Chapter 28 Electromagnetic Induction

1. Chapter 28 Electromagnetic Induction

2. Induction by the relative motion between a coil and a magnet Experiment 28.1 28.1 Induced e.m.f. and induced current • An e.m.f. is induced whenever • the conductor cuts through magnetic field lines due to the relative motion between the conductor and the magnet.

3. Induction by moving a wire across magnetic field lines Experiment 28.2 • An e.m.f. is induced whenever • the conductor cuts through magnetic field lines due to the relative motion between the conductor and the magnet.

4. Induction caused by a changing magnetic field Experiment 28.3 • An e.m.f. is induced whenever • the conductor cuts through magnetic field lines due to the relative motion between the conductor and the magnet. • the magnetic field through a coil changes.

5. induced e.m.f. • This phenomenon is called electromagnetic induction. • Such an e.m.f. is called induced e.m.f. high potential low potential

6. This phenomenon is called electromagnetic induction. • Such an e.m.f. is called induced e.m.f. • The current produced is called an induced current. wire frame induced current induced current if a closed loop is formed

7. Faraday’s law of electromagnetic induction The magnitude of the induced e.m.f. is directly proportional to the rate at which the conductor cuts through the magnetic field lines, or the field through the coil changes. smaller larger

8. Faraday’s law of electromagnetic induction The magnitude of the induced e.m.f. is directly proportional to the rate at which the conductor cuts through the magnetic field lines, or the field through the coil changes. Checkpoint (p.358) O smaller larger

9. N S magnet approaching Ring that cannot get through Lenz’s law An induced current always flows in a direction so as to oppose the change producing it. opposing the change

10. S N magnet withdrawing Ring that cannot get through Lenz’s law An induced current always flows in a direction so as to oppose the change producing it. opposing the change

11. Checkpoint (p.365) O Experiment 28.5 Example 28.2 Conducting loop moving across a magnetic field • Fleming’s right hand rule can also be used to determine the direction of an induced current.

12. 28.2 Faraday’s law and motional e.m.f. Magnetic flux • Magnetic fluxF is a measure of the number of magnetic field lines through a surface.

13. Example 28.3 Checkpoint (p.370) O F = BA cos q • The magneticflux F through a planar surface in a magnetic field B is • Magnetic flux is a scalar quantity with the unit weber (Wb). • The magnetic field is also called the magnetic flux density.

14. Faraday’s law can be expressed in mathematical form: For an N-turn coil, if the magnetic flux F through each turn is the same, the total flux through the coil, called the magnetic flux linkage, is NF. Example 28.4 Example 28.5 Example 28.6 Checkpoint (p.374) O Mathematical form of Faraday’s law The negative sign is another way to state Lenz’s law.

15. – – – – – motional e.m.f. e + + v + + + Motional e.m.f. • A motional e.m.f. is the induced e.m.f. arises from the motion of a conductor in a magnetic field. magnetic force uniform magnetic field B

16. – – – – – l motional e.m.f. e + + v + + + • A motional e.m.f. is the induced e.m.f. arises from the motion of a conductor in a magnetic field. At equilibrium, FB = FE magnetic force FB = qvB e = Blv electric force FE = qE = qV / l uniform magnetic field B

17. v Example 28.7 Checkpoint (p.377) O e = Blv sin f • If the velocity v makes an angle f with the magnetic field B, the motional e.m.f. is f straight conductor rod of length l uniform magnetic field B

18. 28.4 Applications of electromagnetic induction and generators • Electromagnetic induction is used in • moving-coil microphones

19. Electromagnetic induction is used in • moving-coil microphones • magnetic storage Writing data Reading data

20. Electromagnetic induction is used in • moving-coil microphones • magnetic storage • electric guitars

21. Electromagnetic induction is used in • moving-coil microphones • magnetic storage • electric guitars • electrical generators A bicycle alternator Generators in a power station

22. Simple a.c. generator Electrical generator • A generator is a device that converts mechanical energy into electrical energy. • The ones generating alternating currents are called a.c. generators (or alternators). • Those generating direct currents are called d.c. generators (or dynamos). A simple a.c. generator

23. The slip rings in an a.c. generator are used to prevent the twisting of wires during the rotation of the coil. slip rings

24. The slip rings in an a.c. generator are used to prevent the twisting of wires during the rotation of the coil.

25. The slip rings in an a.c. generator are used to prevent the twisting of wires during the rotation of the coil.

26. The slip rings in an a.c. generator are used to prevent the twisting of wires during the rotation of the coil.

27. The slip rings in an a.c. generator are used to prevent the twisting of wires during the rotation of the coil.

28. Variation of the induced e.m.f. in a simple a.c. generator

29. Simple d.c. generator • The commutator of a d.c. generator is used to reverse the connection to the external circuit when the direction of the induced e.m.f. in the coil reverses. A simple d.c. generator

30. The commutator of a d.c. generator is used to reverse the connection to the external circuit when the direction of the induced e.m.f. in the coil reverses. Variation of the induced e.m.f. in a simple d.c. generator

31. Experiment 28.6 Example 28.9 • The induced e.m.f. of a generator can be increased by • increasing the rotational speed of the coil, • using stronger magnets, • winding more turns of wire on the coil, • increasing the area of the coil within the field, and • winding the coil on a soft iron core.

32. In some generators, it is the magnets that rotate but not the coil. A bicycle alternator

33. Checkpoint (p.390) O • For an a.c. generator in a power station, there is an electromagnet, called rotor, rotating in a set of fixed coils, called the stator. Thus, e.m.f. is induced in the stator coils.

34. Falling magnet Coin detector 28.5 Eddy currents • Eddy currents are induced currents circulating through a conducting plate when the plate cuts through magnetic field lines or is placed in a changing magnetic field. eddy currents uniform magnetic field Conducting plate leaving the field

35. Falling magnet Coin detector • Eddycurrents are induced currents circulating through a conducting plate when the plate cuts through magnetic field lines or is placed in a changing magnetic field. eddy currents uniform magnetic field Conducting plate entering the field

36. Experiment 28.7 Braking effect • In general, magnetic braking effect arises whenever eddy currents are induced due to the relative motion between a block of conductor and a magnetic field. external force magnetic force decelerates uniform magnetic field

37. A floating aluminium foil Checkpoint (p.397) O What makes the brass gong rotate Induction heating • Eddy currents can heat up the conductor itself as a result of the heating effect of current. • This phenomenon is called induction heating.