Preview Section 1 Magnets and Magnetic Fields Section 2 Magnetism from Electricity Section 3 Magnetic Force

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.

What do you think? Electromagnets are used every day to operate doorbells and to lift heavy objects in scrap yards. Why is the prefix electro- used to describe these magnets? Is electricity involved in their operation or do they create electricity? Would such a magnet require the use of direct current or alternating current? Why? 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 may be familiar with electromagnets but unclear about their operation. Help them clarify their ideas about how electromagnets operate.

Magnetism from Electricity A compass needle held near a current carrying wire will be deflected. Electric current must produce a magnetic field. Discovered by Hans Christian Oersted Many compasses placed around a vertical current carrying wire align in a circle around the wire.

Right-Hand Rule To find the direction of the magnetic field (B) produced by a current (I): Point your right thumb in the direction of the current Curl your fingers and they will show the direction of the circular field around the wire. Remind students that current (I) is opposite the flow of electrons.

Magnetic Fields C B A Use the right hand rule to decide what direction the magnetic field would be at points A, B, and C. Since magnetic fields are vectors, how would the net field appear in the center of the loop? The fingers of the right hand should always curl around the wire and upward through the middle. This creates a magnetic field that comes out of the top of the loop and into the bottom of the loop. This is demonstrated in the Visual Concepts clip on the next slide.

Magnetic Field of a Current Loop Click below to watch the Visual Concept. Visual Concept

Magnetic Field Around a Current Loop Magnets and loops of wire have magnetic fields that are similar. Solenoids are coils of wire similar to the single loop. More loops strengthens the field Placing an iron rod in the center strengthens the field as well Called an electromagnet For solenoids, all loops must go in the same direction to make the B fields additive. An iron core will become magnetized by the field and thus adds its own B field to that of the solenoid. Do the quick lab in this section of the Student Edition, and hold a compass near the end of the coil with the nail and without the nail to demonstrate the enhancing effect of the iron core. 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 students the “Electromagnet” option. You can show a compass and the field. You can reverse the electron flow (note that it is the opposite the direction of current) by adjusting the potential difference. You can also change the number of loops in the solenoid. There is an AC option as well. Ask students what will happen with each case before running the simulation.

Now what do you think? Electromagnets are used every day to operate doorbells and to lift heavy objects in scrap yards. Why is the prefix electro- used to describe these magnets? Is electricity involved in their operation or do they create electricity? Would such a magnet require the use of direct current or alternating current? Why? An electric current produces a magnetic field. With an iron core, a coil of current-carrying wire can behave just as a magnet would. The advantage in doorbells and scrap yards is the fact that, when the electric current is turned off, the magnetism is dramatically reduced in the iron core, and the device returns to a nearly unmagnetized state. As a result, the doorbell chime springs back to its starting position, and the crane drops the scrap metal. DC current is necessary to align the domains within the iron core. AC would keep switching them back and forth. High-frequency AC current is used as a demagnetizer.

What do you think? When watching a television with a CRT, an image is created on the screen by beams of electrons striking red, green, and blue phosphors on the screen. How are these beams aimed at the right phosphors? Why does holding a magnet near the screen alter the image and sometimes permanently damage the screen? How often does the TV produce a new still image for you to see? How do these still images create movement? 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. There is likely to be a wide range of knowledge regarding this topic. This question is not directed at the newer LCD and plasma TVs, which do not use a CRT (cathode ray tube). Ask students to try to make reasonable assumptions about how a picture might be created with a beam of electrons. Most TVs use three beams, one for each primary color of phosphor. The second question might help students deduce that magnets have something to do with controlling the beam of electrons.

Charged Particles in a Magnetic Field Magnetic fields exert a magnetic force on moving charged particles. Force is greatest when the movement is perpendicular to the magnetic field Force is zero when the particle moves along the field lines Force is in between these values for other directions When the movement is perpendicular, the magnetic force is: Fmagnetic = qvB where q is the charge, v is the velocity, and B is the magnetic field strength. This text limits discussion to situations in which charges move parallel or perpendicular to the magnetic field lines. When the movement is perpendicular, the equation shown on the slide applies. When the movement is parallel, the force is zero.

Charged Particles in a Magnetic Field So, the magnetic field (B) can be determined from the force on moving charged particles as follows: SI unit: Tesla (T) where T = N/(C•(m/s)) = N/(A•m) = (V•s)/m2 Ask students to explain why all three of the units shown for Tesla are equivalent.

Charged Particles in a Magnetic Field The right-hand rule for the force on a moving charged particle Thumb in the direction a positive particle is moving Fingers in the direction of the magnetic field The force will be in the direction of your palm For negative particles, the force is out the back of your hand.

