1 Magnets and Magnetic Fields

Transcription

1 CHAPTER 18 1 Magnets and Magnetic Fields ECTIO Magnetism KEY IDEA As you read this section, keep these questions in mind: What happens when the s of two magnets come close together? What causes a magnet to attract or repel another magnet? How is Earth s magnetic field oriented? What Are ome Properties of Magnets? Magnets get their name from Magnesia, now a part of present-day Greece. Almost 3,000 years ago, people there first found naturally magnetic rocks. These rocks are called lodestones. An example of a lodestone is shown in the figure below. READIG TOOLBOX Compare and Contrast After you read this section, make a chart showing the similarities and differences between Earth s geographic and magnetic s. Lodestones are naturally magnetic rocks. They attract other magnets and objects made of iron. ailors first used magnets as compasses for navigation. Today, magnets play roles in many aspects of our lives. Motors, VCRs, medical imaging machines, and many home alarm systems all rely on magnets. 1. Identify What is a lodestone? MAGETIC POLE Recall that like electric charges repel each other and unlike charges attract. A similar situation occurs for magnetic s, or points that have opposite magnetic properties. All magnets have at least one pair of s, a north magnetic and a south magnetic. A north magnetic attracts a south magnetic on another magnet. In addition, a north magnetic always repels another north magnetic. Magnetic s always exist in pairs. If you cut a magnet in half, each part will have a north and a south. Even the tiniest magnets have two s. 2. Define What are magnetic s? Interactive Reader 395 Magnetism

2 ECTIO 1 3. Identify What is a permanent magnet? What Are Permanent Magnets? ome substances, such as lodestones, stay magnetic all the time. cientists call them permanent magnets. You can use a permanent magnet to make other objects magnetic. For example, if you place an iron nail near a magnet, the nail will become magnetized. If you take away the magnet, the nail will stay magnetic for a time. Even though they are called permanent magnets, the magnetism of permanent magnets isn t really permanent. Heating or hammering a permanent magnet can make it less magnetic, or even remove its magnetism completely. 4. Predict Consequences Magnet A is a stronger magnet than magnet B. Which magnet would be able to lift more nails from the bucket? MAGETIC FORCE Imagine placing a magnet into a bucket filled with iron nails. As you pull the magnet out of the bucket, nails stick to the magnet and to other nails. That happens because each nail that touches the magnet becomes magnetized. The magnetized nails exert magnetic forces on nails below them and pick them up. A magnetic force is a force that one magnet exerts on another. How long is the chain of nails that the magnet can pull from the bucket? That depends on the strength of the magnet and the ability of the nails to become magnetized. The greater the distance between a nail and the magnet, the weaker the magnetic force holding the nail up. Far from the magnet, the magnetic force is weaker than the force of gravity on a nail in the bucket. Therefore, no more nails can be lifted out of the bucket. 5. Explain Why are nails far from the magnet less able to magnetize other nails? The magnet magnetizes the nails it touches. Those nails, in turn, magnetize nails they touch. The farther a nail is from the magnet, the less magnetic it is. Therefore, nails far from the magnet are less able to magnetize other nails. Interactive Reader 396 Magnetism

3 ECTIO 1 What Are Magnetic Fields? Try holding a magnet and moving its south toward the south of a magnet that can move freely. Do not let the magnets touch. You will see the free magnet move away from the one in your hand. It moves because the two magnets exert magnetic forces on each other, even though they never touch. Because it acts at a distance, a magnetic force is a field force. Remember that every electrically charged object produces an electric field. In a similar way, every magnet produces a magnetic field. A magnetic field is the space around a magnet in which another magnet experiences a magnetic force. The strength of a magnetic field depends on the magnetic material and how much it is magnetized. 6. Apply Concepts What would happen if you held the south of one magnet near the north of a magnet that could move freely? The small shavings of iron show the shape of this magnet s magnetic field. 7. Describe What is the shape of the magnetic field around the center of the magnet? What Produces Magnetic Fields? Moving electric charges produce magnetic fields. Atoms have magnetic properties because of the movements of the electrons in the atoms. The magnetic fields of individual atoms balance one another in most materials. Those materials, such as copper and aluminum, are not magnetic. In some materials, including iron, nickel, and cobalt, atoms form larger groups called magnetic domains. The magnetic fields of all the atoms in a magnetic domain line up in the same direction. This forms small magnetized regions in the material. If there is another magnetic field nearby, the small regions align with each other. That increases the material s magnetic field. 8. Explain Why are most materials not magnetic? Interactive Reader 397 Magnetism

