Magnetism & Electromagnetism

Poles and the law of magnetism

• Every magnet has two poles: a north pole and a south pole
• Bringing two magnets close together produces a force between them. The direction of the force follows the law of magnetism:
  • Like poles repel: N–N pushes apart; S–S pushes apart
  • Opposite poles attract: N–S pulls together
• The force between two magnets is a non-contact force, so the magnets feel each other across an air gap without any physical link

Magnetic and non-magnetic materials

• A magnetic material is one that is attracted to a magnet (whether or not it is itself a magnet). The standard list at IGCSE is:
  • iron
  • cobalt
  • nickel
  • steel (an alloy of iron, so it counts)
• Almost every other metal is non-magnetic: copper, aluminium, gold, silver, lead, brass
• Non-metallic materials (plastic, wood, glass, rubber) are non-magnetic
• A magnetic material is attracted to either pole of a permanent magnet; only a magnet itself can be repelled by another magnet. So a quick test for whether an unknown bar of iron-coloured metal is a magnet or just a magnetic material: bring a known magnet near both ends in turn. If one end repels, you've found a magnet; if both ends only attract, it is a magnetic material

Hard and soft magnetic materials

• Magnetically hard materials (e.g. steel) are difficult to magnetise, but once magnetised they hold their magnetism for a long time. They are used for permanent magnets
• Magnetically soft materials (e.g. pure iron) are easy to magnetise, but lose their magnetism almost immediately when the magnetising field is removed. They are used for the cores of electromagnets because you want the magnetism to switch off when the current does