1. Electromagnetic Induction
Lesson 21
Electromagnetic Induction
Eleanor Roosevelt High School
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
Ammeter A

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. Generators
Literally EVERY source of electricity (except for solar) comes from a generator.
And all generators convert motion (kinetic) energy into stored electrical energy.

25. 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

26. 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

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

28. 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

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

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

31. 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

32. Generator & Alternating Current

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

34. 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

35. Generator Example
Hydro power generators

36. Aim: Transformer & Electromagnetic Wave DoNow: Draw the voltage waveform
One complete rotation of the loop produces on complete cycle in voltage and current

37. Aim: Transformer & Electromagnetic Wave DoNow: Draw the voltage waveform
One complete rotation of the loop produces on complete cycle in voltage and current

38. Transformers

39. 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

40. 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

41. 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

42. Transformer Principle
Transformer Symbol:

43. Transformer PrincipleIS
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

44. Transformer PrincipleIS
VP VS
NP NS =

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

46. Transformer PrincipleIS
PP = PS
IP VP = IS VS

47. 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 = NS NP
VS = VP NP NS
IS = IP

48. Application - Power Transmission

49. Electromagnetic Waves

50. 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

51. 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

52. 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

53. 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

54. Electromagnetic Waves
A charge oscillates back and forth in empty space will produce electromagnetic waves in space where vibrating electric and magnetic fields regenerate each other

55. Electromagnetic Waves
Electric field is perpendicular to the magnetic field, and both are perpendicular to the direction of the motion of the wave

56. Electromagnetic Waves
No medium is required
The speed— the speed of light
The wave is continuously self-reinforcing. The changing electric field induced a magnetic field. The changing magnetic field acts back to induce a electric field

57. The End