Magnetism

Jean Brainard, Ph.D.

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Printed: September 7, 2014

AUTHORJean Brainard, Ph.D.

EDITORBradley Hughes, Ph.D.

www.ck12.org Chapter 1. Magnetism

CHAPTER 1 MagnetismCHAPTER OUTLINE

1.1 Magnets and Magnetism

1.2 Earth as a Magnet

1.3 References

The eerie green lights in the sky in this photo are called northern lights, or aurora borealis. They are caused byEarth’s magnetic attraction for electrically charged particles given off by the sun. When the particles collide withgas molecules in Earth’s atmosphere, the molecules give off light. The same thing also occurs in the southernhemisphere, where the lights are called southern lights, or aurora australis. Northern and southern lights are visiblemainly near the poles, and they are typically green like the northern lights shown here. In this chapter, you’ll learnhow magnetic force can cause such spectacular effects and why Earth is a giant magnet.Nick Russill. www. f lickr.com/photos/nickrussill/150410999. CC BY 2.0.

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1.1 Magnets and Magnetism

Lesson Objectives

• Identify properties of magnets.• Explain why some materials are magnetic.

Lesson Vocabulary

• ferromagnetic material• magnet• magnetic domain• magnetic field• magnetic force• magnetic pole• magnetism

Introduction

The futuristic-looking train in Figure 1.1 is called a maglev train. The word "maglev" stands for "magneticlevitation." Magnets push the train upward so it hovers, or levitates, above the track without actually touching it.This eliminates most of the friction acting against the train when it moves. Other magnets pull the train forwardalong the track. Because of these magnets, the train can go very fast. It can fly over the countryside at speeds up to480 kilometers (300 miles) per hour! What are magnets and how do they exert such force? In this lesson, you’ll findout.

You can watch a video introduction to lesson concepts at this URL: http://www.youtube.com/watch?v=8Y4JSp5U82I (10:44).

MEDIAClick image to the left for use the URL below.URL: http://www.ck12.org/flx/render/embeddedobject/5059

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FIGURE 1.1Magnets cause this maglev train to speedalong its track.

Properties of Magnets

A magnet is an object that attracts certain materials such as iron. You’re probably familiar with common barmagnets, like the one in Figure 1.2. Like all magnets, this bar magnet has north and south poles and attracts objectssuch as paper clips that contain iron.

FIGURE 1.2The north and south poles of a bar magnet attract paper clips.

Magnetic Poles

All magnets have two magnetic poles. The poles are regions where the magnet is strongest. The poles are callednorth and south because they always line up with Earth’s north-south axis if the magnet is allowed to move freely.(Earth’s axis is the imaginary line around which the planet rotates.) What do you suppose would happen if you cutthe bar magnet in Figure 1.2 in half along the line between the north and south poles? Both halves would also havenorth and south poles. If you cut each of the halves in half, all those pieces would have north and south poles aswell. Pieces of a magnet always have both north and south poles no matter how many times you cut the magnet.

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Magnetic Force

The force that a magnet exerts on certain materials is called magnetic force. Like electric force, magnetic force isexerted over a distance and includes forces of attraction and repulsion. North and south poles of two magnets attracteach other, while two north poles or two south poles repel each other.

Magnetic Field

Like the electric field that surrounds a charged particle, a magnetic field surrounds a magnet. This is the area aroundthe magnet where it exerts magnetic force. Figure 1.3 shows the magnetic field surrounding a bar magnet. Tinybits of iron, called iron filings, were placed under a sheet of glass. When the magnet was placed on the glass, itattracted the iron filings. The pattern of the iron filings shows the lines of force that make up the magnetic field ofthe magnet. The concentration of iron filings near the poles indicates that these areas exert the strongest force. Tosee an animated magnetic field of a bar magnet, go to this URL: http://elgg.norfolk.e2bn.org/jsmith112/files/68/149/Bar+magnet.swf .

