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Chapter 31

Chapter 31. Faraday’s Law. 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. Michael Faraday.

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Chapter 31

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  1. Chapter 31 Faraday’s Law

  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. Michael Faraday • Started life as a book binder • Discovers that moving magnets will produce an “induced” current in wires that aren’t even connected to a power source • First to describe electric and magnetic forces using “fields” • Remember, the idea that invisible lines are running everywhere, and that we can count them, is not exactly self-evident 1791-1867 29 years old in 1820

  4. Electromagnetic Induction • Induced current—current produced by magnetic fields, not by connection to a battery • Induced emf—potential difference produced by changing magnetic fields • The quicker the B-field changes, the greater the induced emf

  5. Induction

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

  7. Faraday’s Law • Faraday’s Law of Induction • Induced EMF is equal to the rate of change of magnetic flux •  = induced EMF • N = number of loops • Lenz’s Law (that negative sign up there) • Induced emf always produces a current whose B-field opposes the original change in flux

  8. Electric Fields • An induced voltage creates an induced electric field • V = Ed for static electric fields • The induced field only exists while the magnetic flux is changing • This is the “general form” of Faraday’s Law

  9. Generators • Generators convert mechanical energy into electrical energy • Use Faraday’s Law • Electric current induced by a changing magnetic flux • Option 1—rotating wire loops inside stationary magnetic fields • Option 2—rotating magnets inside stationary wire loops • We’ll examine option 1

  10. 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

  11. Generators

  12. Generators • Lenz’s Law • As flux decreases, magnitude of emf increases • As flux increases, magnitude of emf decreases

  13. Generators

  14. Generators • 0 = maximum EMF •  = angular frequency of rotation • Units of radians/second (rad/s)

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