Chapter 19 Magnetism Section 1 Magnets and Magnetic Fields

What do you think? ` An iron nail is attracted to an iron magnet but not to another nail. Two magnets can attract each other. Is either end of the nail attracted to either end of the magnet? Is either end of one magnet attracted to either end of the other magnet? Explain. Both are made of iron, but the magnet behaves differently. Why? How does the nail change when near the magnet so that it is attracted? When asking students to express their ideas, you might try one of the following methods. (1) You could ask them to write their answers in their notebook and then discuss them. (2) You could ask them to first write their ideas and then share them with a small group of 3 or 4 students. At that time you can have each group present their consensus idea. This can be facilitated with the use of whiteboards for the groups. The most important aspect of eliciting students’ ideas is the acceptance of all ideas as valid. Do not correct or judge them. You might want to ask questions to help clarify their answers. You do not want to discourage students from thinking about these questions and just waiting for the correct answer from the teacher. Thank them for sharing their ideas. Misconceptions are common and can be dealt with if they are first expressed in writing and orally. Students likely will know the answers to the first two questions but the last two questions ask for an understanding of WHY this occurs. Encourage them to think about how the particles inside the nail might behave differently than those in the magnet.

Properties of Magnets Magnets attract metals classified as ferromagnetic. Iron, nickel, cobalt Magnets have two poles, north and south. Like poles repel each other. Opposite poles attract each other. When free to rotate, the north pole points toward the north.

Magnetic Poles Click below to watch the Visual Concept. Visual Concept

Magnetic Domains In ferromagnetic materials, groups of atoms form magnetic domains within the material. In a paper clip or nail, the domains are randomly arranged. In a magnet, the domains are more aligned. Students could investigate the model using electron spin to help understand why some materials are ferromagnetic. The domains each have their own magnetic fields. When they are somewhat aligned, the magnetic fields add to give a net magnetic field.

Magnetic Domains What would happen to the domains? They would better align. How would the paper clip be different afterward? It would behave as a magnet. Would it remain magnetized? The domains would gradually become more randomly oriented. Suppose you rubbed a paper clip repeatedly in one direction with the north pole of a magnet. Magnetic materials are classified as hard and soft. With soft magnets, it is easier to align the domains, but they do not hold the alignment as long.

Magnetic Fields What object is used to detect a gravitational field? Any mass - when released it falls in the direction of the field What object was used to detect an electric field? A positively charged test particle - when released it moves in the direction of the field What object would be used to detect a magnetic field? A compass - the north pole points in the direction of the magnetic field Students may have seen iron filings used. Iron filings or paper clips also detect magnetic field,s but they do not tell you which way it is going (because they do not have a north and south pole). They simply align in the field, which could be going either direction.

Magnetic Fields Compass needles show the direction of the field. Out of the north and into the south The distance between field lines indicates the strength of the field. Stronger near the poles The field exists within the magnet as well. The PhET website may be useful at this time. http://phet.colorado.edu/new/index.php Choose “simulations,” then choose “Electricity, Magnets and Circuits,” then choose “Faraday’s Electromagnetic Lab.” At this time, it would be useful to show them the “Bar Magnet” option. You can show a compass and the field. There is also a meter to measure the strength of the field at various points.

Magnetic Flux Flux measures the number of field lines passing perpendicularly through a fixed area. More flux near the poles Have student imagine a circle with a cross- sectional area near the pole, and then the same circle up above the magnet. More field lines pass through near the poles, so there is a greater flux.

Representing the Direction of a Magnetic Field Click below to watch the Visual Concept. Visual Concept

Earth’s Magnetic Field The north pole of a magnet points toward the geographic north pole or Earth’s south magnetic pole. Opposites attract The magnetic poles move around. The magnetic and geographic poles are about 1500 km apart.

Earth’s Magnetic Field Which way would a compass needle point in the U.S.? Toward the north and slightly downward into Earth Field lines go into Earth as seen in the diagram; they are not parallel to the surface. Earth’s poles have reversed many times in the past, as evidenced by core samples showing differing magnetic field directions.

Now what do you think? An iron nail is attracted to an iron magnet but not to another nail. Two magnets can attract each other. Is either end of the nail attracted to either end of the magnet? Is either end of one magnet attracted to either end of the other magnet? Explain. Both are made of iron but the magnet behaves differently. Why? How does the nail change when near the magnet so that it is attracted? As the magnet is brought near the nail, the domains align somewhat because the north pole of the magnet attracts the south pole of each domain. Now the nail has a net magnetic field and is attracted to the magnet. When the magnet is removed, the domains become more random, and the nail is no longer magnetized.