electromagnetism

• • when a current flows through a wire, a magnetic field is created around the wire
  • the field is made up of circles perpendicular to the wire
  • this can be shown by moving a compass around the wire
  • this because the changing direction of the compass needle will show a magnetic field has been produced
  • the strength of the magnetic field produced changes with the current and distance of the wire
  • the larger the current, or closer to the wire, the stronger the magnetic field
• • shaping a wire to form a solenoid will increase the strength of the magnetic field
  • this is because the field lines around each coil of wire line up, so many field lines point in the same direction and are closer to each other
  • the magnetic field inside a solenoid is strong and uniform
  • it has the same strength and direction at every point
  • placing a block of iron in the centre of the coil will also increase the field strength - the iron core becomes an induced magnet when current flows
  • stopping the current will stop the magnetic field from being produced
• • a solenoid with an iron core whose magnetic field can be turned on and off with an electric current is called an electromagnet
• • electromagnets can be useful because they can be turned on and off, so can create a varying force
  • electromagnets are used in cranes to attract and pick up magnetic materials