Magnetic Effect of a Current (Oxford AQA IGCSE Physics)

Magnetic Field of a Current-Carrying 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 magnetic field is made up of concentric circles in a plane perpendicular to the wire

• 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

Magnetic field lines around a current-carrying wire

• The right-hand thumb rule can be used to work out the direction of the magnetic field

Right-hand thumb rule

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

• Increasing the amount of current flowing will increase the strength of the magnetic field

  • This means the field lines will become closer together

Increasing the current through the wire

• Reversing the direction in which the current flows through the wire will reverse the direction of the magnetic field

  • This is made apparent by changing the direction of the thumb in the right-hand rule

Changing current direction

Magnetic Field of a Solenoid

• When a wire is looped into a coil, the magnetic field lines circle each part of the coil, passing through the centre of it

Magnetic field around a loop of wire

• To increase the strength of the magnetic field around the wire it can be coiled to form a solenoid

• The magnetic field around the solenoid is similar to that of a bar magnet

Magnetic field around 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

Direction of current flow in a solenoid