2. Generators

3. Warm-up 1Which of the following coils do you think is harder to be turned in a magnetic field? A. B. No difference.

4. Warm-up 2So, when we generate current by cutting magnetic field lines, electrical energy is produced from nothing. electrical energy comes from the internal energy of the wire. electrical energy is converted from the work done in moving the wire.

5. Bicycle dynamos provide small I to the bicycle light. Huge generators supply mains electricity (large I) to homes, offices & factories. They use electromagneti c induction!! (Photo credit: CLP Power Co. Ltd)

6. 1Induced voltage and current in a rotating coil When the coil is rotated, A B D C motion B-field I centre-zero galvanometer By Fleming’s right-hand rule... I AB moves upwards, CD moves downwards  drives I through coil & the outside circuit voltage is induced across AB and CD

7. 1Induced voltage and current in a rotating coil B-field parallel to... After 1/4 turn, coil passes through the vertical position.   no field lines are cut B-field A B D C motion of AB & CD   no induced voltage   no induced I motion

8. 1Induced voltage and current in a rotating coil A B D C 180 o After 1/2 turn,  induced I flows in opposite direction in the outside circuit AB moves downwards, CD moves upwards motion B-field I I If rotation goes on...  a.c.

9. 2Simple a.c. generator When coil is rotated  induce alternating voltage end of coils are fixed to 2 slip rings carbon brushes  press against the slip rings  connect slip rings to outside circuit  a.c. flows through outside circuit

10. Current in an a.c. generator: 1 2 turn 1 turn no. of turns I coil orientations 2Simple a.c. generator

11. Current in an a.c. generator: 1 2 turn 1 turn no. of turns I I = 0I = 0(coil does not cut through any field line) 2Simple a.c. generator

12. Current in an a.c. generator: 1 2 turn 1 turn no. of turns I max. forward I max. backward I coil cuts through the field lines most rapidly 2Simple a.c. generator

13.  using a stronger magnet,   no. of turns in coil,   the area of coil,  winding the coil on a soft-iron core,  rotating the coil at a higher speed. Note: Frequency of an a.c. generator (current) = frequency of the coil's rotation  induced voltage & I by:

14. Example 2 Current of an a.c. generator This is I-t graph of a simple a.c. generator. (a)How many turns are made by coil of generator within the first 0.04 s? current 0 0.02 0.04 0.06 time / s 1 revolution

15. Example 2 Current of an a.c. generator (b)What is the frequency of the a.c. produced by the generator? current 0 0.02 0.04 0.06 time / s f = 1 T = 25 Hz = 1 0.04 s from (a)

16. Example 2 Current of an a.c. generator (c)When does the coil become (i) parallel, and (ii)  to the magnetic field? current 0 0.02 0.04 0.06 time / s produces max. voltage produces zero voltage (i)0.01 s, 0.03 s, 0.05 s, etc. (ii)0 s, 0.02 s, 0.04 s, 0.06 s, etc.

17. Example 2 Current of an a.c. generator (d)If the coil is rotated twice as fast, what happens to… current 0 0.02 0.04 0.06 time / s (i) frequency, (ii)maximum a.c. produced? Sketch the I-t graph. 25 Hz

18. Example 2 Current of an a.c. generator current 0 0.02 0.04 0.06 time / s (i) frequency = 50 Hz (doubled) (ii)max. I is also doubled. If the coil is rotated twice as fast f = 50 Hz f = 25 Hz 25 Hz

19. 3Simple d.c. generator similar to a.c. generator different from a.c. generator Every time the coil passes through the vertical, it reverses coil's connections with outside circuit.  I in the outside circuit always flows in 1 direction. commutator

20. 3Simple d.c. generator Current in d.c. generator: 1 2 turn 1 turn no. of turns I coil orientations commutator reverses I But, I varies during 1 complete rotation.

21. 4Practical generators aBicycle dynamo bAlternators

22. Experiment 17c The bicycle dynamo Study the voltage produced by a bicycle dynamo. dynamo centre-zero milliameter CRO dynamo

23. friction wheel tyre magnet coil Unlike the simple a.c. generator: coil is fixed on a soft-iron core magnet is rotated instead  rotation causes induced a.c. fixed! waveform of the a.c. produced by dynamo waveform not smooth No problem!  this a.c. is used to light up a lamp, NOT produce a smooth waveform aBicycle dynamo

24. Practical a.c. generator has a rotating electromagnet called the rotor ( 轉子 ). Around the rotor are fixed sets of coils called stator ( 定子 ). Such type of a.c. generator is called alternator ( 交流發電機 ). bAlternators (Photo credit: HK Automobile Assoc.)

25. In power stations, huge alternators are used to generate electricity. The alternator is driven by a steam turbine. powered by coal, oil or nuclear reactor bAlternators (Photo credit: CLP Power Co. Ltd)

26. AMagnet. BMetal coil. CCommutator. DCarbon brush. Which of the following is NOT a part of an a.c. generator? Q1 Which of the following...

27. Q2 What are the turning... I from an a.c. generator changes sign for every _____ revolution(s). Hence, the coil of the a.c. generator takes ____s to complete 1 revolution. By f =, the rotation frequency of the coil of the a.c. generator is ______ Hz. 1/2 0.1 1 T What is the turning frequency of the coil of the a.c. generator? 10 I / A time / s 0.1 0.05

28. Q2 What are the turning... I from a d.c. generator has two peaks in ______ revolution(s). Hence, the coil of the d.c. generator takes _______ s to complete 1 revolution. By f =, the turning frequency of the coil of the d.c. generator is ______Hz. 1 0.1 1 T What is the turning frequency of the coil of the d.c. generator? 10 I / A time / s 0.1 0.05