Magnetism

1. Magnetism

2. Magnetism Unit Plan

3. History of Magnets • (~800 BC) Ancient Chinese and Greeks discovered that certain stones would attract and magnetize iron. • Small slivers of the stone were found to align themselves with the North Pole. • Chinese were the first to use magnets for navigation. • The orienting properties were used to align streets in cities in the North-South / East-West direction.

4. Applications • Computer disc drives (hard and floppy) • VCR and cassette tape • Credit cards • Speakers • Motors (Both AC and DC) • Speed sensors • Solenoids for relays, valves, etc.

5. N S N S N S + Poles of a Magnet • Magnets have a North and South Pole. • Like poles repel. • Unlike poles attract. • What happens if you break a magnet in half? Will you get two monopoles? • No.

6. Magnetic Field Lines • Characteristically similar to electric field lines. • Magnetic field lines point away from the north pole and towards the south pole. • Magnetic field lines are continuous (They do not terminate on the surface!). • Magnetic field lines never cross. • The magnetic field is strongest where the field lines are most concentrated (North and South Pole).

7. Magnetic Flux What is magnetic flux? • Like electric flux • A measure of the strength of the magnetic field, B, passing through a surface perpendicular to the field. • For a bar magnet, the flux is maximum at the poles. • The more magnetic field lines, the higher the flux. =BAcos

8. Oddly shaped magnets still have a north and a south Magnets either attract or repel each other South poles are attracted to north poles

9. Magnetic Field Lines vs. Electric Field Lines N S Electric Dipole Magnetic Dipole

10. Magnets either attract or repel each other Like poles repel South poles are attracted to north poles Unlike poles attract

11. The Earth’s Magnetic Field • The earth has a magnetic field that scientist believe is a result of the dynamo effect due to electrical currents created in the molten iron and nickel outer core. • PHET Magnet and Compass • The Earth's Magnetic Field • Bar Magnet - 3D

12. Sometimes the field completely flips. The north and the south poles swap places. Such reversals, recorded in the magnetism of ancient rocks, are unpredictable.  They come at irregular intervals averaging about 300,000 years; the last one was 780,000 years ago. Are we overdue for another? No one knows.

13. Magnetic Domains • A: Iron absent of a magnetic field. This material is magnetic but not a magnet. • B: Iron in the presence of a magnetic field. This is a magnet, either temporary or permanent. • C: A non-magnetic material. No domains.

14. Magnetic Domains = groups of atoms with aligned poles Magnets can be temporary (like the needle used in the compass). This nail has its atoms aligned, but the effect is only temporary.  You can get this affect by rubbing the nail on a magnet. Neat fact:  Hitting the nail can demagnetize it, you are basically scrambling the atoms.

15. Ferromagnetism • Soft Ferromagnets: (Silicon-steels and Iron-Nickel alloys) When the domains align themselves when exposed to an external magnetic field and re-randomize in its absence. • Hard Ferromagnets: (ALNICO, ferrite and neodymium iron boron) Magnetic field persists even in the absence of an external field. • Domains may realign themselves when exposed to an external magnetic field. • Shocking them may re-randomize the domains, such as by dropping. • Heat at or above the Curie point will re-randomize the domains. Ferromagnets lose their ferromagnetism when heated above a specific temperature , because the thermal energy melts the magnetic alignment.

16. Metals that are ferromagnetic:  nickel, iron, cobalt Things that are not magnetic:  aluminum, plastic, glass

17. The Earth’s Magnetic Field • How does a compass behave in Earth’s Magnetic field? N Earth’s Magnetic Field

18. S N S Magnetism of Soft Ferromagnetic Materials How does a magnet attract screws, bolts nails, paperclips, etc. when they are not magnetic to start with? • Soft ferromagnetic material align their domains in the presence of an external magnetic field creating a magnetic dipole. • When the magnetic field is removed, the domains re-randomize resulting in no magnetic attraction. They are temporary • Soft ferromagnetic material is attracted to both the North pole and South pole.

19. Example 1: Application of Magnetism What type of ferromagnetic material would you use for video cassette tapes, audio cassette tapes, credit card strips, hard drives or floppy discs? • Soft Ferromagnetic • Hard Ferromagnetic • Diamagnetic • Paramagnetic Diamagnetism and paramagnetism are too weak, and soft ferromagnetic material is temporary while the external field exists.

