Magnetic Fields in Wires & Solenoids (OCR GCSE Physics A (Gateway))

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

• 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

• 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

• 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