electromagnetism

• when a current flows through a conducting wire, a mangetic field is produced around the wire
• the strength of the magnetic field depends on the current through the wire and the distance from the wire.
• the magnetic field around a wire is made up of concentric circles, and a circular field pattern indicates that the magnetic field around a current-carrying wire has no poles
• as the distance from the wire increases, the circles get further apart which shows the magnetic field is strongest closest to the wire
• the right hand thumb rule can be used to work out the direction of the magnetic field:

  • thumb points along the direction of the current
  • fingers wrap around what would be the wire to give the direction of the field
• reversing the direction in which the current flows through a wire will reverse the direction of the magnetic field
• if there's no current flowing through a wire, there will be no magnetic field
• a solenoid is a long current-carrying coil and the magnetic field within it is strong and uniform
• shaping a wire to form a solenoid increases the strength of the magnetic field created by current through the wire
• increasing an solenoid's strength:

  • increase the current flowing through the wire
  • increasing the number of turns on the coil while keeping the length the same
  • decreasing the length while keeping the number of turns the same (more densely packed coils)
  • adding an iron core
• the strength of the field inside a solenoid is greater because the small magnetic fields caused by the current in each turn add up to create an overall larger magnetic field
• electromagnets are useful because:

  • they can be switched on and off by turning the power supply on or off
  • the strength of an electromagnet can be adjusted by changing the current flowing through it