Magnetism (Cambridge (CIE) O Level Physics): Revision Note

Magnetism

Properties of Magnets

• The ends of a magnet are called poles

• Magnets have two poles: a north and a south

Poles of a Magnet

A magnet has a north and a south pole

• Magnets are objects which experience attraction and repulsion

  • Like poles repel (push each other apart)

    • For example a north pole will repel a north pole and a south pole will repel a south pole

  • Unlike poles attract (move towards each other)

    • For example a north pole will be attracted to a south pole 

• When two magnets are held close together, there will be a force between the magnets:

Forces Between Magnets

Opposite poles attract; like poles repel

• Magnetic materials:

  • Experience a force when placed in a magnetic field

  • Are attracted to a magnet when unmagnetised

  • Can be magnetised to form a magnet

• Only a magnet can repel another magnet (This can be a useful test for a magnet)

• Non-magnetic materials do not experience a force when placed in a magnetic field

Uses of Magnets

Uses of Permanent Magnets

• Permanent magnets are usually (but not always) made from steel

  • They tend to stay magnetised

Examples of Uses of Permanent Magnets

Permanent magnets are often in found in compasses and children's toys

• Permanent magnets have many uses including

  • Compasses: for thousands of years humans have used compasses for navigation, since the needle always points north

  • School lab experiments: the magnets used in school science demonstrations are permanent magnets

  • Toys: toy trains and trucks often have magnets which attach the carriages or trailers to the engine or cab

  • Fridge magnets: these are made either of flexible magnetic material or by sticking a magnet to the back of something

Uses of Electromagnets    

• Electromagnets use electricity to create a magnet from a current-carrying wire

  • They have the advantage that they can be magnetised and demagnetised, literally at the flick of a switch

  • They can be switched on and off

• Soft iron is the metal normally used for this

  • It can easily become a temporary magnet

Uses of Temporary Magnets

Temporary magnets like electromagnets are used in industry and medicine

• Electromagnets have many uses including

  • MRI scanners: in hospitals, an MRI scanner is a large, cylindrical machine using powerful electromagnets to produce diagnostic images of the organs of the body

  • Speakers and earphones: the loudspeakers, microphones and earphones used in phones and laptops use electromagnets to sense or send soundwaves

  • Recycling: because steel is a magnetic material it can be easily separated from other metals and materials using electromagnets. Once recovered the steel is re-used and recycled, reducing mining for iron ore and processing ore into steel

  • Mag-Lev Trains: the ability of Mag-Lev trains to hover above the rails is due to them being repelled by large electromagnets on the train and track. This reduces friction and allows speeds of nearly 400 miles per hour