Fleming's Left-Hand Rule
- 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)
Using Fleming’s left-hand rule:
B = into the page
F = vertically downwards
I = from right to left
Exam Tip
Don’t be afraid to use Fleming’s left-hand rule during an exam. Although, it is best to do it subtly in order not to give the answer away to other students!