20. Types of Magnets • Temporary: When charged particles move through space, they induce a magnetic field (Electromagnets). • Permanent: Electrons have an intrinsic magnetic field that may add together in certain matter to create a magnetic field (Speakers). Temporary Permanent

21. Types of Magnetism • Ferromagnetism: Ferromagnetic materials (Iron, Cobalt, Nickel) exhibit a long-range ordering phenomenon at the atomic level which causes the unpaired electron spins to line up parallel with each other in a region called a domain. (Bind ~ Bapp x 105) • Paramagnetism: Paramagnetic materials (Aluminum, Tungsten, Oxygen) form weak magnetic dipoles at the atomic level when exposed to a magnetic field (Bind ~ Bapp x 10-5). Thermal motion results in randomization of the dipoles and a weak net magnetic field. • Diamagnetism: Diamagnetic materials (Gold, Copper, Water) respond to magnetic fields by developing a weakly opposing magnetic field (Bind ~ -Bapp x 10-5). Bind = Induced Magnetic Field, Bapp = Applied Magnetic Field

22. Key Ideas • All magnets have North and South Poles • Magnetic field lines originate in the North and end at the south pole. • Magnetic field lines do not cross. • Magnetism exists at the atomic level. • Magnetism is the result of moving charges. • Some magnets are temporary while others are permanent. • Types of Magnetism. • Ferromagnetism. • Paramagnetism. • Diamagnetism.

23. Magnetic Fields due to Current

24. Source of Magnetic Fields • Electrical Charge in motion. • Currents occur at the atomic level in atoms due to the orbits of electrons around the nucleus. • The intrinsic spin (+1/2, -1/2) is critical in the case of magnetism.

25. A Surprising Discovery • In 1820, Hans Christian Oersted discovered that moving charges create a magnetic field.

26. I B a r Magnetic Field of a Current Carrying Wire • Hans Christian Oersted discovered that a wire carrying current influenced the needles of nearby compasses. • By applying right-hand-rule #2, the direction of the magnetic field can be determined around the wire. • For an infinitely long straight wire: • B is proportional to I and inversely proportional to r. Magnetic Field due to a wire.

27. NI B a R Magnetic Field in a Loop of Wire • For the center of a circular loop, the magnetic field is: Where: N = number of turns of wire. R = Radius of loop.

28. L Magnetic Field of a Solenoid • For a solenoid, the magnetic field is given by: B aNI L B anI Where: n = the number of turns per length of coil = N/L

29. F F F F (a) (b) Magnetic Force on Current Carrying Wires • When two current carrying wires have current flowing in the same direction, they will be attracted to one another (a). • When two current carrying wires have current flowing in opposite directions, they will repel (b).

30. x x x Magnetic Force on Current Carrying Wires(cont.) -The influence of the magnetic field of wire (a) on wire (b). -Using RHR #1, we see that the force by wire (a) on wire (b) is such that it is attracted to wire (b). -The same is true for wire (b) on wire (a). -However, even if the current is flowing in opposite directions, won’t the conductors be attracted to one another? (a) (b) x B

31. Magnetic Force on Current Carrying Wires(cont.) -No. Note that the magnetic fields cancel each other between the conductors while they add outside for two parallel conductors with current moving in the same direction. -As a result the conductors are attracted to one another. -In the case where the conductors have current flowing in opposite directions, the field lines add between them while they cancel outside. This results in a net repulsion between the two conductors.

32. Key Ideas • The strength of a magnetic field created by current in a wire is inversely proportional to the distance from the wire. • Two current carrying wires will attract each other if the current flows in the same direction • Two current carrying wires will repel each other if the current is in opposite directions. • The strength of the magnetic field of current in a loop is proportional the current in the loop and the number of loops.

33. Magnetic Forces

34. Forces in Magnetism • The existence of magnetic fields is known because of their affects on moving charges. • What is magnetic force (FB)? • How does it differ from electric force (FE)? • What is known about the forces acting on charged bodies in motion through a magnetic field? • Magnitude of the force is proportional to the component of the charge’s velocity that is perpendicular to the magnetic field. • Direction of the force is perpendicular to the component of the charge’s velocity perpendicular to the magnetic field(B).

