1. Electromagnetic Induction

2. Electric Fields Electric fields are created bycharges A charge in an electric field always has a force on it + force

3. Magnetic Field Magnetic fields are created by moving charges This can happen in magnets or current carrying wires.

4. Magnetic Fields only affect moving charges ++ velocity FORCE is…ZERO FORCE is… towards you

6. strong field weak field

7. strong field weak field

8. NS electron beam electron gun x y z

9. What direction is the magnetic field?

10. NS wire Wire's velocity x y z

11. Magnetic Flux Magnetic flux is the amount of magnetic field. It depends on the field strength and the area

13. NS There is a large flux through this loop And a smaller flux through this loop Even smaller flux through this loop

15. Which ring has the biggest magnetic flux in it? Which ring has the strongest magnetic field strength in it? Magnetic field lines

16. Which ring has the largest magnetic flux in it?

17. Magnetic = Magnetic X Area Flux Field Strength Webers = Tesla x m 2

18. Magnetic field strength is also called flux density.

19. Which ring has the biggest magnetic flux ?

20. Area is same Field strength is same Which ring has the biggest magnetic flux ?

21. Year 12: A wire cutting across a magnetic field has an induced EMF (or voltage) Faraday’s Law When the magnetic flux through a loop changes, there is an induced EMF (voltage) The faster the change, the bigger the EMF

22. Changing the Flux You can change the flux by changing the field strength or the area perpendicular

23. NS Changing the field strength

24. NS Changing the area

25. NS Changing the area perpendicular

26. Flux Change in a Generator

27. faraday-mx.swf

28. So for many loops, becomes,

29. Changing the actual area Changing the angle Changing the Area

30. Flux change in a moving loop flux time

31. voltage time Induced EMF in a moving loop

32. voltage time flux

33. Close switch… current increases …field increases Flux change through 2 nd coil Induced EMF in 2 nd coil Creates current in 2 nd coil Creates magnetic field that opposes the cause. N N

34. Lenz’s Law Electron flow Force on roller 1: electrons in roller are moving 2: causing them to be pushed 3: electrons in roller are now flowing 4: causing them (and roller) to be pushed

35. Faraday: A flux change causes an induced EMF (voltage) Lenz’s Law states that the induced voltage opposes the flux change that caused it.

36. N Falling magnet creates a flux change in the pipe. This creates induced EMF This creates induced current This creates induced magnetic field magnetic field opposes flux change

37. So how does your electric toothbrush charge up?

38. Increasing current in Primary Causes flux change in core Causes flux change in Sec Causes induced EMF in Sec Causes induced current in Sec This is called Mutual Induction P S

39. AC Generator http://www.walter- fendt.de/ph11e/generator_e.htmhttp://www.walter- fendt.de/ph11e/generator_e.htm

40. B A C D Voltage Angle Flux Angle Coil position (End on)

41. Voltage Angle Flux Angle

42. ..\faraday.jar

43. Transformers

44. A transformer is used to increase or decrease the voltage. Mains voltage in NZ is 240 V AC. Your cellphone charger needs about 4.0 V. It uses a transformer to reduce the voltage.

45. Increasing current in P Causes flux change in P Causes flux change in S Causes induced EMF in S Causes induced current in S This is called Mutual Induction PS

46. transformer.jar

47. Primary current Magnetic flux in core Induced EMF in Secondary

48. The AC current in the primary coil is changing sinusoidaly So the magnetic flux is changing sinusoidally So the induced EMF is sinusoidal

49. The output voltage (Vs) depends on: the input voltage the ratio of turns or VsVp primary secondary Iron core NpNs

50. Mutual Induction This is when a changing current in one coil induces a voltage (EMF) in a second coil.

51. Small Mutual Inductance between coils

52. Iron cores produce stronger, more concentrated magnetic field  larger Mutual Inductance

53. Toroidal core produces even larger Mutual Inductance

54. M is the mutual inductance, measured in Henries (H)

55. Using transformers to save energy

56. You’ve seen these around the streets. What do they do?

57. Transformers are very important for power transmission. The power transmitted along wires is given by: Power = V x I The power wasted as heat in the wire is given by : Power = I 2 x R

58. To minimise the power lost as heat, the wire’s resistance and the current must be a small as possible. Power lost = I 2 x R To transmit the same power, the voltage must be very large. Power transmitted = V x I

59. Power station generators produce electricity at about 4000 V Transformers increase it to about 400 000 V. (and reduce the current)

60. Transmission lines then carry the electricity at 400 000 V

61. Transformers in Auckland reduce the voltage to about 4000 V

62. This is then reduced to 240 V in local transformers around the streets

63. 4000 V 400 000 V 4000 V 240 V

64. Your cellphone charger changes the 240 V current to about 4 V

65. http://phet.colorado.edu/web- pages/simulations-base.htmlhttp://phet.colorado.edu/web- pages/simulations-base.html

66. Inductors An inductor is a wire coil usually wrapped around an iron core

67. Self Inductance A coil can induce a voltage in itself !!!?? A

68. Predict what happens when the switch closes. What does happen when the switch closes? A

69. What happens when the switch opens? A

70. An inductor is designed to oppose a changing current This is because it can induce an EMF in itself. This is called Self Inductance.

71. Increasing current Causes increasing magnetic field There is a flux change through coil Causes induced EMF Direction of EMF opposes current change Increasing current Induced EMF Current Increasing

72. Self Inductance A coil can induce a voltage in itself. L is called the self inductance Self Inductance (L) is measured in Henries (H)

73. This induced EMF (or back EMF) opposes the increase in current, so the current rises… SLOWLY This is called Self Inductance (The coil induces an EMF in itself)

74. current timeClose switch I = V/R R is the ohmic resistance of the inductor

75. current Close switch Back EMF Close switch Recall:

76. time Close switch I max Recall the meaning of time constant for a capacitor and resistor? It means the same for an inductor and resistor. 0.63 I max

77. Larger inductance, longer time to reach maximum current Larger resistance, smaller current so shorter time to reach maximum current

78. current Close switch How would the graph change if: Inductor had higher inductance? Inductor had higher resistance (careful)

79. Decreasing current Causes decreasing magnetic field This is a flux change Causes induced EMF Direction of EMF opposes current change. Current Decreasing

80. This induced EMF (or back EMF) opposes the decrease in current, so the current drops… SLOWLY This is called Self Inductance

81. When the switch opens, the current drops to zero rapidly This causes a large flux change This induces a very large EMF This causes a spark across the switch

83. Mutual Inductance http://phet.colorado.edu/web- pages/simulations-base.html http://phet.colorado.edu/web- pages/simulations-base.html

84. http://www.walter- fendt.de/ph11e/osccirc.htm Extension: LC oscillation