Electromagnetic induction

  • Created by: emma
  • Created on: 16-05-13 13:48

Faraday's law and Lenz's law

  • The induced EMF in a circuit is equal in size to the rate of change of magnetic flux linkage through the circuit
  • an induced EMF acts in such a direction as to oppose the change that produces it.
  • faraday's law can be investigated by dropping a magnet through a coil of wire connected to a voltage data logger.
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dropping a magnet through a coil

Graph produced:

  • positive region - flux through coil increases as magnet approaches
  • negative region - flux decreases as magnet exits the coil
  • maximum emf when approaching is lower than maximum emf when leaving as it accelerates as it falls ( induced emf is rate of change of flux linkage and rate of change of flux through the coil is greater as magnet leaves)
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Lenz's law

  • drop magnet down copper tube
  • it falls more slowly than a non-magnetic piece of metal of similar dimensions
  • friction force should be indetical for the two falling objects
  • falling magnet induces emf in each copper circlet
  • small current flows around the tube
  • current causes magnetic field which interact with the falling magnet
  • depending on direction of field, it will repel or attract magnet
  • if it attracted it it would end up with more energy than the GPE it originaly had.
  • this is not possible (law of conservation of energy)
  • it must act in such a direction as to repel the magnet
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  • pair of coils linked by soft iron core
  • ac in primary coil produces changing magnetic field
  • this is carried to secondary coil by iron core
  • changing magnetic field through secondary coil induces emf in coil

Ratio of the voltages between primary and secondary coil is the same as the ration of the number of turns.

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how can the amount of EMF induced in a coil be cha

  • changing the angle between the coil and the field: The more aligned the coil is to the field, the fewer lines it cuts through and the smaller the induced e.m.f
  • changing the number if turns:  the grater the number of turns, the more points in each coil cut each flux line so the e.m.f induced is greater
  • changing the area of the coil: the larger the area, the more flux lines are cut through and the greater the induced e.m.f
  • changing the magnetic field strength: the greater the flux density, the greater the number of flux lines per unit area , so the coil  cuts more flux and more emf is induced
  • changing the angular speed of the coil: increasing the rate of rotation of a coil increases the number of flux lines cut by the coil in a given time, increasing the induced e.m.f
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  • coil rotates
  • flux linkage = 0 when coil is parallel with field
  • flux linkage = maximum when coil is perpendicular to the field
  • alternating emf generated
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