Fleming’s left-hand rule
Forefinger B, middle finger I, thumb Force (FBI).
Step 1 — Study the figure
Cover the label key on the image, name each numbered part, then check the explanations below.
Step 2 — Diagram details for students
What this diagram shows
Fleming’s left-hand rule gives the force direction on a current-carrying conductor in a magnetic field. Forefinger is field B, middle finger is current I, and thumb is force F (FBI). It is the motor rule and needs mutually perpendicular fingers.
Numbered parts — mark & remember
Match each number on the figure with the meaning below. Say the function aloud once.
1 Forefinger = B
Point the forefinger in the magnetic field direction (N to S outside a magnet). Field direction must be identified from the diagram first. Wrong B direction reverses predicted force.
2 Middle finger = I
Point the middle finger in conventional current direction. Keep it perpendicular to the forefinger. For electron flow questions, convert carefully to conventional current.
3 Thumb = Force
The thumb then gives the direction of force or motion of the conductor. This is why the rule is used for electric motors. Force is maximum when current is perpendicular to the field.
4 Perpendicular setup
The three fingers must be mutually perpendicular. Practise the hand posture until it is automatic. Write FBI as a memory key in rough work.
How to read this figure
- Set B and I on fingers, read F from thumb.
- Use for motors (force on conductor), not for induced current rules beyond syllabus needs.
- Check perpendicular condition for maximum force statements.
Redraw for board marks
- Draw conductor, field lines, and current arrow.
- Sketch hand with three labelled fingers.
- Mark force direction on the wire.
Board answer tip: State Fleming’s left-hand rule with FBI, and apply it once to a drawn wire in a field.
Exam tips — don’t lose marks
- Left hand for force on conductor (motor); do not mix with right-hand thumb rule for wire field.
- State mutually perpendicular condition.
- Maximum force when I ⊥ B.