FIGURE 1.3Lines of magnetic force are revealed bythe iron filings attracted to this magnet.

When two magnets are brought close together, their magnetic fields interact. You can see how in Figure 1.4. Thedrawings show how lines of force of north and south poles attract each other whereas those of two north poles repeleach other. The animations at the URL below show how magnetic field lines change as two or more magnets movein relation to each other.

http://www.coolmagnetman.com/magmotion.htm

You can take an animated quiz to check your understanding of magnetic field interactions at this URL: http://elgg.norfolk.e2bn.org/jsmith112/files/68/151/Law+of+magnetism.swf .

Magnetism and Materials

Magnetism is the ability of a material to be attracted by a magnet and to act as a magnet. No doubt you’ve handledrefrigerator magnets like the ones in Figure 1.5. You probably know first-hand that they stick to a metal refrigerator

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FIGURE 1.4When it come to magnets, there is a forceof attraction between opposite poles anda force of repulsion between like poles.

but not to surfaces such as wooden doors and glass windows. Wood and glass aren’t attracted to a magnet, whereasthe steel refrigerator is. Obviously, only certain materials respond to magnetic force.

FIGURE 1.5Refrigerator magnets stick to a refrigerator door because it contains iron.Why won’t the magnets stick to wooden cabinet doors?

What Makes a Material Magnetic?

Magnetism is due to the movement of electrons within atoms of matter. When electrons spin around the nucleusof an atom, it causes the atom to become a tiny magnet, with north and south poles and a magnetic field. Inmost materials, the electrons orbiting the nuclei of the atoms are arranged in such a way that the materials haveno magnetic properties. Also, in most types of matter, the north and south poles of atoms point in all differentdirections, so overall the matter is not magnetic. Examples of nonmagnetic materials include wood, glass, plastic,paper, copper, and aluminum. These materials are not attracted to magnets and cannot become magnets.

In other materials, electrons fill the orbitals of the atoms that make up the material in a way to allow for each atom tohave a tiny magnetic field, giving each atom a tiny north and south pole. There are large areas where the north andsouth poles of atoms are all lined up in the same direction. These areas are called magnetic domains. Generally,the magnetic domains point in different directions, so the material is still not magnetic. However, the material can

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be magnetized by placing it in a magnetic field. When this happens, all the magnetic domains become aligned,and the material becomes a magnet. This is illustrated in Figure 1.6. Materials that can be magnetized are calledferromagnetic materials. They include iron, cobalt, and nickel.

FIGURE 1.6Magnetic domains must be aligned by an outside magnetic field for mostferromagnetic materials to become magnets.

Temporary and Permanent Magnets

Materials that have been magnetized may become temporary or permanent magnets. An example of each type ofmagnet is described below. Both are demonstrated in Figure 1.7.

• If you bring a bar magnet close to pile of paper clips, the paper clips will become temporarily magnetized, asall their magnetic domains align. As a result, the paper clips will stick to the magnet and also to each other.However, if you remove the paper clips from the bar magnet’s magnetic field, their magnetic domains will nolonger align. As a result, the paper clips will no longer be magnetized or stick together.

• If you stroke an iron nail with a bar magnet, the nail will become a permanent (or at least long-lasting) magnet.Its magnetic domains will remain aligned even after you remove it from the magnetic field of the bar magnet.Permanent magnets can be demagnetized, however, if they are dropped or heated to high temperatures. Theseactions move the magnetic domains out of alignment.

FIGURE 1.7Paper clips become temporary magnetswhen placed in a magnetic field. An ironnail becomes a permanent magnet whenstroked with a bar magnet.

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Some materials are natural permanent magnets. The most magnetic material in nature is the mineral magnetite, alsocalled lodestone. The magnetic domains of magnetite naturally align with Earth’s axis. Figure 1.8 shows a chunkof magnetite attracting iron nails and iron filings. The magnetic properties of magnetite have been recognized forthousands of years. The earliest compasses used magnetite pointers to show direction. The magnetite spoon compassin Figure 1.8 dates back about 2000 years and comes from China.

