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
• Using this, we can draw the pattern of magnetic field lines of a current carrying solenoid

Magnetic field around and through a solenoid. This is similar to the field of a bar magnet.

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 right hand rule can be adapted for this situation, with fingers following the direction of current and the thumb pointing in the direction of the central magnetic field lines.

Factors affecting magnetic field strength 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