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Unit 6 · Lesson 6.4.2

Fleming’s Left-Hand Rule

Apply Fleming's left-hand rule to find the direction of the force on a current-carrying conductor in a magnetic field, using the correct convention for current and field.

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The Rule

6.4.2 Fleming's Left-Hand Rule
Figure 1 6.4.2 Fleming's Left-Hand Rule
First finger (field)
  • Points in the direction of the magnetic field, from the north pole towards the south pole.
Second finger (current)
  • Points in the direction of the conventional current, from positive to negative.
Thumb (force)
  • Points in the direction of the force, or motion, on the conductor, held at right angles to both fingers.
Memory aid
  • The letters F, B and I stand for force, field and current, with the thumb for force and the first finger for field.
Previous lesson

Using the Rule Step by Step

1
Draw the magnetic field direction from north to south, and mark the direction of the conventional current on the conductor.
2
Hold the left hand with the first finger along the field and the second finger along the current, with both fingers at right angles.
3
The thumb then points in the direction of the force on the conductor.
4
Check the answer by predicting how the conductor moves when the current is reversed, which should reverse the thumb's direction.

Notation and Checks

Notation and Checks
Dot notation
  • A dot in a circle shows a current or field pointing out of the page, towards the reader.
Cross notation
  • A cross in a circle shows a current or field pointing into the page, away from the reader.
Right angles
  • When the field and current are at right angles, the force is at right angles to both of them.
Reversing
  • Reversing either the current or the field reverses the force, while reversing both returns the force to its original direction.

Worked Example

Question: A wire carries a current into the page. A magnetic field points from left to right across the wire. Use Fleming's left-hand rule to find the direction of the force on the wire.

  1. Field: left to right, so the first finger points to the right.
  2. Current: into the page, so the second finger points into the page, at right angles to the first finger.
  3. Thumb: at right angles to both fingers, so it points down the page.

Answer: The force on the wire is downwards, towards the bottom of the page.

Exam Checklist & Tips

6.4.2 – Key Points for the Exam

  • Fleming's left-hand rule: A rule using the thumb, first finger and second finger to find the direction of the motor-effect force.
  • First finger: The finger that points in the direction of the magnetic field.
  • Second finger: The finger that points in the direction of the conventional current.
  • Conventional current: The flow of current from positive to negative.
  • Motor effect: The force on a current-carrying conductor in a magnetic field.

Common Mistakes

  • Using the right hand instead of the left hand when applying Fleming's rule for the motor effect.
  • Using electron flow instead of conventional current for the second finger.
  • Putting the current along the thumb rather than along the second finger.
  • Holding the fingers at angles other than right angles to each other.
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