FIGURE 1.8Magnetite naturally attracts iron nails and filings. Its natural magnetism was discovered thousands of years ago.

Lesson Summary

• A magnet is an object that attracts certain materials such as iron. All magnets have two magnetic poles anda magnetic field over which they exert force. Opposite magnetic poles attract each other, and like magneticpoles repel each other.

• Magnetism is the ability to be attracted by a magnet and to act as a magnet. Only ferromagnetic materialshave this property. They include iron, cobalt, and nickel. When these materials are magnetized, they becometemporary or permanent magnets. Magnetite is a natural permanent magnet.

Lesson Review Questions

Recall

1. What is a magnet?2. Define magnetic force.3. Give examples of objects that are attracted by magnets.4. Identify ferromagnetic materials.

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Apply Concepts

5. Draw magnetic field lines around the bar two magnets pictured below.

6. Sasha dropped a magnet on the sidewalk. Now it no longer attracts paper clips. Apply lesson concepts toexplain why.

Think Critically

7. Explain how and why a ferromagnetic material can be magnetized.

Points to Consider

The northern lights that you saw in the opening photo of this chapter are caused by Earth’s magnetic field. You willread about Earth’s magnetic field in the next lesson, "Earth as a Magnet."

• If you could see Earth’s magnetic field, what do you think it would look like? (Hint: Look at Figure 1.3.)• After reading this lesson, can you predict why northern lights are most likely to be visible near Earth’s poles?

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1.2 Earth as a Magnet

Lesson Objectives

• Describe Earth as a magnet.• State how Earth’s magnetism benefits living things.

Lesson Vocabulary

• magnetosphere

Introduction

Did you ever use a compass like the car compass in Figure 1.9? The pointer in a compass always points northbecause Earth is a giant magnet. In this lesson, you’ll learn why Earth is a magnet and how Earth’s magnetismbenefits living things.

FIGURE 1.9A compass pointer is aligned by Earth’smagnetism to point north.

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Magnet Earth

Imagine a huge bar magnet passing through Earth’s axis, as illustrated in Figure 1.10. This is a good representationof Earth as a magnet. Like a bar magnet, Earth has north and south magnetic poles and a magnetic field.

FIGURE 1.10Earth is like a giant bar magnet.

Earth’s Magnetic Poles

Although a compass always points north, it doesn’t point to Earth’s geographic north pole, which is located at 90°north latitude (see Figure 1.11). Instead, it points to Earth’s magnetic north pole, which is located at about 80° northlatitude. Earth’s magnetic south pole is also located several degrees of latitude away from the geographic south pole.

A compass pointer has north and south poles, and its north pole points to Earth’s magnetic north pole. Why does thishappen if opposite poles attract? Why doesn’t the compass needle point south instead? The answer may surpriseyou. Earth’s magnetic north pole is actually the south pole of magnet Earth! It’s called the magnetic north pole toavoid confusion. Because it’s close to the geographic north pole, it would be confusing to call it the magnetic southpole.

Earth’s Magnetic Field

Like all magnets, Earth has a magnetic field. Earth’s magnetic field is called the magnetosphere. It is a huge regionthat extends outward from Earth for several thousand kilometers but is strongest at the poles. You can see the extentof the magnetosphere in Figure 1.12. For an animated version of the magnetosphere, watch the video at this URL:

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FIGURE 1.11Earth’s magnetic north pole is close to thegeographic north pole.

http://www.youtube.com/watch?v=5SXgOWYyn84 (2:48).

MEDIAClick image to the left for use the URL below.URL: http://www.ck12.org/flx/render/embeddedobject/5060

FIGURE 1.12The magnetosphere extends outwardfrom Earth in all directions.

