Magnetic Fields in Wires & Solenoids

• Magnetic Field Around a Wire

  • When a current flows through a conducting wire a magnetic field is produced around the wire
  • A conducting wire is any wire that has current flowing through it
  • The shape and direction of the magnetic field can be investigated using plotting compasses
  • The compasses would produce a magnetic field line pattern that looks like the following
• Diagram showing the magnetic field around a current-carrying wire

  
• Magnetic Field Around a Wire:

  • The magnetic field is made up of concentric circles
  • 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
  • This shows that the magnetic field is strongest closest to the wire and gets weaker as the distance from the wire increases
  • The right-hand thumb rule can be used to work out the direction of the magnetic field
• The right-hand thumb rule shows the direction of current flow through a wire and the direction of the magnetic field around the wire

  
• • Reversing the direction in which the current flows through the wire will reverse the direction of the magnetic field
• Side and top view of the current flowing through a wire and the magnetic field produced

  • If there is no current flowing through the conductor there will be no magnetic field

  
• Factors Affecting Magnetic Field Strength

  • The strength of the magnetic field around a wire depends on:
  • The size of the current
  • The distance from the long straight conductor (such as a wire)
  • A larger current will produce a larger magnetic field and vice versa
  • The greater the distance from the conductor, the weaker the magnetic field and vice versa
• The greater the current, the stronger the magnetic field. This is shown by more concentrated field lines

  
• Magnetic Field Around a Solenoid

  • When a wire is looped into a coil, the magnetic field lines circle around each part of the coil, passing through the centre of it. Diagram showing the magnetic field around a flat circular coil

  
• Magnetic Field Around a Solenoid

  • To increase the strength of the magnetic field around the wire it should be coiled to form a solenoid
  • The magnetic field around the solenoid is similar to that of a bar magnet
• Magnetic field around and through a solenoid

  
• • The magnetic field inside the solenoid is strong and uniform
  • One end of the solenoid behaves like the north pole of a magnet; the other side behaves like the south pole
  • To work out the polarity of each end of the solenoid it needs to be viewed from the end
  • If the current is travelling around in a clockwise direction then it is the south pole
  • If the current is travelling around in an anticlockwise direction then it is the north pole
• • If the current changes direction then the north and south poles will be reversed
  • If there is no current flowing through the wire then there will be no magnetic field produced around or through the solenoid
• Poles of a Solenoid

  
• Magnetic Field Strength Around a Solenoid

  • The strength of the magnetic field produced around a solenoid can be increased by:
  • Increasing the size of the current which is flowing through the wire
  • Increasing the number of coils
  • Adding an iron core through the centre of the coils
• Magnetic Field Strength Around a Solenoid

  • The iron core will become an induced magnet when current is flowing through the coils
  • The magnetic field produced from the solenoid and the iron core will create a much stronger magnet overall