35. Magnetic Force (Lorentz Force) FB = |q|vB sinθ • Because the magnetic force is always perpendicular to the component of the charge’s velocity perpendicular to the magnetic field, it cannot change its speed. • Force is maximum when the charge is moving perpendicular to the magnetic field ( = 90). • The force is zero if the charge’s velocity is in the same direction as the magnetic field ( = 0). • Also, if the speed is not changing, KE will be constant as well.

36. What is the magnetic field (B)? • The magnetic field is a force field just like electric and gravitational fields. • It is a vector quantity. • Hence, it has both magnitude and direction. • Magnetic fields are similar to electric fields in that the field intensity is directly proportional to the force and inversely related to the charge. E = FE/q B = FB/(|q|v) Units for B: N•s/C•m = 1 Tesla

37. Right Hand Rules • Right hand rule is used to determine the relationship between the magnetic field, the velocity of a positively charged particle and the resulting force it experiences.

38. Right Hand Rules #2 #1 #3 FB = |q|v x B

39. V vsinθ Uniform B θ + q The Lorentz Force Equation & RHR FB = qvB sinθ What is the direction of force on the particle by the magnetic field? • Right b. Left c. Up d. Down • Into the page f. Out of the Page

40. v1 + x x x x x x x x x x x x v2 + Lorentz Force Two protons are launched into a magnetic field with the same speed as shown. What is the difference in magnitude of the magnetic force on each particle? a. F1 < F2 b. F1 = F2 c. F1 > F2 F = qv x B = qvBsinθ Since the angle between B and the particles is 90o in both cases, F1 = F2. How does the kinetic energy change once the particle is in the B field? a. Increase b. Decrease c. Stays the Same Since the magnetic force is always perpendicular to the velocity, it cannot do any work and change its KE.

41. x x x x x x x x x x x x x x x x x x x x x x x x v + Right Hand Rule – What is the Force? What is the direction of the magnetic force on the charge? a) Down b) Up c) Right d)Left

42. Right Hand Rule – What is the Charge? Particle 1: • Positive • Negative • Neutral Particle 2: • Positive • Negative • Neutral Particle 3: • Positive • Negative • Neutral

43. Right Hand Rule – What is the Direction of B What is the direction of the magnetic field in each chamber? • Up • Down • Left • Right • Into Page • Out of Page 1 4 2 3 What is the speed of the particle when it leaves chamber 4? • v/2 b. -v • v d. 2v Since the magnetic force is always perpendicular to the velocity, it cannot do any work and change its KE.

44. v1 + x x x x x x x x x x x x v2 + Example 2: Lorentz Force Two protons are launched into a magnetic field with the same speed as shown. What is the difference in magnitude of the magnetic force on each particle? a. F1 < F2 b. F1 = F2 c. F1 > F2 F = qv x B = qvBsinθ Since the angle between B and the particles is 90o in both cases, F1 = F2. How does the kinetic energy change once the particle is in the B field? a. Increase b. Decrease c. Stays the Same Since the magnetic force is always perpendicular to the velocity, it cannot do any work and change its KE.

45. x x x x x x x x x x x x x x x x x x x x x x x x + v Trajectory of a Charge in a Constant Magnetic Field • What path will a charge take when it enters a constant magnetic field with a velocity v as shown below? • Since the force is always perpendicular to the v and B, the particle will travel in a circle • Hence, the force is a centripetal force.

46. x x x x x x x x x x x x x x x x x x x x x x x x + R v Fc Radius of Circular Orbit What is the radius of the circular orbit? Lorentz Force: F = qv x B Centripetal Acc: ac = v2/R Newton’s Second Law: F = mac qvB = mv2/R R = mv/qB

47. Crossed Fields in the CRT • How do we make a charged particle go straight if the magnetic field is going to make it go in circles? • Use a velocity selector that incorporates the use of electric and magnetic fields. • Applications for a velocity selector: • Cathode ray tubes (TV, Computer monitor)

48. x x x x x x x x x x x x + + + + + FE FB E - - - - - B into page Phosphor Coated Screen - - - v v v Crossed Fields • E and B fields are balanced to control the trajectory of the charged particle. • FB = FE • Velocity Selector • Velocity Selector qvB = qE v = E/B

49. Earth’s Magnetosphere • Magnetic field of Earth’s atmosphere protects us from charged particles streaming from Sun (solar wind)

50. Aurora • Charged particles can enter atmosphere at magnetic poles, causing an aurora