• The direction of the force on a charge moving in a magnetic field is determined by the direction of the magnetic field and the current
• Recall that the direction of the current is the direction of conventional current flow (positive to negative)
• When the force, magnetic field and current are all mutually perpendicular to each other, the directions of each can be interpreted by Fleming’s left-hand rule:
  • On the left hand, with the thumb pointed upwards, first finger forwards and second finger to the right ie. all three are perpendicular to each other
  • The thumb points in the direction of motion of the rod (or the direction of the force) (F)
  • The first finger points in the direction of the external magnetic field (B)
  • The second finger points in the direction of conventional current flow (I)

Worked example: Using the left-hand rule

Using Fleming’s left-hand rule:

B = into the page

F = vertically downwards

I = from right to left