Electromagnetic induction

1. Electromagnetic induction Topic 12

2. Remember? • An electron is moving downward with a velocity, v, in a magnetic field directed within the page, determine direction of force.

3. Moving conductor • Conductors contain free electrons • So when a conductor moves downward, electron will experience a magnetic force pulling them to the left. • Lattice atoms on the right become positive… • There is a potential difference now

4. Moving conductor • E force produced wanting to push electrons to the right • Forces are now balanced and electrons will stop moving • What will happen if you connect a resistor to the metal conductor?

5. Moving conductor R – – – + + + • E force produced wanting to push electrons to the right • Forces are now balanced and electrons will stop moving • What will happen if you connect a resistor to the metal conductor? • Current will flow from high to low potential

6. Moving conductor R – – – + + + • What did we define emf previously as? • Amount of chemical energy converted to electrical energy per unit charge

7. Moving conductor R – – – + + + • What did we define emf previously as? • Amount of chemical energy converted to electrical energy per unit charge • We don’t have chemical energy here!!! • Where does our energy come from here?

8. Moving conductor R – – – + + + • Which direction were we pushing this conductor? • Another force acting on conductor?

9. Moving conductor R – – – + + + • Which direction were we pushing this conductor? • Another force acting on conductor? • Yes magnetic force… which direction is it acting?

10. Moving conductor Magnetic force R – – – + + + • Which direction were we pushing this conductor? • Another force acting on conductor? • Yes magnetic force… which direction is it acting? – upward • So when you’re pushing it downward, what energy is it gaining? Force applied

11. Moving conductor Magnetic force R – – – + + + • Which direction were we pushing this conductor? • Another force acting on conductor? • Yes magnetic force… which direction is it acting? – upward • So when you’re pushing it downward, what energy is it gaining? – EPE • You are doing work • If forces are equal, conductor is moving at constant v. Force applied

12. Moving conductor Magnetic force R – – – + + + • EPE will be converted into heat • Energy is conserved • Now let’s define INDUCED EMF? • Amount of mechanical energy converted into electrical energy per unit charge. Force applied

13. Calculating induced EMF • Maximum p.d. in conductor is when the magnetic force ON ELECTRON is equal to the electrical force L –

14. Calculating induced EMF • Maximum p.d. in conductor is when the magnetic force ON ELECTRON is equal to the electrical force • Electron will stop moving.. Therefore greatest p.d. • Equate equations L –

15. Induced EMF equation * * The induced EMF will be the same as p.d. across conductor

16. Question • If magnetic field not perpendicular to direction of motion… what will you do? B v

17. Question • If magnetic field not perpendicular to direction of motion… what will you do? • Take the B that is perpendicular to v

18. QUESTION

19. QUESTION

20. THREE HAND RULES RIGHT LEFT

21. Faraday’s law • What generated induced emf? • What did it depend on?

22. Faraday’s law • What generated induced emf? – moving conductor in a magnetic field • What did it depend on? • Faraday’s law: • The induced emf is equal to the rate of change of flux

23. Flux VS flux density • Let’s look at this analogy

24. Flux VS flux density • How much grass do you have? • Is it taking lots of area?

25. Flux VS flux density • Pieces of grass is flux density (B) • Area over which grass takes over is flux (Φ)

26. Flux VS flux density Flux unit: Tm2, Wb

27. Flux VS flux density • If area at angle from B • Find component of B that will be perpendicular to area Normal to surface θ

28. Lenz’s law • Moving conductor in magnetic field causes a force to oppose the direction of motion as seen earlier (if not true than energy will not be conserved) • Lenz’s law is an extension to Faraday by stating that: • the induced current will be in such a direction as to OPPOSE THE CHANGE IN FLUX that created the current.

29. Question

30. Question

31. Question • Determine direction of current

32. Question

33. Question

34. Question

35. Question

36. Question • Rail gun… how does it work? resistor

37. Question • What will happen if you remove the magnetic field suddenly? resistor

38. Question • BRING BACK THE FIIIEEEELLLDDD!!! LENZ’S LAW resistor

39. Question • to get the field back, which direction should the induced current be? resistor

40. Question • Current upward…. Force???? resistor

41. Question • Current upward…. Force to the right… and off it goes….. resistor

42. Question

43. Question http://science.howstuffworks.com/rail-gun1.htm

44. Read • Applications of EM induction on page 213

45. Alternating current (AC) • What’s the difference between a motor and a generator?

46. Alternating current (AC) • What’s the difference between a motor and a generator? • Motor  electrical to mechanical energy • Generator  mechanical to electrical

47. Alternating current (AC) • What’s the direction of current induced here?

48. Alternating current (AC) • What’s the flux going to be at angle 0?

49. Alternating current (AC) • What’s the flux going to be at angle 0? • Maximum since • So how will the graph look like?

50. Alternating current (AC) Label the positions in graph