A: A compass needle is attracted by magnet Earth. It always points north because Earth acts as a giant magnet.

Transcription

1 5.80 Earth as a Magnet Describe how Earth is a magnet. Distinguish between Earth s geographic and magnetic poles. Describe the magnetosphere. Did you ever use a compass like the one in this picture? Even if you ve never used a compass, you probably know that the needle of a compass always points north. That s because a compass needle is magnetized, so it is attracted by a magnet. Q: What magnet attracts a compass needle? A: A compass needle is attracted by magnet Earth. It always points north because Earth acts as a giant magnet. Earth s Magnetic Poles Imagine a huge bar magnet passing through Earth s axis, as in the Figure This is a good representation of Earth as a magnet. Like a bar magnet, Earth has north and south magnetic poles. A magnetic pole is the north or south end of a magnet, where the magnet exerts the most force.

2 Two North Poles FIGURE Although the needle of a compass always points north, it doesn t point to Earth s north geographic pole. Find the north geographic pole in the sketch below. As you can see, it is located at 90 north latitude. Where does a compass needle point instead? It points to Earth s north magnetic pole, which is located at about 80 north latitude. Earth also has two south poles: a south geographic pole and a south magnetic pole. FIGURE 5.181

3 Q: The north end of a compass needle points toward Earth s north magnetic pole. The like poles of two magnets repel each other, and the opposite poles attract. So why doesn t the north end of a compass needle point to Earth s south magnetic pole instead? A: The answer may surprise you. The compass needle actually does point to the south pole of magnet Earth. However, it is called the north magnetic pole because it is close to the north geographic pole. This naming convention was adopted a long time ago to avoid confusion. Earth s Magnetic Field Like all magnets, Earth has a magnetic field. Earth s magnetic field is called the magnetosphere. You can see a model of the magnetosphere below. It is a huge region that extends outward from Earth in all directions. Earth exerts magnetic force over the entire field, but the force is strongest at the poles, where lines of force converge. For an animated model of the magnetosphere, watch this video: eature=related and Summary FIGURE Earth acts as a giant magnet with magnetic poles and a magnetic field over which it exerts magnetic force. Earth has north and south magnetic poles like a bar magnet. Earth s magnetic poles are not the same as the geographic poles. Earth s magnetic field is called the magnetosphere. It is strongest at the poles. Vocabulary

4 magnetic pole: North or south end of a magnet where the magnet exerts the most force. magnetosphere: Region surrounding Earth that is affected by Earth s magnetic force; name for Earth s magnetic field. Practice At the following URL, use the simulation to explore Earth s magnetic field. Then write a paragraph to describe your observations. le_interacti ve.html Review 1. How does Earth act as a bar magnet? 2. The compass in a car shows that the car is moving north. Does this mean that the car is moving toward 90 north latitude? Why or why not? 3. Describe the magnetosphere Magnetic Field Reversal Describe a magnetic field reversal. Identify evidence for magnetic field reversals. Do you like science fiction? This is a scene from a science fiction movie that is set in the future. Science fiction writers are so creative! They might even describe a time when compasses point south instead of north. Actually, this idea isn t fiction it s a fact. Changing Places

5 Earth s magnetic poles have switched places repeatedly in the past. As you can see in the Figure 5.183, each time the switch occurred, Earth s magnetic field was reversed. The magnetic field is the region around a magnet over which it exerts magnetic force. We think of today s magnetic field direction as normal, but that s only because it s what we re used to. You can watch a simulation of a magnetic field reversal at this URL: nova/earth/when-ourmagnetic-field-flips.html Fact, not Fiction Scientists don t know for certain why magnetic reversals occur, but there is hard evidence that they have for hundreds of millions of years. The evidence comes from rocks on the ocean floor. Look at Figure They show the same ridge on the ocean floor during different periods of time. A. At the center of the ridge, hot magma pushes up through the crust and hardens into rock. Once the magma hardens, the alignment of magnetic domains in the rock is frozen in place forever. Magnetic domains are regions in the rocks where all the atoms are lined up and pointing toward Earth s north magnetic pole. B. The newly hardened rock is gradually pushed away from the ridge in both directions as more magma erupts and newer rock forms. The alignment of magnetic domains in this new rock is in the opposite direction, showing that a magnetic reversal has occurred. FIGURE FIGURE 5.184

6 C. A magnetic reversal occurs again. It is frozen in rock to document the change. Rock samples from many places on the ocean floor show that the north and south magnetic poles reversed hundreds of times over the last 330 million years. The last reversal was less than a million years ago. Q: What might happen if a magnetic reversal occurred in your lifetime? How might it affect you? A: For ideas, learn more about Earth s magnetic reversals at this URL: our-magnetic-field-flips.html Summary Earth s magnetic poles have switched places repeatedly over hundreds of millions of years, each time reversing Earth s magnetic field. Magnetic domains in rocks on the ocean floor provide evidence for magnetic field reversals in the past. Vocabulary magnetic field: Area around a magnet where it exerts magnetic force. Practice At the following URL, listen to the streaming audio (or read the article) about magnetic field reversals. Then answer the questions below How has Earth s magnetic field changed during the past 200 years? 2. How often do magnetic field reversals occur? When did the last one take place? 3. How long does it take for a magnetic field reversal to occur? Describe what happens to Earth s magnetic field during that time.