4 ECTIO 1 How Can You Draw Magnetic Fields? Remember that you can use electric field lines to show the shape of the electric field around a charged object. In a similar way, you can use magnetic field lines to show the magnetic field of a magnet. Magnetic field lines start at the north of a magnet and end at its south. They always form closed loops. The field lines actually continue inside the magnet itself to form the loops. 9. Describe In which direction do the arrowheads on the magnetic field lines point? Compass Magnetic field line Magnetic field lines show the shape of a magnetic field. You can use a compass to determine how to draw magnetic field lines. 10. Identify Which of Earth s magnetic s is the north of a magnet attracted to? Where Are Earth s Magnetic Poles? Earth has a magnetic field. A compass is a device that can show the direction of Earth s magnetic field. The needle of a compass lines up with Earth s magnetic field. The compass points along Earth s magnetic field lines at any particular point. Earth has two geographic s, or points where Earth s axis of rotation meets Earth s surface. Earth also has two magnetic s. However, Earth s magnetic s are not in the same places as its geographic s. A compass needle is a small magnet. Like all magnets, it has a north and a south. Remember that north and south magnetic s attract each other. Therefore, a compass needle s north must be attracted to Earth s south magnetic. The north of a compass needle points toward Earth s north geographic. Therefore, the magnetic in Earth s northern hemisphere is actually a south magnetic. The magnetic in the southern hemisphere is a magnetic north. In other words, Earth s magnetic and geographic s are opposite one another. This is shown in the figure at the top of the next page. Interactive Reader 398 Magnetism

5 ECTIO 1 outh magnetic orth geographic Magnetic field lines outh geographic orth magnetic 11. Identify Which of Earth s magnetic s is located in the northern hemisphere? Earth s north geographic is closest to Earth s south magnetic. FIDIG THE MAGETIC POLE To find the exact locations of the magnetic north and magnetic south s, scientists use a magnetic dip needle. This is a compass needle that can point up and down. The needle points in the direction of the Earth s magnetic field at its location. In most parts of the world, the needle points slightly down. At the south and north magnetic s, however, a dip needle points straight down. What Produces Earth s Magnetic Field? cientists do not know for certain what makes Earth magnetic. They know that Earth s core consists mostly of iron. However, the core is too hot for the iron to retain any magnetic properties. Many scientists think that ions or electrons moving in the liquid layer of Earth s core cause Earth s magnetism. Others think that several different factors produce Earth s magnetic field. The sun has its own magnetic field, and ejects charged particles into space. Earth s magnetic field deflects most of those particles. If Earth did not have a magnetic field, these particles could strike Earth. They could harm living things. Many scientists think that some living things use Earth s magnetic field to guide them as they migrate. They have found magnetic particles in the tissues of migrating animals such as birds, bees, and fish. 12. Describe In which direction does a magnetic dip needle point at Earth s north magnetic? Interactive Reader 399 Magnetism

6 ection 1 Review ECTIO VOCABULARY magnetic field a region where a magnetic force can be detected magnetic one of two points, such as the ends of a magnet, that have opposing magnetic qualities 1. Draw Label the north and south magnetic s in the illustration below. 2. Apply Concepts Which pair of magnets below will move toward each other? Explain your answer. A: B: C: 3. Identify What produces magnetic fields? 4. Explain A student places a magnet into a box of paper clips. When she lifts the magnet, some of the paper clips are pulled out of the box. However, some of the clips remain in the box. Why can t the student pull all of the paper clips out of the box? Interactive Reader 400 Magnetism