2. 9 電磁感應 How did the electric guitar revolutionize rock? From acoustic to electric guitars

3. 9-1 Two symmetric Situations l current loop + magnetic field → torque l torque + magnetic field → current

4. 9-2 Two Experiments First Experiment A Magnet is moving with respect to the loop.

5. Second Experiment The switch S is closed or opend.

6. 9-3 Faraday’s Law of Induction l An emf is induced when the number of magnetic field lines that pass through the loop is changing. – A Quantitative Treatment – The magnetic flux Electromotive force

7. Faraday’s law emf magnetic flux induced emf induced electric field

8. 例 1 A solenoid contains a coil l i =1.5 A n s = 2.2×10 4 turns / m l A = 3.46 × 10 -4 m 2 N c = 130 turns l 電流在 25ms 內穩定降至 0

9. 計算 l i =1.5 A n = 2.2×10 4 turns / m l A = 3.46 × 10 -4 m 2 N = 130 turns l 電流在 25ms 內穩定降至 0 e

10. 感應電流所生的磁場與感應出該電流的磁場反向 9-4 Lenz’s Law l Opposition to pole Movement l Opposition to flux change l Electric Guitars

11. 例 2 B = 4.0 t 2 +2.0 t + 3.0 Ebat = 2.0V r = 0.20m R = 2.0Ω increasing clockwise e

12. 例 3 B = 4 t 2 x 2 W = 3.0m H = 2.0m t = 0.1s increasing counterclockwise e

13. 9-5 Induction and Energy Transfers The work done by the applied force and the thermal energy produced in the wire. e e

14. The Power

15. Eddy currents – induction stoves and EM braking

16. Eddy currents –EM braking

17. Eddy currents – transformers

18. Eddy currents – metal detectors

19. The microphone

20. The tape recorder

21. Speaker

22. Voice-coil positioner of a HDD

23. 9-6 Inductors and Inductance

24. Inductance of a solenoid:

25. Inductance of a toroid

26. 9-7 Self-Induction An induced emf appears in any coil in which the current is changing.

27. 9-8 Mutual Induction

28. Two circular close-packed coils