Magnetism

• Magnets


  • Have a magnetic field around them
  • Have two opposite poles (North and South)
  • Poles exert forces on other magnets
  • Like poles repel and unlike poles attract
• Magnets facing each other with opposite poles

  Will come together
• Magnets


  • Attract magnetic materials by inducing (permanent or temporary) magnetism in them
  • Exert little or no force on non-magnetic materials
• Electric field direction


  Direction of the force on a positive charge at that point
• Induced Magnetism


  • Magnets attract materials by inducing magnetism in them; the material becomes a magnet as well
  • The side of the material facing the magnet will become the opposite pole as the magnet
• Methods of inducing magnetism

  1. Placing a piece of steel near a magnet

  2. Stroking a piece of steel with one end of a magnet

  3. Placing it in a solenoid and passing a large, direct current through the coil
• Solenoid and iron core

  • Most effective method of inducing magnetism
  • The iron core can be easily magnetised
• How a transformer works


  1. Primary coil connected to AC with high voltage

  2. Current in primary coil creates changing magnetic fields

  3. Magnetic field passes through iron core to secondary coil

  4. Current in secondary coil changes the voltage or potential difference
• Transformers


  • Only work with AC because we need changing magnetic fields to induce potential difference
  • If same number of turns, potential difference remains the same
  • More turns in secondary coil = step-up transformer, higher potential difference
  • Less turns in secondary coil = step-down transformer, lower potential difference
• Input power = output power, assuming no energy is wasted
• Methods of demagnetisation


  1. Hammering a magnet

  2. Heating a magnet to high temperature

  3. Stroking with another magnet

  4. Placing in a coil with opposite DC current

  5. Placing in a solenoid with AC current
• Soft iron


  • Gets magnetised faster but loses magnetism when inducing magnet is removed
  • High susceptibility but low retentivity
  • Used for transformer cores
• Steel


  • Slow to be magnetised but retains acquired magnetism for a long time
  • Low susceptibility but high retentivity
  • Used for making magnets