Fleming's Left Hand Rule (AQA A Level Physics): Revision Note

Direction of force on a current-carrying conductor

• When a current-carrying conductor is placed in a magnetic field, the force, $B$-field and current are all mutually perpendicular to each other

  • Their directions can be determined using Fleming’s left-hand rule

• To use Fleming's left-hand rule, point the thumb, first finger and second finger at right angles to each other

  • The thumb points in the direction of motion or force $F$ of the conductor

  • The first finger points in the direction of the applied magnetic field $B$

  • The second finger points in the direction of the flow of conventional current $I$ (from positive to negative)

Representing magnetic fields in 3D

• When solving problems in three-dimensional space, the current, force or magnetic field could be directed into or out of the page

• When the magnetic field is directed into or out of the page, the following symbols are used:

  • Dots (sometimes with a circle around them) represent the magnetic field directed out of the plane of the page

  • Crosses represent the magnetic field directed into the plane of the page

• The way to remember this is by imagining an arrow used in archery or darts:

  • If the arrow is approaching head-on, such as out of a page, only the very tip of the arrow can be seen (a dot)

  • When the arrow is receding away, such as into a page, only the cross of the feathers at the back can be seen (a cross)