Electromagnetic Induction

1
Chapter 20

• Electromagnetic Induction

2
Electromagnetic Induction

• 1820-1821, discovery of magnetic fields due to
  wires and forces on current carrying wires in a
  magnetic field
• Joseph Henry (American) and Michael Faraday
  (Englishman) pursued making an electric current
  with magnetic fields

3
Electromagnetic Induction

• Induced currentcurrent produced by magnetic
  fields, not by connection to a battery
• Induced emfpotential difference produced by
  changing magnetic fields
• The quicker the B-field changes, the greater the
  induced emf

4
Faradays Law

• Faraday used field lines (vectors) to represent
  B-fields
• The more field lines in a given area, the greater
  the magnitude of B

5
Faradays Law

• Magnetic flux (?B)number of field lines passing
  through the given area
• Units of tesla-meter2 (Tm2), renamed the Weber
  (Wb)
• ?B is greatest when ? 0 (loop is ? to B-field)
• ?B is zero when ? 90 (loop is parallel to
  B-field)
• ? angle between B and a line drawn ? to coil

6
Faradays Law

• Faradays Law of Induction
• Induced EMF is equal to the rate of change of
  magnetic flux
• ? induced EMF
• N number of loops
• Lenzs Law (that negative sign up there)
• Induced emf always produces a current whose
  B-field opposes the original change in flux

7
Generators

• Generators convert mechanical energy into
  electrical energy
• Use Faradays Law
• Electric current induced by a changing magnetic
  flux
• Option 1rotating wire loops inside stationary
  magnetic fields
• Option 2rotating magnets inside stationary wire
  loops
• Well examine option 1

8
Generators

• As wire loop spins
• Magnetic flux increases when the loop becomes
  more perpendicular to B-field
• Magnetic flux decreases when the loop becomes
  more parallel to B-field

9
Generators
10
Generators

• Lenzs Law
• As flux decreases, magnitude of emf increases
• As flux increases, magnitude of emf decreases

11
Generators
12
Generators

• ?0 maximum EMF
• ? angular frequency of rotation
• Units of radians/second (rad/s)

13
Electromagnetic Waves

• Changing B fields induce changing E fields
• They also send out EM waves
• Visible light is just one kind

14
EM Spectrum
c 3 x 108 m/s
c f?