Magnetism

1. Magnetism

2. N S Magnetic south pole Magnetic north pole 磁北極 磁南極

3. Like poles repel. 同性磁極相斥。 異性磁極相吸。 Unlike poles attract Law of magnetism:

4. stronger field weaker field (磁場的概念) Magnetic field Magnetic field :N- pole to S- pole. 磁場線是從磁北極繞向磁南極的。 field lines closer 磁場愈強 磁場線愈密 field lines further apart 磁場線愈疏 磁場愈弱

5. Plotting compasses. 用多個指南針把磁場顯示出來。 line up along the direction of magnetic field lines 指南針的指針自動排列並顯示出磁場。

6. X Magnetic field pattern of 2 opposite poles 兩根異極相對的磁棒所產生的磁場圖形： Magnetic field pattern of 2 like poles 兩根同極相對的磁棒所產生的磁場圖形 neutral pointX : magnetic field = 0 中和點 :磁場的合量為零

7. direction of current direction of magnetic field right-hand grip rule. 右手握拳定則 電流方向 磁場方向

8. B-field due to a straight current wire--1 載電流直導線周圍的磁場圖形--1

9. B-field due to a straight current wire--2 載電流直導線周圍的磁場圖形--2

10. B-field inside a solenoid uniform magnetic field 磁場線分佈均勻 螺線管中央 CW: P.101 current flow 電流 方向 • Outside the solenoid螺線管外 • B ~ bar magnet (與磁棒周圍的磁場線相類似) • Solenoid ~ bar magnet

11. Electromagnet 電磁鐵 can be ‘switched’ on and off. 能夠隨時 啟動 和 關閉 magnetic field direction can be reversed 磁場方向 隨電流方向改變。

12. Strength of an electromagnet can be increased by: 以下方法加強電磁鐵的磁場: • a soft iron core • Increasing current • 3. Increasing the number of turns per unit length (插入軟鐵心) (電流 ) 每單位長度的線圈匝數

13. Application of electromagnet: spring push button battery metal contact electromagnet hammer gong (電鈴) 1. Electric bells 簧片 按鈕開關 電池組 金屬接點 電磁鐵 鐵錘 銅鑼

14. The push button is pressed 按下按鈕開關 The spring moves back to original position. Electromagnet is switched on. 電流通過電磁鐵 簧片彈回原位 The contact is closed. 簧片和金屬接點再次接觸 The spring is attracted by the electromagnet. 電磁鐵吸引簧片 The process repeats. 這個過程不斷重複 The contact is broken. 簧片與金屬接點斷開

15. electromagnet metal plate 2.Telephones 電話 Varying electric current passes the electromagnet 通過電磁鐵的電流不斷改變 電磁鐵 Varying magnetic field is produced. 產生不斷改變 的磁場 The metal plate vibrates. 金屬片振動 金屬片 Sound is produced 產生聲音

16. Magnetic force on a current

17. Fleming’s left-hand rule 弗林明左手定則 magnetic force(F) 磁力的方向 magnetic field(B) 磁場的方向 Current (i) 電流的方向

18. S-pole A current in a magnetic field: 置於磁場內的載電流導線會感受到磁力 F is increased when: 在下列情況下，磁力均會增加： B i is increased. 電流增加 B is increased. 磁場增強 3. length of the wireis increased. 導線較長 F I F = 0 when B // I

19. Magnetic field at 900 with i, F is maximum 電流的方向與磁場的方向成直角，磁力 B // i, F = 0 當電流與磁場平行時，磁力等於零。

20. Moving-coil loudspeakers 動圈式揚聲器 Varying current (a.c.)through the coil. 不斷改變的電流 通過線圈 varying I 不斷改變的電流 paper cone(錐形紙盤) The coil experiences varying magnetic force. 線圈受不斷改變 的磁力作用 Coil (線圈) Magnet 磁鐵 Paper cone vibrates producing sound 產生聲波

21. The paper cone vibrates. 使錐形紙盤振動 Sound is produced. 產生聲波 coil moves inwards and outwards. 線圈振動 N S S N S S

22. Turning effect of a current carrying coil 載電流線圈在磁場中的轉動效應 Downward force on AB magnetic field 磁場 AB 邊 所受的力向下 C upward force on AB B 所受的力向上 CD 邊 force on AB = 0 沒有磁力作用 BC 邊 When coil vertical, F = 0 It rotates due to inertia D A Current電流 Coil oscillates for d.c. current The coil rotates. 線圈轉動

23. The turning effect can be increased by: 以下方法可增加載電流線圈的轉動效應： • increasing i (增加電流的大小) • increasing B (增強磁場) • winding more turns (增加線圈的匝數) • increasing the area of the coil (增加線圈的面積)

