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Lesson 2 How Are Electricity and Magnetism Related? Fast fact Mighty Magnets! The world's strongest magnets are made from iron, boron, and neodymium, a metal. A tiny 50-mm (2-in.) magnet made of these materials can lift up to 30 kg (66 lb)! This sculpture uses pairs of these strong magnets. In the Investigate, you'll learn how to make a different kind of magnet. Investigate Can Electricity Make a Magnet? Materials bar magnet D-cell battery small compass 30-cm length of insulated wire with stripped ends sheet of cardboard tape Procedure 1. Move a bar magnet around a compass. Observe what the compass needle does. Put the magnet away. 2. Tape the battery to the cardboard. Tape one end of the wire to the flat end of the battery. Leave the other end loose. 3. Tape the wire to the cardboard in a loop, as shown in the picture. 4. Place the compass on top of the loop. Observe the direction in which the compass needle points. 5. Touch the loose end of the wire to the pointed end of the battery. Observe what the compass needle does. Record your observations. Draw Conclusions 1. How does a magnet affect a compass needle? 2. How does an electric current affect a compass needle? 3. Inquiry Skill Compare electricity and magnetism. How are they alike? Investigate Further Repeat steps 4 and 5, with the compass under the loop. Compare your observations. 151 Reading in Science VOCABULARY magnet p. 152 magnetic pole p. 154 magnetic field p. 155 electromagnet p. 156 generator p. 158 electric motor p. 159 SCIENCE CONCEPTS how electric currents are like magnets how generators and motors work READING FOCUS SKILL COMPARE AND CONTRAST Look for ways in which magnets and electric currents are alike and different. Magnets Have you ever played with a magnet? If you haven't, you may want to now! As you move a magnet around and bring it close to objects, you can discover what a magnet does. A magnet is an object that attracts iron and a few (not all) other metals. Magnets attract steel because it contains iron. When you bring an iron object or a steel object close to a magnet, the object moves toward the magnet. Try this for yourself. Place a steel paper clip near a magnet. What happens? Try the same thing with a plastic paper clip. How do your results compare? All magnets attract iron, but they may not look alike. Some magnets are shaped like bars. Others are U-shaped. Some magnets that stick to refrigerator doors are thin, flat shapes. ---see pictures A magnet inside the plastic base holds these steel pieces together. The horseshoe magnet attracts these metal objects. What metal do the objects contain? 152 ---see pictures Some magnets are shaped like bars. Why do these plastic letters stick to this refrigerator door? Distance affects the strength of a magnet's attraction. A small steel object that is close to a magnet moves toward it. However, if the same object is farther away, it will not move toward the magnet. Other forces can overcome the force of a magnet. Refrigerator magnets stick well to the door, but you can easily pull them off. Barriers can interfere with a magnet's pull, too. A refrigerator magnet may hold one or two sheets of paper to the door, but if you put too many sheets under it, the magnet will fall. Magnets can make some other objects magnetic. For example, if you rub a needle over a magnet several times in the same direction, the needle will become magnetic enough to pick up other needles. COMPARE AND CONTRAST How are all magnets alike? How are they different? 153 Magnetic Poles and Magnetic Fields A magnet has two places at which its force is the strongest. Each of these is called a magnetic pole, or pole for short. If you tie a string around the middle of a bar magnet and let it swing, one end will point north. That end is the magnet's northseeking pole. It is often marked with an N. The end that points south is often marked S. Forces between magnetic poles act like forces between electric charges. Opposite poles attract, and like poles repel. If you hold two S poles or two N poles near each other, they push apart. If you hold an N pole and an S pole near each other, they attract. Magnets of every shape have N and S poles. Try holding two round refrigerator magnets close together. If you turn them in one direction, they attract each other. If you turn them in the other direction, they repel each other. Magnets keep their poles even if you change their shape. If you cut a bar magnet into pieces, each piece would be a magnet with both an N pole and an S pole. Magnetic Poles ---see pictures Putting the opposite poles of magnets near each other causes the magnets to pull toward each other. Putting the like poles of magnets near each other causes the magnets to push away from each other. All magnets have N and S poles. Poles that are the same repel, or push apart. Poles that are opposite attract, or pull together. 154 Magnetic Field ---see pictures A bar magnet moves iron filings into the shape of the magnet's field. The shape of a magnetic field depends on the shape of the magnet. A magnetic field forms loops all around this bar magnet. You can use a compass to see a magnet's force. If you put a compass close to a bar magnet, the needle will point toward the magnet's N pole and away from the magnet's S pole. A magnetic field is the space around a magnet in which the force of the magnet acts. If you put iron filings around the magnet, you will be able to see the shape of the magnet's field. The filings will form circles that start and end at the poles, where the magnet's pull is the strongest. COMPARE AND CONTRAST How are a magnet's magnetic force and magnetic field different? Insta-Lab Needle Dance Place a compass near one pole of a magnet. Then move it all around the magnet. What does the needle do in each position? What does this tell you? 155 Electromagnets In the Investigate, you saw that a current of electricity causes a magnetic force. You showed that electricity and magnetism are related. Actually, an electric current produces a magnetic field around a wire. You can't see the field, but it circles the wire. The field around a single wire is weak. The field around many wires close together is strong. When coils wrap around an iron core, such as a nail, the core becomes an electromagnet. An electromagnet is a temporary magnet. It has a magnetic force only when an electric current moves through the wire. The electromagnet does not work if the current is switched off. With many coils of wire and a strong current, electromagnets can be made very strong. In junkyards, such electromagnets lift many tons of scrap iron and steel. COMPARE AND CONTRAST How are a magnet and an electromagnet alike? How are they different? ---see picture A steel nail is not normally a magnet, but it becomes one when an electric current runs around it. How can you tell that this nail is now a magnet? What would happen if you took one of the wire ends off the battery? 