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Magnetic Field Reversals

Do you like to read science fiction? Science fiction writers are really creative. For example, an author might writeabout a time in the distant past when compasses pointed south instead of north. Actually, this idea isn’t fiction—it’sa fact! Earth’s magnetic poles have switched places repeatedly over the past hundreds of millions of years, each timereversing Earth’s magnetic field. This is illustrated in Figure 1.13.

FIGURE 1.13We think of today’s magnetic pole orientation as "normal" only because that’s what we are used to.

Scientists don’t know for certain why magnetic reversals occur, but there is hard evidence showing that they haveoccurred. The evidence comes from rocks on the ocean floor. Look at Figure 1.14, which shows a ridge on theocean floor. At the center of the ridge, hot magma pushes up through the crust and hardens into rock. Once themagma hardens, the alignment of magnetic domains in the rock is frozen in place forever. The newly hardened rockis then gradually pushed away from the ridge in both directions as more magma erupts and newer rock forms. Rocksamples from many places on the ocean floor reveal that magnetic domains of rocks from different time periods arealigned in opposite directions. The evidence shows that Earth’s magnetic field reversed hundreds of times over thelast 330 million years. The last reversal was less than a million years ago. What might happen if a magnetic reversaloccurred in your lifetime? How might it affect you? You can learn more about Earth’s magnetic reversals at thisURL: http://www.pbs.org/wgbh/nova/earth/when-our-magnetic-field-flips.html .

Why Is Earth a Magnet?

The idea that Earth is a magnet is far from new. It was first proposed in 1600 by a British physician named WilliamGilbert. However, explaining why Earth acts like a magnet is a relatively recent discovery. It had to wait until thedevelopment of technologies such as seismographs, which detect and measure earthquake waves. Then scientistscould learn about Earth’s inner structure (see Figure 1.15). They discovered that Earth has an inner and outer coreand that the outer core consists of liquid metals, mainly iron and nickel. Scientists think that Earth’s magnetic fieldis generated by the movement of charged particles through the molten metals in the outer core. The particles move

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FIGURE 1.14The alignment of magnetic domains inrocks on the ocean floor provide evidencefor Earth’s magnetic reversals.

as Earth spins on its axis. The video at the URL below takes a closer look at how this occurs.

http://www.youtube.com/watch?v=O-V3yR2RZUE (9:34)

MEDIAClick image to the left for use the URL below.URL: http://www.ck12.org/flx/render/embeddedobject/5061

FIGURE 1.15Charged particles flow through Earth’sliquid outer core, making Earth a giantmagnet.

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Benefits of Earth’s Magnetic Field

Earth’s magnetic field helps protect Earth and its organisms from harmful particles given off by the sun. Most of theparticles are attracted to the north and south magnetic poles, where Earth’s magnetic field is strongest. This is alsowhere relatively few organisms live.

Another benefit of Earth’s magnetic field is its use for navigation. People use compasses to detect Earth’s magneticnorth pole and tell direction. Many animals have natural "compasses" that work just as well. Birds like the gardenwarbler in Figure 1.16 use Earth’s magnetic field to guide their annual migrations. Recent research suggests thatwarblers and other migrating birds have structures in their eyes that let them see Earth’s magnetic field as a visualpattern. You can learn more about animals and Earth’s magnetic field, including the potential effects of magneticfield reversals, at this URL: http://www.pbs.org/wgbh/nova/nature/magnetic-impact-on-animals.html .

FIGURE 1.16The garden warbler flies from Europe tocentral Africa in the fall and returns to Eu-rope in the spring. Its internal "compass"helps it find the way.

KQED: Illuminating the Northern Lights

Northern California residents may not be able to see the northern lights like people in Alaska can, but Bay Areascientists are playing a key role in understanding them. Find out more about the spectacular light shows up northand what scientists at UC Berkeley are discovering about the Earth’s magnetic field. For more information on thenorthern lights, see http://science.kqed.org/quest/video/illuminating-the-northern-lights/ .

