Preview

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

2. 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? `

3. 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.

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

5. 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.

6. 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 Domains

7. 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

8. 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.

9. Magnetic Flux • Flux measures the number of field lines passing perpendicularly through a fixed area. • More flux near the poles

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

11. 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.

12. 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.

13. 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?

14. 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?

15. 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.

16. 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.

17. Magnetic Field of a Current-Carrying Wire Click below to watch the Visual Concept. http://www.youtube.com/watch?v=bSge-qDcS4Y&feature=endscreen&NR=1

18. 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?

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

20. 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

21. 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?

22. Homework • p.682/1-4 and p.686/1-3

23. 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?

24. 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.

25. 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

26. 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.

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

28. 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

29. 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.

30. 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:

31. 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.

32. 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

33. 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.

34. 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.

35. Galvanometer Click below to watch the Visual Concept. Visual Concept

36. 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?