1 Magnets and Magnetic Fields

Name Class Date

CHAPTER 18 Magnetism SECTION 1 Magnets and Magnetic Fields

KEY IDEAS As you read this section, keep these questions in mind: • What happens when the poles 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 Some Properties of Magnets? Magnets get their name from Magnesia, now a part of READING TOOLBOX present-day Greece. Almost 3,000 years ago, people there Compare and Contrast first found naturally magnetic rocks. These rocks are After you read this section, called lodestones. An example of a lodestone is shown in make a chart showing the similarities and differences the figure below. between Earth’s geographic and magnetic poles.

Lodestones are naturally magnetic rocks. They attract other magnets and objects made of iron. EHHDBG@K 1. Identify What is Sailors first used magnets as compasses for navigation. a lodestone? Today, magnets play roles in many aspects of our lives. Motors, VCRs, medical imaging machines, and many home alarm systems all rely on magnets.

MAGNETIC POLES Recall that like electric charges repel each other and unlike charges attract. A similar situation occurs for magnetic poles, or points that have opposite magnetic properties. All magnets have at least one pair of poles, a north magnetic pole and a south magnetic pole. A north magnetic pole attracts a south magnetic pole on another READING CHECK magnet. In addition, a north magnetic pole always repels 2. Deﬁne What are another north magnetic pole. magnetic poles? Magnetic poles always exist in pairs. If you cut a magnet in half, each part will have a north pole and a south pole. Even the tiniest magnets have two poles.

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What Are Permanent Magnets? Some substances, such as lodestones, stay magnetic all the time. Scientists 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 READING CHECK the magnet, the nail will stay magnetic for a time. 3. Identify What is a Even though they are called permanent magnets, the permanent magnet? 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.

MAGNETIC FORCES 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. 8g^i^XVaI]^c`^c\ How long is the chain of nails that the magnet can pull 4. Predict Consequences from the bucket? That depends on the strength of the Magnet A is a stronger magnet magnet and the ability of the nails to become magnetized. than magnet B. Which magnet would be able to lift more The greater the distance between a nail and the magnet, nails from the bucket? 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.

EHHDBG@K 5. Explain Why are nails far from the magnet less able to The magnet magnetizes the nails it touches. magnetize other nails? 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.

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What Are Magnetic Fields? Try holding a magnet and moving its south pole toward 8g^i^XVaI]^c`^c\ the south pole of a magnet that can move freely. Do not 6. Apply Concepts What let the magnets touch. You will see the free magnet move would happen if you held the south pole of one away from the one in your hand. It moves because the magnet near the north pole two magnets exert magnetic forces on each other, even of a magnet that could though they never touch. move freely? 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.

The small shavings of iron show the shape of this magnet’s magnetic ﬁeld. EHHDBG@K 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 move- ments 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. READING CHECK In some materials, including iron, nickel, and cobalt, 8. Explain Why are most atoms form larger groups called magnetic domains. The materials not magnetic? 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.

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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 pole of a magnet and end at its south pole. They always form closed loops. The field lines actually continue inside the magnet itself to form the loops.

N Magnetic field lines EHHDBG@K show the shape of a 9. Describe In which direc- magnetic field. You can use a compass to tion do the arrowheads determine how to draw on the magnetic field lines magnetic field lines. point? Compass Magnetic S field line

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 poles, or points where Earth’s axis of rotation meets Earth’s surface. Earth also has two magnetic poles. However, Earth’s magnetic poles are not in the same places as its geographic poles. A compass needle is a small magnet. Like all magnets, it has a north pole and a south pole. Remember that north and south magnetic poles attract each other. Therefore, a compass needle’s north pole must be READING CHECK attracted to Earth’s south magnetic pole. 10. Identify Which of Earth’s The north pole of a compass needle points toward magnetic poles is the north Earth’s north geographic pole. Therefore, the magnetic pole pole of a magnet attracted in Earth’s northern hemisphere is actually a south magnetic to? pole. The magnetic pole in the southern hemisphere is a magnetic north pole. In other words, Earth’s magnetic and geographic poles are opposite one another. This is shown in the figure at the top of the next page.

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South North magnetic geographic pole pole

Magnetic South North ﬁeld lines geographic EHHDBG@K magnetic Identify Which of Earth’s pole pole 11. magnetic poles is located in the northern hemisphere?

Earth’s north geographic pole is closest to Earth’s south magnetic pole.

FINDING THE MAGNETIC POLES To find the exact locations of the magnetic north and magnetic south poles, 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 poles, however, a dip needle points straight down. READING CHECK 12. Describe In which direc- What Produces Earth’s Magnetic Field? tion does a magnetic dip Scientists do not know for certain what makes Earth needle point at Earth’s north magnetic pole? 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.

Copyright © by Holt, Rinehart and Winston. All rights reserved. Interactive Reader 399 Magnetism Name Class Date Section 1 Review SECTION VOCABULARY magnetic ﬁeld a region where a magnetic force magnetic pole one of two points, such as can be detected the ends of a magnet, that have opposing magnetic qualities

1. Draw Label the north and south magnetic poles in the illustration below.

2. Apply Concepts Which pair of magnets below will move toward each other? Explain your answer.

A: N B: N C: S

S S N

S N N

N S S

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?