1. Topic 12 Electromagnetic Induction

2. Electromagnetic induction • Make a coil using wire. The coil should be wide enough to easily move a magnet inside

3. Electromagnetic induction • Put your coil in this circuit. The multimeter should be on the μA scale. μA

4. Electromagnetic induction • MOVE a magnet in and out of the coil. Watch the meter! μA

5. Electromagnetic induction If a magnet is moved inside a coil an electric current is induced (produced)

6. Generator/dynamo A generator works in this way by rotating a coil in a magnetic field (or rotating a magnet in a coil)

7. Motor = generator If electric energy enters a motor it is changed into kinetic energy, but if kinetic energy is inputted (the motor is turned) electric energy is produced!

8. The Motor Effect When a current is placed in a magnetic field it will experience a force (provided the current is not parallel to the field). This is called the motor effect. Can you copy this sentence into your books please.

9. The Motor Effect The direction of the force on a current in a magnetic field is given by Flemming’s left hand rule. Thumb = Motion First finger = Field direction Centre finger = Conventional Current

10. Can you copy this please? WITH DIAGRAM! The Motor Effect The direction of the force on a current in a magnetic field is given by Flemming’s left hand rule. Thumb = Motion First finger = Field direction Centre finger = Conventional Current

11. Sample question In this example, which way will the wire be pushed? (red is north on the magnets)

12. Sample question In this example, which way will the wire be pushed? (red is north on the magnets) Current Field

13. IB Level!

14. Electromagnetic Induction Imagine a wire moving with velocity v in a magnetic field B out of the page. Wire moving with velocity v L v Region of magnetic field B out of page

15. The electrons in the wire feel a force (the motor effect) which pushes the electrons to the right. This creates a potential difference in the wire. Electrons pushed this way (left hand rule) L v

16. The field in the wire that produces this potential difference is given by E = V/L e.m.f. (voltage) across the wire in the magnetic field L + - v

17. The force produced by this field E = V/L would push the electrons back again, but this is opposed by the force on the electrons due to the magnetic filed F = Bev L + - v

18. There exists a balance between the force on the electrons due to the field in the wire and the force due to the field eE = Bev L v

19. eE = Bev since E = V/L, V = vBL L v

20. V = vBL This means that a conducting wire of length L moving with speed v normally to a magnetic field B will have a e.m.f. of vBL across its ends. This is called a motional e.m.f. Wire moving with velocity v L v Region of magnetic field B out of page

21. Faraday’s Law My hero!

22. A Faraday’s Law Consider a magnet moving through a rectangular plane coil of wire. N S

23. A Faraday’s Law A current is produced in the wire only when the magnet is moving. N S

24. A Faraday’s Law The faster the magnet moves, the bigger the current. N S

25. N S A Faraday’s Law The stronger the magnet, the bigger the current.

26. A Faraday’s Law The more turns on the coil (same area), the bigger the current. N S

27. A Faraday’s Law The bigger the area of the coil, the bigger the current. N S

28. N S A Faraday’s Law If the movement is not perpendicular, the current is less.

29. Magnetic Flux (Ф) Imagine a loop of (plane) wire in a region where the magnetic filed (B) is constant. B

30. The magnetic flux (Ф) is defined as Ф = BAcosθ where A is the area of the loop and θ is the angle between the magnetic field direction and the direction normal (perpendicular) to the plane of the coil. B

31. If the loop has N turns, the flux is given by Ф = NBAcosθ in which case we call this the flux linkage. B The unit of flux is the Weber (Wb) (= 1 Tm2)

32. It can help to imagine the flux as the number of lines of magnetic field going through the area of the coil. We can increase the flux with a larger area, larger field, and keeping the loop perpendicular to the field. B

33. Faraday’s law (at last!) I built the first electric motor and generator too. I refused all prizes and awards because that would detract from God’s glory. As we seen, an e.m.f. is only induced when the field is changing. The induced e.m.f. is found using Faraday’s law, which uses the idea of flux.

34. Faraday’s law The induced e.m.f. is equal to the (negative) rate of change of magnetic flux, E = -ΔФ/Δt

35. Example question The magnetic field through a single loop of area 0.2 m2 is changing at a rate of 4 t.s-1. What is the induced e.m.f? “Physics for the IB Diploma” K.A.Tsokos (Cambridge University Press)

36. Example question The magnetic field (perpendicular) through a single loop of area 0.2 m2 is changing at a rate of 4 t.s-1. What is the induced e.m.f? Ф = BAcosθ = BA E = ΔФ = ΔBA = 4 x 0.2 = 0.8 V Δt Δt

37. Another example question! There is a uniform magnetic filed B = 0.40 T out of the page. A rod of length L = 0.20 m is placed on a railing and pushed to the right at a constant speed of v = 0.60 m.s-1. What is the e.m.f. induced in the loop? v L

38. The area of the loop is decreasing, so the flux (BAcosθ) must be changing. In time Δt the rod will move a distance vΔt, so the area will decrease by an area of LvΔt v L LvΔt

39. An important result, you may be asked to do this! E = ΔФ = BΔA = BLvΔt = BLv Δt Δt Δt E = 0.40 x 0.20 x 0.60 = 48 mV v L LvΔt

40. Lenz’s Law The induced current will be in such a direction as to oppose the change in magnetic flux that created the current (If you think about it, this has to be so…….)

41. Alternating current A coil rotating in a magnetic field will produce an e.m.f. N S

42. Alternating current The e.m.f. produced is sinusoidal (for constant rotation) e.m.f. V

43. Slip ring commutator To use this e.m.f. to produce a current the coil must be connected to an external circuit using a split-ring commutator. Slip-rings lamp

44. Increasing the generator frequency? e.m.f. V

45. Root mean square voltage and current It is useful to define an “average” current and voltage when talking about an a.c. supply. Unfortunately the average voltage and current is zero! To help us we use the idea of root mean square voltage and current.

46. Root mean square voltage e.m.f. V

47. Root mean square voltage First we square the voltage to get a quantity that is positive during a whole cycle. e.m.f. V

48. Root mean square voltage Then we find the average of this positive quantity e.m.f. V

49. Root mean square voltage We then find the square root of this quantity. e.m.f. V

50. Root mean square voltage We then find the square root of this quantity. e.m.f. V This value is called the root mean square voltage