8.2 - Permanent & Induced Magnets

Permanent and induced magnets

• A permanent magnet produces its own magnetic field all the time (e.g. a bar magnet).
• An induced magnet is a magnetic material that becomes a magnet only when placed in a magnetic field.

The force is always attractive

• The force between a permanent magnet and an induced magnet is always attractive.
• A pole on a permanent magnet induces the opposite pole in the nearest part of the magnetic material:
  • A north pole induces a south pole.
  • A south pole induces a north pole.
• The closer the magnet is to the material, the stronger the induced magnetism.

Soft and hard magnetic materials

• When removed from a magnetic field, an induced magnet loses its magnetism.
• Magnetically soft materials (e.g. pure iron) are easily magnetised and lose their magnetism very quickly.
• Magnetically hard materials (e.g. steel) are harder to magnetise but keep their magnetism. Permanent magnets are made from magnetically hard materials.

Which materials are magnetic

• There are three magnetic elements: iron, nickel and cobalt.
• Some alloys of these metals are magnetic too (e.g. steel, because it contains iron).
• When a magnetic material is placed in a magnetic field, it always experiences an attractive force.
• Non-magnetic materials (e.g. plastic, rubber, glass and wood) feel no force in a magnetic field.

Uses of magnetic materials

• Fridge doors - A permanent magnetic strip keeps the door closed.
• Cranes - Scrap-yard cranes use electromagnets to attract and move magnetic materials, switching the current off to release the load.
• Doorbells - An electromagnet switches on and off rapidly, repeatedly attracting and releasing an arm that strikes a bell.
• Magnetic separators - Used in recycling plants to sort magnetic metals (such as steel cans) from non-magnetic waste.
• Maglev trains - Use magnetic repulsion to float the train above the track, reducing friction, and to propel it forward.
• MRI machines - Use strong magnetic fields to produce images of the inside of the body without using ionising radiation.
• Loudspeakers and microphones - Rely on magnetic fields to convert between sound waves and electrical signals.