156 Math in Science Interpret Data In one experiment, a student built and tested an electromagnet. This graph shows the results. What hypothesis was the student testing? How did the student measure the magnet's strength? What conclusion can you draw from this graph? ---see graph ---see pictures This powerful electromagnet is being used to move scrap iron in a junkyard. This is an MRI (magnetic resonance imaging) machine in a hospital. It uses a magnetic field to take pictures of the brain, the muscles, and other soft tissues inside the body. 157 Generators and Motors Electricity can produce a magnetic field. Luckily for us, the reverse is also true—a magnetic field can produce electricity. If you move a coil of wire near a magnet, current electricity flows in the wire. That is how a generator, a device that produces an electric current, works. Any source of energy that can turn a coil of wire in a magnetic field can produce electricity. Hand-cranked generators use human power to turn the coil. During a power failure, you might use a gasoline-powered generator to produce electricity for lights. Power plants in large cities use many huge generators to produce enough electricity to meet people's needs. Most power plants burn coal, oil, or natural gas. The fuel heats water until it turns to steam. The steam's pressure turns a turbine, (TER•bin) a machine that produces a spinning motion. The turbine spins a coil inside the field of a magnet to produce electricity. Then the electricity is sent out along power lines to homes and businesses. Generators use motion to produce electricity. Can electricity produce motion? If you have ever seen someone 158 use a mixer or an electric drill, you know that the answer is yes. Such tools have an electric motor in them. An electric motor changes electrical energy into mechanical energy. In some motors, an electromagnet lies between the poles of a permanent magnet. Like all magnets, the electromagnet has an N pole and an S pole. The poles are pushed away from the like poles of the permanent magnet. They are pulled toward the opposite poles of the permanent magnet. The motor's shaft turns until the poles of the electromagnet are near the opposite poles of the permanent magnet. Then the current of the electromagnet reverses. Its N pole becomes its S pole, and its S pole becomes its N pole. The shaft turns again. The current keeps reversing, and the shaft keeps spinning. Electric motors do many useful things. They start cars. They run CD players. The next time you turn on a fan, thank the inventors of electric motors! COMPARE AND CONTRAST How are a generator and an electric motor alike? How are they different? ---see pictures pg.158 Generators This small, simple generator doesn't turn a coil inside a magnet. Instead, it turns a magnet around a coil. The result is the same—an electric current is generated. This hand-cranked generator uses "people power" to produce electricity. ---see pictures pg.159 Motors This simple electric motor turns because a permanent magnet pushes and pulls on an electromagnet. An electric motor sends this toy car racing across the floor. 159 Other Uses of Magnets Generators and motors aren't the only devices that work because of magnets. Compasses point north because they respond to Earth's natural magnetic field. This helps people find their way on land and at sea. Magnets are used in computers, compact disc players, and magnetic recording devices, such as VCRs. They are also inside headphones, stereo speakers, and telephone receivers. Doorbells and phones ring because of magnets. Even the strip on the back of a credit card is a magnet. Magnets are used for recording information. In a computer hard drive, for example, electromagnets move across a disk's surface. They make one disk area more magnetic than another. The disk spins under a part called the head. Later, the head reads the magnetic information COMPARE AND CONTRAST How are a videotape and a computer hard drive alike? ---see pictures When the head inside the VCR records, an electromagnet "writes" a magnetic field on the videotape. When the tape is played back, the magnetic field on the tape produces an electrical signal. In this computer hard drive, information is stored as a pattern of magnetic fields. Computers have magnets inside. A VCR (videocassette recorder) uses electricity and magnetism to record sounds and pictures. 160 Reading Review 1. COMPARE AND CONTRAST Draw and complete this graphic organizer to explain how electricity and magnetism are related. ---see chart 2. SUMMARIZE Use the completed graphic organizer to write a lesson summary. 3. DRAW CONCLUSIONS Tell why the relationship between electricity and magnetism is important to people. 4. VOCABULARY Use the terms electricity and magnet in a sentence to explain how a generator works. Test Prep 5. Critical Thinking Why is an electromagnet not a permanent magnet? 6. If you turn the N poles of two magnets toward each other, what will they do? A. attract B. produce C. repel D. spin electricity Writing Expository Writing Write a paragraph for a friend. Tell your friend how an electric motor works. Math Make a Bar Graph Test the strength of one magnet by measuring how many paper clips it can pick up. Then test two, three, and four magnets stuck together. Does it matter how the magnets are put together? Make a bar graph of your results. Social Studies History of Names Electricity is measured in amperes, coulombs, ohms, watts, and volts. Find out more about the famous scientists whose names are used for these units of measurement. Share what you learn with the class. For more links and activities, go to www.hspscience.com 161