Magnetic Field Patterns (Edexcel IGCSE Physics)

• Magnetic field line patterns are all slightly different around:
  • Straight wires
  • Flat circular coils
  • Solenoids

Magnetic Field in a Straight Wire

• When a current flows through a conducting wire a magnetic field is produced around the wire
• The shape and direction of the magnetic field can be investigated using plotting compasses

• 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 direction of the magnetic field around a wire is given by the right-hand thumb rule

• Reversing the direction in which the current flows through the wire will reverse the direction of the magnetic field
• If there is no current flowing through the conductor there will be no magnetic field
• Increasing the amount of current flowing through the wire will increase the strength of the magnetic field
  • This means the field lines will become closer together

Magnetic Field in a Flat Circular Coil

• 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

The magnetic field around a flat circular coil

• 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

Magnetic Field in a Solenoid

• The magnetic field inside the solenoid is strong and uniform
• Inside a solenoid (an example of an electromagnet) the fields from individual coils
  • Add together to form a very strong almost uniform field along the centre of the solenoid
  • Cancel to give a weaker field outside the solenoid

• 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

• 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

• 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