24. Structure of moving-coil ammeter (動圈式檢流計的結構) Pointer(指針 ) scale cylindrical magnet (弧形磁鐵) I N Coil (線圈) S soft iron core (軟鐵心) hair spring (游絲)

25. The pointer stops when the magnetic turning effect = restoring torque of the hair springs. • (線圈的轉動效應與游絲的反向轉動效應互相抵消, 線圈停止轉動) • Deflection increases with i • (電流,偏轉)

26. d.c motor (直流電動機能沿單一方向持續轉動): coil線圈 + – axis of rotation Commutator / half-rings: connected to coil. Magnet 磁鐵 換向器:兩個連接線圈的半圓環 Carbon brush 電刷

27. at vertical position... 在垂直的位置 At the start... 開始時 B A B A C D C 2 1 D + + – – F = 0! The coil keeps turning due to inertia. • 線圈因自己的慣性而保持轉動。 coil rotates as current flows (i from B to C). 線圈在通電後轉動

28. half-rings and the brushes interchanges. 兩個半圓環都會與另一個電刷接觸。 After ½ cycle... B A C 3 D Current in the coil is reversed (i from C to B). 改變電流的方向 + - Magnetic forces acting on the coil is reversed too. 作用在線圈的磁力逆轉 The coil keeps on rotating in the same direction. 線圈繼續沿單一方向轉動。

29. A practical motor (實際的電動機) magnetic field strength 磁場強度 磁場強度 magnetic field strength 1. It contains several coils at an angle to each other.(擁有多組線圈，以不同角度互相圍繞。) 2. wires are wound around a soft iron core. 線圈繞在軟鐵心上 3. Electromagnets are used instead of permanent magnets. 使用電磁鐵取代永久磁鐵

30. Electromagnetic induction: Generation of electricity from magnetism.

31. Experiment: move up move down The galvanometer deflects to 2 sides. A current flows through the wire called induced current . Wire at rest, i = 0. wire is moved up and down repeatedly, a.c. results.

32. Induced emf / current can be produced by: • Cutting B field lines by a moving conductor or, • Changing B field inside a resting conductor. B increasing/decreasing XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

33. Fleming’sright - hand rule(弗林明右手定則): motion of the conductor(F) magnetic field(B) induced current(i)

34. south pole induced current 感生電流 wire e.g. a wire cuts through the magnetic field lines. 當導體切割磁場線時... Induced voltage/ induced current 便會產生感生電壓。

35. The direction of induced current can also be found by: N S Lenz’s law 我們可利用下列方法找到感生電流的方向：楞次定律 An induced current flows in a direction so as to oppose the change producing it. 感生電流總是跟產生它的改變抗衡。 Induced currenti opposes magnet’s motion into coil. 感生電流 i抗衡磁棒進入線圈

36. N S Induced currenti opposes magnet’s motion moving away coil. 感生電流i抗衡磁棒移離線圈。

37. e.g. v X X X X X X X X V/i t

38. rate at which :(1)the conductor cuts through the magnetic field.(2)B is changing 導體切割磁場線的速率  induced voltage/ i induced voltage When(以下方法令感生電壓增加): conductor or the magnet moving faster 導體或磁鐵的運動速率 lengthof the conductor in the magnetic field 導體在磁場中的長度 magnetic field changing more rapidly 磁場的改變速率

39. Application of electromagnetic induction: a.c. diaphragm vibrates. sound waves coil vibrates. Varying induced current flows in the coil. The signal is converted into sound by loudspeaker. 1. microphone diaphragm sound waves strike the diaphragm. S N S magnet movable coil

40. coil slip rings Generators 1. a.c. generators B I flows from A to B A carbon brush

41. 1 When the coil is at vertical position... B • coil does not cut the magnetic field lines. • No induced current. C A D

42. 2 When the coil has rotated 1800 C B D A induced current flows in the coil(B to A).

43. A A A induced current number of revolution 1 The induced current =0 when the coil passes the vertical position.

44. P.162 1-5

45. similar to a.c. generator coil Simple d.c. generators i flows clockwisely commutator / half - ring.

46. Induced current / voltage in generators can be increased by: 1. use a stronger magnet 2. wind more turns 3.use a soft iron core 4. Increase the coil area 5. coil rotating at a higher speed

47. driving wheel axle soft iron C-core case coil (fixed) permanent magnet to head lamp magnet spins Current is induced The head lamp lights up Bicycle dynamo wheel rotates

48. soft iron core The structure of a transformer secondary coil a.c. source/VP primary coil a.c. Output/ VS

49. turn ratio Formula: where Vp = input voltage Vs = output voltage Np = number of turns in the primary coil Ns = number of turns in the secondary coil

50. input voltage < output voltage called step-up transformer. input voltage > output voltage calledstep-down transformer.