Topic 12: Magnetism

• Every magnet has 2 poles (North/South pole) and a magnetic field around it which will exert a force on magnetic material placed in the field
• Magnetic materials include nickel, cobalt, iron and steel.

  Some magnetic materials can be used to make permanent magnets while others are used in temporary magnets
• Like poles repel (N-N/S-S) and unlike poles attract (N-S).

  Fields are stronger at the poles and weak when field lines are far apart (concentrated lines are strong)

  
• Magnet Uses:

  • door locks
  • MRI scans
  • compasses

  Magnet - any object that produces a magnetic field and the ones influenced by it are magnetic material
• Permanent Magnet - produces its own magnetic field all the time
• Induced Magnet - has a temporary magnetic field that is either magnetically hard which slowly uses field so it lasts longer or magnetically soft which loses magnetism quickly
• Field lines:

  • continuous
  • goes from North to South
  • never crosses or overlaps
• Electromagnetism - electric currents that produce their own magnetic field where its strongest at the centre and direction depends on the current (up is anti-clockwise)
• You can induce a magnet by wrapping a nail in a wire attached to a mains supply (AC) where it can get stronger by increasing the number of turns:

  • Soft Iron Core - acts as temporary magnet and amplifies magnetic field
  • Insulated Copper Wire - conductor of electricity, flexible for turns and prevents electric shocks

  
• Motor Effect - a current carrying wire, in the presence of a magnetic field, will experience a force
• The force acting on a wire can be increased:

  • larger current
  • stronger magnetic field
  • greater length of wire in the field (more turns)

  The wire vibrates as the magnetic field of the permanent magnet reacts with that of the wire.

  It tends to be catapulted outwards

  
• The electric motor changes direction every half turn so it never does a full rotation (360') which is useless so its modified with split-ring commutators
• Electric Motor:

  • split-ring commutator swaps the positive and negative connections every half turn so the current direction swaps
  • forces acting on coil will act in the same direction (rotate one way)
  • when coil is vertical (90'), the current stops because of the the commutator gaps and momentum causes it to keep turning
  • when brushes make contact again, the coil and commutator has turned so the wire near North is towards us (force is still up)