ee lek troh MAG nit) A temporary magnet caused by an electrical current (10, 15) generator (JEN er ayt er) A device that produces an electric current (11) insulator (IN suh layt er) A material that does not carry current electricity well (6, 7, 15) kinetic energy (kih NET ik EN er jee) Energy of motion (12, 13, 15) magnet (MAG nit) An object that attracts iron, stell, and a few other (but not all) metals (8, 9, 10, 11) magnetic poles (mag NET ik POHLZ) The parts of a magnet at which its force is strongest (9) mechanical energy (muh KAN ih kuhl EN er jee) Energy that moves a machine (15) parallel circuit (PAR uh lel SER kit) A circuit that has more than one path for an electric current to follow (4, 5) potential energy (poh TEN shuhl EN er jee) Energy that an object has because of its position or its condition (12, 13, 15) series circuit (SIR eez SER kit) A circuit that has only one path for an electric current to follow (4, 5)

19 What is the relationship between currents and circuits? If a circuit changes from a closed circuit to an open circuit, what will happen next? What is the difference between an object that is magnetic and one that is not? Write a short paper that gives ideas about ways to use electromagnets. Explain how they work and how they could be used. Ask your teacher to loan you a magnet. Go to different parts of your classroom and test to see which materials are magnetic. Write down your results, and note if you were surprised to find that certain objects were magnetic or nonmagnetic. Ask the older members of your community if they have always had electricity. If not, ask them about the other energy sources they used to use for light and heat. Take notes and share your information with the class.

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