Magnetic Effect of a Current (Oxford AQA IGCSE Combined Science Double Award)

Revision Note

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

Current in a wire travels up the page. Magnetic field lines are drawn around the wire on a perpendicular plane, for IGCSE & GCSE Physics revision notes
Diagram showing the magnetic field 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

Thumb points upwards, fingers curl to make a fist. When the thumb points in the direction of the wire, the fingers point in the direction of the circular field lines.
The right-hand thumb rule shows the direction of current flow through a wire and the direction of the magnetic field around the wire
  • 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

The field lines around a wire become further apart away from the wire. The greater the current through the wire then the greater the magnetic field, for IGCSE & GCSE Physics revision notes
The greater the current, the stronger the magnetic field. This is shown by more concentrated field lines.
  • 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

Side and top view of current and the magnetic field produced. Where the current flowing out of the page is shown with a dot and into the page with a cross, for IGCSE & GCSE Physics revision notes
Side and top view of the current flowing through a wire and the magnetic field produced

Examiner Tips and Tricks

When drawing these field lines around a wire, make sure it is clear the lines become further apart with increasing distance from the wire, it is better to exaggerate this for the examiner.

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

A magnetic field is present around a loop of current-carrying wire, for IGCSE & GCSE Physics revision notes
Diagram showing the magnetic field around a flat circular coil
  • 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

A solenoid is a cylindrical coil of wire which has a magnetic field similar to that of a bar magnet. Arrows on the field lines show the direction from the north pole to the south pole, for IGCSE & GCSE Physics revision notes
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

Direction of current flow in a solenoid

The current at the north pole of a solenoid flows anti-clockwise and at the south pole clockwise, for IGCSE & GCSE Physics revision notes
Poles of a Solenoid

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