physics magnetism

• Magnetism


  An example of a non-contact force
• Contact force


  A force that acts when two objects are physically touching each other
• Non-contact force


  A force that acts between objects that are not touching each other
• Non-contact forces are caused by field lines
• Bar magnet


  • Consists of a North pole and a South pole
  • Magnetic field lines point away from the North pole and towards the South pole
  • The field lines give the direction of force on the North pole of another magnet
• Like poles repel, unlike poles attract
• The magnetic field is stronger near the ends (poles) of the bar magnet
• Permanent magnet


  Produces its own magnetic field
• Magnetic metals


  • Iron
  • Nickel
  • Cobalt
• Steel is also a magnetic metal as it contains iron
• Metals other than iron, nickel and cobalt are not magnetic
• Magnetic metals are attracted by permanent magnets, but never repelled
• Demagnetising a permanent magnet

  1. Heating it

  2. Hitting it with a hammer

  3. Placing it in a reversed magnetic field
• Visualising the magnetic field around a bar magnet

  • Using iron filings
• Electromagnet


  Formed when current flows through a wire
• Increasing the current


  Increases the strength of the magnetic field
• If the current stops flowing, the magnetic field also stops
• The magnetic field around a current carrying wire is in concentric circles around the wire
• Solenoid


  A coil of wire that creates a magnetic field in the same pattern as a bar magnet
• Ways to increase the strength of the magnetic field produced by a solenoid

  • Increase the number of turns in the coil
  • Increase the current
  • Add an iron core
• Soft magnetic material


  Can be easily magnetised and demagnetised
• Hard magnetic material


  Difficult to magnetise and demagnetise
• A hard material shouldn't be used in the core of a solenoid as it would retain some magnetism
• Plotting compass


  A small bar magnet that is free to rotate, the north pole points towards magnetic south
• Using plotting compasses to show the direction of a magnetic field

  1. Place a bar magnet on paper

  2. Place a plotting compass near one of its poles

  3. Mark the direction of the compass

  4. Move the compass to various positions and join the arrows to show the overall field shape
• Compasses are used to help navigate
• Earth's magnetic field


  Caused by the molten iron and nickel in the outer core, in the same shape as a bar magnet
• The geographic North pole of the Earth is the magnetic South pole
• A compass points to magnetic South, so it points to geographic North