Electromagnetic Induction

1. Lesson 18 Electromagnetic Induction Chin-Sung Lin

2. Electromagnetic Induction & Faraday’s Law

3. Electromagnetic Induction • In 1831, Michael Faraday (England) and Joseph Henry (US) independently discovered that magnetism could produce current in a wire

4. Electromagnetic Induction Faraday’s law— Electromagnetic induction • Electromagnetic induction— any change in the magnetic field around a conductor induces a voltage (or emf) • Faraday’s law— The induced voltage (or emf) in a coil is proportional to the product of the number of loops and the rate of change of the magnetic field within those loops

5. Electromagnetic Induction How can we change the magnetic field around a conductor to induces a voltage (or emf)?

6. Electromagnetic Induction • The change could be produced by • relative motion of a wire with respect to the magnetic field

7. Electromagnetic Induction • The change could be produced by • moving the coil into or out of the magnetic field

8. Electromagnetic Induction • The change could be produced by • rotating the coil relative to the magnet

9. Electromagnetic Induction • The change could be produced by • changing the magnetic field strength

10. Electromagnetic Induction • A magnet moving past a stationary conductor, or • A conductor moving through a stationary magnetic field

11. Electromagnetic Induction • The work done to the magnet is equal to the energy generated in the circuit to which the coil is connected Wmechanical = Welectric

12. Induced Voltage • Induced voltage depends on: • Speed of the wire traversing the magnetic field lines. Quicker motion induces a greater voltage (V ~ v) • Number of loops of wire that moves in a magnetic field. The voltage is proportional to the number of loops (V ~ N)

13. Induced Voltage • If the coil does not form a complete circuit, what will happen?

14. Induced Voltage • Induced voltage without current, no work to plunge the magnet into the coil + -

15. Induced Current • If the coil forms a complete circuit, what is the direction of the induced current? A Ammeter

16. Induced Current • The induced magnetic field is repelling, the current will flow in a way to create such a repelling field A Ammeter

17. Induced Current • The more loops of the coil, the more voltage induced (V ~ N) • The more voltage induced in the coil, the more current through the resistor in the circuit (I ~ V) • The more current through the coil, the stronger the magnetic field it generated (B ~ I) • The stronger the magnetic field generated, the stronger the repelling force acting back to your magnet (F ~ B) • A coil with more loops is a stronger electromagnet and push back harder

18. Induced Current • What factors will affect the induced current? A Ammeter

19. Induced Current • Induced current depends on • the induced voltage • the resistance of the coil and the • the “reactance” of the coil A Ammeter

20. Reactance • Reactance • similar to resistance • depends on • the number of loops in the coil • the frequency of the AC source

21. Reactance • Reactance The counter-emf is the source of the opposition to current flow change • A constant DC current has a zero rate-of-change, and sees an inductor as a short-circuit • An AC current has a time-averaged rate-of-change that is proportional to frequency, this causes the increase in inductive reactance with frequency

22. Moving Conductor in a Magnetic Field • Induced voltage of a moving conductor in a magnetic field V = v B L

23. Generators &Alternating Current

24. Generator & Alternating Current • The movement of a magnet is alternating, the induced voltage alternates on direction • The greater the frequency of the field change, the greater the induced voltage

25. Generator & Alternating Current • The frequency of the induced alternating voltage equals the frequency of the alternating magnetic field within the loops High Frequency Low Frequency

26. Generator & Alternating Current • Generator— a device that converts mechanical energy to electrical energy • Motor— a device that converts electrical energy to mechanical energy

27. Flemming’s Right Hand Generator Rule • When a closed conductor loop is moved in a magnetic field, an induced current flows through it • The direction of induced current is given by the Flemming's right hand generator rule

28. Generator & Alternating Current • What’s the direction of the induced current? N S

29. Generator & Alternating Current • Given by the Flemming's right hand generator rule N S

30. Generator & Alternating Current • As the number of magnetic field lines within the loop changes, the magnitude and direction of the induced voltage and current change

31. Generator & Alternating Current

32. Generator & Alternating Current • One complete rotation of the loop produces on complete cycle in voltage and current

33. Generator & Alternating Current • The voltage induced by the generator alternates, and the current produced is alternating current (AC) • The standard alternating current is 60 Hz

34. Generator Example • Hydro power generators

35. Transformers

36. Transformer Definition • A a static device that transfers electrical energy from one circuit to another through inductively coupled conductors • A static device that transfers electrical energy to magnetic energy, and to electric energy again • A device with which we can raise (for transmission) and lower (for use) the AC voltage in a circuit • Transformer only works for AC

37. Transformer Principle • Primary and secondary coils • Use AC voltage source (primary coil) • AC voltage is induced (secondary coil) • Frequency AC voltage source = Frequency Induced AC voltage Primary Secondary

38. Transformer Principle • Iron core (high permeability) is inserted into the coils to intensify the magnetic field • Iron core forms a complete loop to guide all magnetic field lines through the secondary

39. Transformer Principle • Transformer Symbol:

40. Transformer Principle Np no. of turns of primary coil Ns no. of turns of secondary coil Vp voltage of primary coil Vs voltage of secondary coil Ip current of primary coil Is current of secondary coil IS IP

41. Transformer Principle IS IP VP VS NP NS =

42. Transformer Principle • Step-up transformer • NP < NS • VP < VS • Step-down transformer • NP > NS • VP > VS

43. Transformer Principle IS IP PP = PS IP VP = IS VS

44. Transformer Principle • VP / NP = VS / NS • IP VP = IS VS • VP / VS = IS / IP = NP / NS • VS = VP (NS / NP ) • IS = IP (NP / NS ) VP IS NP VS IP NS = = NP NS NS NP IS = IP VS = VP

45. Application - Power Transmission

46. Electromagnetic Waves

47. Induction of Electric Field • Faraday’s law: • An electric field is created in any region of space in which a magnetic field is changing with time • The magnitude of the created electric field is proportional to the rate at which the magnetic field changes • The direction of the created electric field is at right angles to the changing magnetic field

48. Induction of Magnetic Field • Maxwell’s law: • A magnetic field is created in any region of space in which an electric field is changing with time • The magnitude of the created magnetic field is proportional to the rate at which the electric field changes • The direction of the created magnetic field is at right angles to the changing electric field

49. Electromagnetic Waves • In 1861 Scottish physicist James Clerk Maxwell discovered the theory of electromagnetism • Maxwell united all previously unrelated observations and equations of electricity, magnetism and optics into a consistent electromagnetic field theory

50. Electromagnetic Waves • German physicist Heinrich Rudolf Hertz was the first to satisfactorily demonstrate the existence of electromagnetic waves by building an apparatus to produce and detect VHF or UHF radio waves