MEDIAClick image to the left for use the URL below.URL: http://www.ck12.org/flx/render/embeddedobject/116508

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Lesson Summary

• Earth is a giant magnet with north and south magnetic poles and a magnetic field called the magnetosphere.Evidence in rocks shows that Earth’s magnetic poles switched positions hundreds of times in the past. Scien-tists think that Earth’s magnetic field is caused by the movement of charged particles through molten metalsin the outer core.

• Earth’s magnetic field helps protect Earth’s surface and its organisms from harmful solar particles by pullingmost of the particles toward the magnetic poles. Earth’s magnetic field is also used for navigation by humansand many other animals.

Lesson Review Questions

Recall

1. What is the magnetosphere?2. Identify evidence for magnetic reversals in Earth’s past.3. List two benefits to organisms of Earth’s magnetic field.

Apply Concepts

4. Use a bar magnet, a globe or large ball, and any other props you need to demonstrate to another student howEarth is like a bar magnet.

Think Critically

5. What is the relationship between Earth’s magnetic poles and Earth’s geographic poles?6. Explain why Earth is a magnet.

Points to Consider

In this chapter, you learned that Earth is a magnet because of moving charged particles in its outer core. In thechapter "Electricity," you learned that moving charged particles create electric current. The next chapter explainshow electric current and magnetism are related.

• Based on what you now know about electricity and magnetism, can you predict how they are related?• Do you think electric current could be used to create a magnet? How might this be done?

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1.3. References www.ck12.org

1.3 References

1. Max Talbot-Minkin. http://www.flickr.com/photos/maxtm/6823667554/ . CC-BY 2.02. Christopher Auyeung. CK-12 Foundation . CC BY-NC 3.03. Marc Spooner. http://www.flickr.com/photos/marcspooner/5287991784/ . CC BY 2.04. Christopher Auyeung. CK-12 Foundation . CC BY-NC 3.05. Neil Piddock (Flickr: piddy77). http://www.flickr.com/photos/piddysplace/145287597/ . CC BY 2.06. Christopher Auyeung. CK-12 Foundation . CC BY-NC 3.07. Magnet and paperclip: User:Havelock/He.Wikipedia; Magnet and nail: Christopher Auyeung. Magnet and

paperclip: http://commons.wikimedia.org/wiki/File:Magpins1.JPG; Magnet and nail: CK-12 Foundation .Magnet and paperclip: The copyright holder of this file allows anyone to use it for any purpose, provided thatthe copyright holder is properly attributed; Magnet and nail: CC BY-NC 3.0

8. Left: User:Teravolt/Wikipedia; Right: User:Yug/Wikimedia Commons. Left: http://commons.wikimedia.org/wiki/File:Lodestone_%28black%29.jpg; Right: http://commons.wikimedia.org/wiki/File:Antic_chinese_-Compass.jpg . Left: CC BY 3.0; Right: Public Domain

9. Flickr:Alberto.... http://www.flickr.com/photos/albertoalerigi/2886121661/ . CC BY 2.010. Laura Guerin. CK-12 Foundation . CC BY-NC 3.011. Laura Guerin. CK-12 Foundation . CC BY-NC 3.012. Laura Guerin. CK-12 Foundation . CC BY-NC 3.013. Christopher Auyeung (CK-12 Foundation), using Earth clip art by yeKcim from Open Clip Art Library. Eart

h: http://commons.wikimedia.org/wiki/File:Earth_clip_art.svg . CC BY-NC 3.0 (Earth svg released into thepublic domain)

14. Courtesy of the U.S. Geological Survey. http://commons.wikimedia.org/wiki/File:Oceanic.Stripe.Magnetic.Anomalies.Scheme.gif . Public Domain

15. Laura Guerin. CK-12 Foundation . CC BY-NC 3.016. Flickr:Biillyboy. http://commons.wikimedia.org/wiki/File:Sylvia_borin_%28%C3%96rebro_County%29.jpg

. CC BY 2.0

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