7 Review 1. What is a magnetic field reversal? 2. Explain how rocks on the ocean floor provide hard evidence that magnetic reversals occurred in the past Why Earth Is a Magnet Describe how Gilbert discovered that Earth is a magnet. Relate Earth s inner structure to its magnetic field. Did you ever see a globe like this one? Magnets in the globe and its stand repel each other, allowing the globe to hover in midair. The globe is a good model for Earth the magnet. Earth the Magnet Like the real Earth, the globe pictured above is a magnet. A magnet is an object that has north and south magnetic poles and a magnetic field. The magnetic globe is a modern device, but the idea that Earth is a magnet is far from new. It was first proposed in 1600 by a British physician named William Gilbert. He used a spherical magnet to represent Earth. With a compass, he demonstrated that it the spherical magnet causes a compass needle to behave the same way that Earth causes a compass needle to behave. This showed that a spherical magnet is a good model for Earth and therefore that Earth is a magnet. Q: Can you describe Earth s magnetic poles and magnetic field? A: Earth has north and south magnetic poles. The North Pole is located at about 80 degrees north latitude. The magnetic field is an area around Earth that is affected

8 by its magnetic field. The field is strongest at the poles, and lines of magnetic force move from the north to the south magnetic pole. Spinning Like a Top Although the idea that Earth is a magnet is centuries old, the discovery of why Earth is a magnet is a relatively new. In the early 1900s, scientists started using seismographic data to learn about Earth s inner structure. A seismograph detects and measure earthquake waves. Evidence from earthquakes showed that Earth has a solid inner core and a liquid outer core (see the diagram below). The outer core consists of molten metals, mainly iron and nickel. Scientists think that Earth s magnetic field is generated by the movement of charged particles through these molten metals in the outer core. The particles move as Earth spins on its axis. The video at the URL below takes a closer look at how this occurs. Summary FIGURE In the 1600s, William Gilbert demonstrated that Earth is basically a spherical magnet, with north and south poles and a magnetic field. In the 1900s, scientists used earthquake data to determine that Earth has a solid inner core and molten outer core. Scientists think that Earth is a magnet because of charged particles moving through the molten outer core as Earth spins on its axis. Vocabulary magnet: Object that attracts ferromagnetic materials such as iron. Practice

9 At the following URL, watch the video to gain a deeper understanding of the production of Earth s magnetic field. Then write three points that extend the content of this article. Review 1. How did Gilbert show that Earth is a magnet? 2. Which would be a better model of Earth s inner structure: a peach or an orange? Explain your answer? 3. Explain why Earth generates a magnetic field Using Earth s Magnetic Field Describe Earth s magnetic field. Explain how Earth s magnetic field protects organisms from solar radiation. Describe how some animals use Earth s magnetic field for navigation. You may recognize the eerie green glow in this cold northern sky as the northern lights, or aurora borealis. But do you know what causes the northern lights? Earth s magnetic field is a major factor. Earth s Magnetic Field Like a bar magnet, planet Earth has north and south magnetic poles and a magnetic field over which it exerts magnetic force. Earth s magnetic field is called the magnetosphere. You can see it in the Figure

10 Like an Umbrella The sun gives off radiation in solar winds. You can see solar winds in the model above. Notice what happens to solar winds when they reach the magnetosphere. They are deflected almost completely by Earth s magnetic field. Radiation in solar wind would wash over Earth and kill most living things were it not for the magnetosphere. It protects Earth s organisms from radiation like an umbrella protects you from rain. Q: Now can you explain the northern lights? A: Energetic particles in solar wind collide with atoms in the atmosphere over the poles, and energy is released in the form of light. The swirling patterns of light follow lines of magnetic force in the magnetosphere. Finding the Way Another benefit of Earth s magnetic field is its use for navigation. People use compasses to detect Earth s magnetic north pole and tell direction. Many animals have natural compasses that work just as well. For example, the loggerhead turtles in the Figure sense the direction and strength of Earth s magnetic field and use it to navigate along migration routes. Many migratory bird species can also sense the magnetic field and use it for navigation. FIGURE Recent research suggests that they may have structures in their eyes that let them see Earth s magnetic field as a visual pattern. You can learn more at this URL: nd-their-way- Home.html

11 Q: In the past, Earth s magnetic poles have switched places and reversed Earth s magnetic field. How might a magnetic reversal affect loggerhead turtle navigation? A: You can find out at this URL: Summary Earth has north and south magnetic poles and a magnetic field called the magnetosphere. The magnetosphere protects Earth s organisms from solar radiation. Some organisms including humans with compasses use Earth s magnetic field for navigation. Vocabulary magnetic field: Area around a magnet where it exerts magnetic force. Practice At the following URL, explore the interaction between a compass and Earth s magnetic field. Move the compass around the planet and observe what happens to the needle. Also observe what happens to the compass when you flip polarity (in other words, when a magnetic field reversal occurs). Describe your observations. Review 1. Make a sketch of Earth s magnetic field. 2. Explain the northern lights in terms of Earth s magnetic field. 3. How is a loggerhead turtle like a compass? FIGURE 5.187

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