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6.4.2Chemical Reactions

Fleming’s Left-Hand Rule — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions

5 min read 5 parts Lesson 13 of 21 Unit quiz

📚 Cambridge IGCSE Chemistry 0620 Hub / Topical Revision
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Part 1 of 5The Rule

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.

Part 2 of 5Using 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.

Part 3 of 5Notation 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.

Part 4 of 5Worked 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. Fieldleft to right, so the first finger points to the right.
  2. Currentinto the page, so the second finger points into the page, at right angles to the first finger.
  3. Thumbat 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.

Part 5 of 5Exam 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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