Force on a Charge Moving in a Magnetic Field Click below to watch the Visual Concept. Visual Concept

Classroom Practice Problems An electron moving north at 4.5  104 m/s enters a 1.0 mT magnetic field pointed upward. What is the magnitude and direction of the force on the electron? What would the force be if the particle was a proton? What would the force be if the particle was a neutron? Answers: 7.2  10-18 N west 7.2  10-18 N east 0.0 N For problems, it is a good idea to go through the steps on the overhead projector or board so students can see the process instead of just seeing the solution. Allow students some time to work on problems and then show them the proper solutions. Do not rush through the solutions. Discuss the importance of units at every step. Problem solving is a developed skill and good examples are very helpful. This is a good application of the equation and the right-hand rule. Students will need to know the charge on an electron. Help them use the right-hand rule appropriately for a negative charge like the electron. You can also ask students what the force would be if the magnetic field was north or south instead of upward. (Zero, because the velocity and B field are parallel)

Magnetic Force as Centripetal Force Use the right-hand rule to determine the direction of the force. Which direction would the force be when the charge is at the top? the left side? the bottom? Always directed toward the center Because of this magnetic force, the charge moves in a circle. The force is centripetal. Charged particles entering Earth’s atmosphere get trapped into circular motion by the magnetic field and spiral toward the poles. This phenomenon causes the northern and southern lights.

Current-Carrying Wires Magnetic forces also exist on the moving charges in current-carrying wires. The right-hand rule to is used to determine the direction, as shown in the diagram. The magnitude of the force is as follows:

Parallel Current-Carrying Wires Current carrying wires create a magnetic field which interacts with the moving electrons in the nearby wire. Currents in the same direction produce attraction. Currents in opposite directions cause the wires to repel. Use the-right hand rule to verify the direction of the force for each of the four wires shown. Remind students that the direction for I is that of positive charge. The electrons are moving in the opposite direction. In order to verify the direction of the force, students will need to imagine the B field extending over to the other wire. For example, in the top diagram, the B field from the left wire at the right wire would be into the page. Therefore, since the positive charge is moving upward, the force would be to the left.

Classroom Practice Problem A 4.5 m wire carries a current of 12.5 A from north to south. If the magnetic force on the wire due to a uniform magnetic field is 1.1  103 N downward, what is the magnitude and direction of the magnetic field? Answer: 2.0  101 T to the west For problems, it is a good idea to go through the steps on the overhead projector or board so students can see the process instead of just seeing the solution. Allow students some time to work on problems and then show them the proper solutions. Do not rush through the solutions. Discuss the importance of units at every step. Problem solving is a developed skill and good examples are very helpful. Using the right-hand rule may be difficult for students. They need to point their thumb to the south (direction of the positive charge) and palm downward (direction of the force). This will leave their fingers pointed to the west for the magnetic field.

Applications - Cathode Ray Tube Televisions and computer monitors use CRTs. A magnetic field deflects a beam of electrons back and forth across the screen to create an image. Have students look up more information about how a television produces an image (a CRT television, not LCD or plasma). “HowStuffWorks.com” is a web site with good information.

Applications - Speakers The forces on electrons as they move back and forth in the coil of wire cause the coil to vibrate. The coil is attached to the paper cone, so sound waves are produced by the vibration.

Galvanometer Click below to watch the Visual Concept. Visual Concept

Now what do you think? When watching a television with a CRT, an image is created on the screen by beams of electrons striking red, green, and blue phosphors on the screen. How are these beams aimed at the right phosphors? Why does holding a magnet near the screen alter the image and sometimes permanently damage the screen? How often does the TV produce a new still image for you to see? How do these still images create movement? Students should be able to apply their knowledge from the section to describe the operation of a CRT in terms of electron deflection and magnetic fields, and they should also be able to predict what happens when a magnet is held near the screen. Use the information below to help them fill out their answers for the content that was not covered in the section. TV images are produced on CRT televisions with a scanning beam of electrons (3 beams on most TVs, one for each color). The beam direction is controlled by coils of wire creating a magnetic field. As the negatively-charged electrons move through the magnetic field, the force deflects them. One coil controls the horizontal deflection while another controls the vertical deflection. A magnet near the screen deflects the many electrons coming toward it. As a result, many strike the same spot on the TV screen, rather than the pixel they were supposed to strike. This distorts the picture and can permanently damage the screen. The electron beam in a TV scans and creates a new still picture 30 times per second. Each picture consists of 525 lines (except on HDTV). These rapidly-changing images appear to produce smooth motion because our eyes do not see them changing (due to the rapid rate of change).