Physics›The Motor Effect
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P5 MagnetismAQA GCSE Physics · 8463 (Triple)

The Motor Effect

Force on a current-carrying conductor, Fleming's left-hand rule, electric motors.

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CognitoThe motor effect and Fleming's left-hand rule
Key facts & equations

Force on a current-carrying conductor

  • A current-carrying conductor in a magnetic field experiences a force (the motor effect).
  • Force is largest when current is perpendicular to the field.
  • Force = 0 when current is parallel to the field.
  • F = BIL: force = magnetic flux density × current × length of conductor.
  • Units: B in tesla (T), I in amps (A), L in metres (m), F in newtons (N).

Fleming's left-hand rule

  • Used to find the direction of the force (motor effect).
  • Left hand: First finger = Field direction. Second finger = Current direction. Thumb = Motion (force) direction.
  • Mnemonic: FBI (Force-field-current) or SpAce (Sweep fingers: field, current, force up).
  • If current reverses: force reverses. If field reverses: force reverses.
  • Both reversed simultaneously: force direction unchanged.

Electric motor

  • Uses the motor effect to convert electrical energy to kinetic energy.
  • A coil carrying current is placed in a magnetic field.
  • Forces act on opposite sides of the coil in opposite directions → torque (turning effect).
  • Split-ring commutator: reverses current in the coil every half-turn → keeps coil rotating in the same direction.
  • Without commutator: coil would oscillate, not rotate continuously.

Increasing motor speed/force

  • Stronger magnet (higher B).
  • Greater current (higher I).
  • More turns in the coil.
  • Soft iron core: concentrates magnetic field.
  • Higher frequency alternating supply (for AC motors).
Exam questions — 4 questions · 10 marks · AQA 8463 style
⚡ Show all working — the AI marks against the AQA mark scheme. Method marks awarded for correct working even if the final answer is wrong.
3 marksF = BIL calculationAQA 8463 P2 style 🔢 Calculator

A wire of length 0.15 m carries a current of 4 A in a magnetic field of flux density 0.3 T (perpendicular to the field). Calculate the force on the wire. (2 marks)

Hint: F = BIL. All units are already standard. Multiply directly.
+30 XP
1 markFleming left hand ruleAQA 8463 style

In Fleming's left-hand rule, what does the first finger represent?

A The direction of the current
B The direction of the force (motion)
C The direction of the magnetic field
D The direction of the magnetic flux
3 marksMotor operationAQA 8463 style

Explain why the split-ring commutator is essential for a DC electric motor to rotate continuously. (3 marks)

Hint: Explain what would happen without the commutator (the coil would oscillate). Then explain what the commutator does (reverses current at the right moment to maintain rotation direction).
+30 XP
1 markForce directionAQA 8463 style

If both the current direction and magnetic field direction are both reversed simultaneously, what happens to the force on the conductor?

A The force doubles in magnitude
B The force reverses direction
C The force remains in the same direction (double reversal = same direction)
D The force becomes zero

Module complete! 🎉

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