This topic summary covers Knowledge Organiser: The Motor Effect within The Motor Effect for GCSE Physics. Revise The Motor Effect in Magnetism for GCSE Physics with 19 exam-style questions and 12 flashcards. This topic appears regularly enough that it should still be part of a steady revision cycle. It is section 12 of 12 in this topic. Use this topic summary to connect the idea to the wider topic before moving on to questions and flashcards.
Knowledge Organiser: The Motor Effect
Key Terms
- Motor effect: force on current-carrying conductor in magnetic field
- Fleming's Left-Hand Rule: thumb=force, 1st=field, 2nd=current
- Split-ring commutator: reverses current every half turn
- Brushes: connect supply to spinning coil
Key Facts
- Use LEFT hand for motor effect
- Force is max when current ⊥ field
- Force is zero when current ∥ field
- Commutator is essential for continuous rotation
- DC motor has 4 parts: coil, magnets, commutator, brushes
Key Equations
- F = BIL
- F in N, B in T, I in A, L in m
- Increase F: increase B, I, or L
- Reverse current → force reverses direction
Exam Tips
- Left hand for motors — FBI (Force, B-field, I-current)
- Commutator reverses current every half turn
- Max force when perpendicular, zero when parallel to field
- Reverse current or reverse field = force reverses
Common Mistakes
- Using the right hand instead of left hand: Fleming's left-hand rule applies to motors (current-carrying conductors in a magnetic field) — the right-hand rule is for generators/dynamos
- Forgetting the commutator's role: The split-ring commutator reverses the current direction every half turn — without it, the coil would oscillate rather than spin continuously
- Saying force is maximum when conductor is parallel to field: Force is maximum when the conductor is perpendicular to the magnetic field; force is zero when parallel to the field
- Confusing the three directions in Fleming's rule: In the left-hand rule — First finger = Field direction, seCond finger = Current direction, thuMb = Motion (force) direction — each finger points in a different direction
- Reversing both current and field keeps force the same: Reversing both current and field direction simultaneously does not change the force direction — you must reverse only one to change the force direction
Practice questions for The Motor Effect
What is the motor effect?
Explain how Fleming's left-hand rule is used to find the direction of the force on a current-carrying conductor in a magnetic field.
Quick recall flashcards
Fleming's Left Hand: for?
Motors (force on current-carrying conductor)
Left hand: thumb =?
Motion/Force