Magnetism

Basics of Magnetism

  • All magnets have two poles: a north and a south.
  • Like poles of two magnets repel, while opposite poles attract.
  • The strongest part of a magnet is at the poles.

Magnetic Fields

  • Magnetic fields are regions around a magnet where the force of magnetism acts.
  • The direction of the magnetic field is always from the north pole to the south pole.
  • Field lines are used to represent the strength and direction of the magnetic field.

Magnetisation and Demagnetisation

  • Iron, steel, cobalt, and nickel can be magnetised, demonstrating the property of ferromagnetism.
  • Magnetisation can be achieved by stroking a ferromagnetic material with a magnet, or by placing it within a coil carrying a direct current.
  • Demagnetisation can occur when a material is heated or knocked, or when it is placed within a coil carrying a rapidly changing current.

Earth’s Magnetic Field

  • Earth behaves like a giant bar magnet, with a magnetic field extending into space.
  • The Earth’s geographic north corresponds to the magnetic south pole.
  • The Earth’s magnetic field causes a compass needle to point North-South, providing a navigation tool.

How Magnets Interact with Materials

  • Ferromagnetic materials are strongly attracted to magnets.
  • Paramagnetic materials, such as aluminium and copper, align with a magnetic field but do not stay magnetised.
  • Diamagnetic materials, such as copper and silver, slightly repel magnetic fields.

Use of Magnets

  • Magnets are used in many everyday items including refrigerators, doorbells, electric motors, and MRI scanners.
  • Magnetic fields can generate electricity in a wire, a principle harnessed by generators and transformers.

Interaction with Electrical Currents

  • An electric current moving through a wire generates a magnetic field around that wire.
  • The direction of the magnetic field can be determined using the right-hand grip rule: if the thumb points in the direction of the current, the fingers curl in the direction of the magnetic field.
  • A solenoid (a coil of wire) carrying a current generates a strong magnetic field inside the coil.
  • Changing the amount of current flowing through a solenoid alters the strength of its magnetic field.
  • The principle of electromagnetism is utilised by devices such as loudspeakers, electric motors, and relays.