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Transcript

  • 1. Magnetism
    Magnets and electromagnets
    Catherine Mortimer-Hawkins, IEST
  • 2. Magnets
    A magnet has 2 poles, a N-pole and a S-pole.
    The magneticfieldgoes from N to S.
  • 3. Attraction or repulsion?
  • 4. The earth’smagneticfield
    Ifwestand with loads of magnets on the surface of the earththeywould all pointtheir N poletowards the north!
    And theirS-pole
    towards the south.
    This is because the
    earth has a magnetic
    field.
    The earth’ssouthmagneticpole is near the earth’snorthgeographicpole!!
  • 5. Magneticfield round a wire
    The danish scientist Hans Christian Örstednoticed that if you have a current in a wire therewill be a magneticfield.
    The thumbrule is:
    Imaginegrabbing the wire
    with your right hand.
    Thumb in the currents
    direction, your finger will
    thenpoint in the same direction
    as the fieldlines.
  • 6. The magneticfield round a wire:
    The magneticfieldlines are circles.
    The field is strongestnearest the wire.
    Increasing the currentincreases the strength of the field.
  • 7. Coils and solenoids
    A magneticfield round a wire will be strongerif it is woundinto a coil (spole).
    A longcoil is called a solenoid.
    The magneticfieldwillthen look like this:
  • 8. The magneticfield of a coil:
    is increased by increasing the current.
    is increased by the number of turnsaround the coil.
    is reversedif the currentchangesdirection.
  • 9. Thumbrule nr 2
    Imaginegripping the coil with your right hand so that your fingers are in the same direction as the current.
    Your thumbwillthenpoint at the north of your electromagnet.
  • 10. Different uses of a electromagnets:
    Relay
    Computer hard drive
    The curcuitbreaker
    A magnetic door lock
    Hugeelectromagnets on cranes
    Door bells