2 sources of magnetism

1. 2 sources of magnetism Permanent magnets Electromagnets

2. Basics of magnetism • Magnet: An object that creates a magnetic field and exerts a magnetic force • All magnets have a north and south pole (Not “positive/negative” or “plus/minus”) • If you cut a magnet in half, you get two magnets, each with a north and south pole

3. Magnets are strongest at the poles A horseshoe magnet is just a bar magnet bent into a U N S

4. Permanent Magnets • Certain minerals (lodestone) are naturally magnetic • Other materials can be magnetized and retain their magnetism → ferromagnetic materials • These materials (iron) get temporarily magnetized by placing them near magnets • Some materials have essentially no magnetic properties: copper, aluminum, plastics... • Heat can destroy magnetism

5. Magnets have magnetic fields • Magnets can attract or repel without touching because their magnetic fields are interacting with each other • The magnetic field is the region where a magnet can exert a force

6. Magnetic field, B • B is the symbol for magnetic field • units: Teslas (T)

7. ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? N S ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? Map a magnetic field A magnet produces a magnetic field in the space around it ? ?

8. The magnetic field of a bar magnet

9. The magnetic field of a bar magnet

10. The magnetic field of a bar magnet

11. The magnetic field of a bar magnet • Magnetic field lines are always closed loops, even if we don’t have room to draw them that way • (external) Field lines point North to South • Like electric field lines, the magnetic field is strongest where the lines are closer

12. S N Earth’s magnetic field resembles that of a bar magnet A compass is just a magnet that aligns itself with earth’s magnetic field

13. Geographic north is actually a magnetic south pole (north) • Your compass points toward the magnetic pole, not the geographic pole • This error needs to be adjusted for when navigating (south) • Declination: angle between magnetic and geographic poles

14. The magnetic poles are moving

15. Earth’s magnetic poles switch North and south reverse roughly every 250,000 years (we might be experiencing a reversal now) http://www.pbs.org/wgbh/nova/magnetic/timeline.html

16. Magnetism comes from electrons in motion • Each electron has its own magnetic field associated with its “spin” • In most materials, paired electrons cancel each other’s magnetic field • The result is no overall magnetic field for this kind of material magnetic field

17. Ferromagnetic materials • A few materials like iron, nickel, and cobalt, have electrons spins which don’t cancel entirely • A domain is a large group of atoms (drawn as a single arrow below) whose spins are aligned • Ferromagnetic materials are not necessarily magnetized if the domains are disorganized

18. Think of a domain as a really tiny bar magnet N S

19. Pin 1 Pin 2 Pin 3 How do domains get aligned? • Stroke the magnetic material with a magnet • Put the magnetic material in a strong magnetic field Domains in pin getting aligned or Domains in pins getting aligned Domains in bar magnets getting aligned better

20. Domains getting aligned by the presence of a strong magnetic field

21. A magnetic field will deflect a stream of electrons

22. q = 8 C q = 8 C q q q What is magnitude of force? v = 3 m/s v = 3 m/s v = 3 m/s B = 5 tesla S N q = 8 C

23. Magnetic force on a moving charge • A magnetic field exerts a force on moving electrically-charged particles • The AMOUNT of force depends on four things: F = force on charged particle (newton, N) q = charge (coulomb, C) v = velocity (m/s) B = magnetic field (tesla, T) θ = angle between magnetic field and motion

24. q = 8 C q = 8 C q q q What is magnitude of force? v = 3 m/s v = 3 m/s v = 3 m/s B = 5 tesla S N q = 8 C F = 103.9 N F = 103.9 N F = 120 N

25. q q What two ways could a proton move through the magnetic field and have zero force exerted on it? B = 5 tesla N S

26. Right-Hand Rule #1 The DIRECTION of the magnetic force on a positive charge moving through a magnetic field can be determined using the “right-hand” rule: • Point your four fingers in the direction of magnetic field (N to S) • Point your thumb in the direction of motion (“velocity”) • Palm shows the direction of the force on the positive charge Magnetic field, B Velocity, v Force on wire

27. Force is “into screen” or “away from you” Which way is proton pushed? Velocity is upward B south pole north pole

28. Which way is proton pushed? B south pole north pole Velocity is away from you Force pushes proton downward

29. Which way is proton pushed? south pole B Velocity is toward you Force pushes proton to the left north pole

30. Which way is proton pushed? Velocity is upward B Force pushes proton to the left

31. A magnetic field deflects a stream of electrons

32. Magnetic forces on electrons in a TV picture tube

33. B θ v F Two variations seen in homework problems • If the charge is negative rather than positive, the force will be directed opposite what your right hand shows • If θ < 90º, the force will be less (as solved by F=qvBsinθ), but direction of force will still be in same direction as when θ = 90º

34. S N Example 1 • QUESTION • A proton moves due north with a speed of 1.5 x 106 m/s at the Earth’s equator. What is the magnetic force exerted on it? ANSWER

35. S N Example 2 • QUESTION • A proton moves due east with a speed of 1.5 x 106 m/s at the Earth’s equator. What is the magnetic force exerted on it? ANSWER , upward (away from surface of earth)

36. Circular motion of particles in magnetic fields • When the velocity of a charged particle is perpendicular to a uniform magnetic field, it causes the particle to move in a circular path • The diagram shows a magnetic field (pointing directly into the screen) and a positively charged particle moving in a circular path • Its motion started when the particle was fired into the field along the surface of the page • Why is the motion circular? First, recall that the magnetic force is always perpendicular to the velocity vector • This means it neither increases nor decreases the speed of the particle. It only changes the direction of its motion

37. = F qvB sin q = F IlB sin q Magnetic Force on a Current-Carrying Wire • A magnetic field exerts a force on a moving charge • It follows that a magnetic field should also exert a force on a wire carrying current • And the relationship follows from the equation for the magnetic force on a single moving charge becomes

38. = q sin F IlB Resulting force on wire Length, L Magnetic field, B θis angle between wire and field lines (0º < θ<90º) Current, I

39. = F qvB sin q = F IlB sin q Equivalent formulae (compare units)

40. Magnetic field current Force on wire Magnetic forces on wire

41. Torque on a loop of wire • Magnetic fields and loops of current-carrying wire are essential components of electric motors • In this diagram, the B-field points right and the conventional current runs counterclockwise • Only left and right sides of this loop experience torque • Current in front and back sections run parallel to the external B-field

42. Magnets make motors work When a current is present in a coil, it experiences a force from the field of the horseshoe magnet N S

43. Where: F = Force on the Moving Charge (Newtons, N)B = Strength of Magnetic Field [Teslas, T) - [1 T = 1 N / A m] I = Current in Wire (Amps)l = Length of wire exposed to magnetic field q = Magnitude of the charge (Coulombs, C)v = Velocity of the Moving Charge (m/s) = Angle between magnetic field and wire = F qvB sin q I B  Review The Deflective Force on a Current-Carrying Wire The Deflective Force on a Moving Charge = F IlB sin q

44. F B I F I I I B B F I F I F F B B B Evaluating Force, Mag Field, and Current Direction for wire Find the direction of the missing element (I, B, F)(answers in red)