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Magnetism Preview Section 1 Magnets and Magnetic Fields Section 2 Magnetism from Electricity Section 3 Magnetic Force © Houghton Mifflin Harcourt Publishing Company Section 1 Magnetism TEKS Section 1 The student is expected to: 5D identify examples of electric and magnetic forces in everyday life © Houghton Mifflin Harcourt Publishing Company Magnetism What do you think? Section 1 ` • 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? © Houghton Mifflin Harcourt Publishing Company Magnetism Section 1 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. © Houghton Mifflin Harcourt Publishing Company Magnetism Section 1 Magnetic Poles Click below to watch the Visual Concept. Visual Concept © Houghton Mifflin Harcourt Publishing Company Magnetism 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. © Houghton Mifflin Harcourt Publishing Company Section 1 Magnetism Section 1 Magnetic Domains • What would happen to the domains? – They would better align. • How would the paper clip be different afterward? • Suppose you rubbed a paper clip repeatedly in one direction with the north pole of a magnet. © Houghton Mifflin Harcourt Publishing Company – It would behave as a magnet. • Would it remain magnetized? – The domains would gradually become more randomly oriented. Magnetism Section 1 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 © Houghton Mifflin Harcourt Publishing Company Magnetism Section 1 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. © Houghton Mifflin Harcourt Publishing Company Magnetism Magnetic Flux • Flux measures the number of field lines passing perpendicularly through a fixed area. – More flux near the poles © Houghton Mifflin Harcourt Publishing Company Section 1 Magnetism Section 1 Representing the Direction of a Magnetic Field Click below to watch the Visual Concept. Visual Concept © Houghton Mifflin Harcourt Publishing Company Magnetism Section 1 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. © Houghton Mifflin Harcourt Publishing Company Magnetism Section 1 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. © Houghton Mifflin Harcourt Publishing Company Magnetism Section 1 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? © Houghton Mifflin Harcourt Publishing Company Magnetism Section 2 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? © Houghton Mifflin Harcourt Publishing Company Magnetism 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. © Houghton Mifflin Harcourt Publishing Company Section 2 Magnetism Section 2 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. © Houghton Mifflin Harcourt Publishing Company Magnetism Section 2 Magnetic Fields C B A © Houghton Mifflin Harcourt Publishing Company • 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? Magnetism Section 2 Magnetic Field of a Current Loop Click below to watch the Visual Concept. Visual Concept © Houghton Mifflin Harcourt Publishing Company Magnetism Section 2 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 © Houghton Mifflin Harcourt Publishing Company Magnetism Section 2 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? © Houghton Mifflin Harcourt Publishing Company Magnetism TEKS Section 3 The student is expected to: 5D identify examples of electric and magnetic forces in everyday life © Houghton Mifflin Harcourt Publishing Company Magnetism Section 3 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? © Houghton Mifflin Harcourt Publishing Company Magnetism Section 3 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. © Houghton Mifflin Harcourt Publishing Company Magnetism Section 3 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 © Houghton Mifflin Harcourt Publishing Company Magnetism Section 3 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. © Houghton Mifflin Harcourt Publishing Company Magnetism Section 3 Force on a Charge Moving in a Magnetic Field Click below to watch the Visual Concept. Visual Concept © Houghton Mifflin Harcourt Publishing Company Magnetism Section 3 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 © Houghton Mifflin Harcourt Publishing Company Magnetism Section 3 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. © Houghton Mifflin Harcourt Publishing Company Magnetism 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: © Houghton Mifflin Harcourt Publishing Company Section 3 Magnetism Section 3 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. © Houghton Mifflin Harcourt Publishing Company Magnetism Section 3 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 © Houghton Mifflin Harcourt Publishing Company Magnetism Section 3 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. © Houghton Mifflin Harcourt Publishing Company Magnetism Section 3 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. © Houghton Mifflin Harcourt Publishing Company Magnetism Section 3 Galvanometer Click below to watch the Visual Concept. Visual Concept © Houghton Mifflin Harcourt Publishing Company Magnetism Section 3 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? © Houghton Mifflin Harcourt Publishing Company