DC Motor — Flashcards & Quiz

Key Points

• Motor effect: F = BIL sinθ on a current-carrying conductor in a magnetic field.
• Torque on a coil: τ = nBIA sinθ, where n is the number of turns, A is the coil area, and θ is the angle between the coil normal and B.
• Split-ring commutator reverses the current in the coil every half turn, keeping the torque pushing the coil in the same rotational direction.
• Torque is maximum when the coil is parallel to B (normal perpendicular to B); zero when the coil is perpendicular to B (normal parallel to B).
• Right-hand rule: fingers along current, curl toward B, thumb gives force direction.
• Parallel currents attract, antiparallel currents repel — the definition of the ampere.

Common Mistakes to Avoid

1. Forgetting the split-ring commutator — without it, the coil oscillates rather than rotates continuously.
2. Using the wrong hand for the right-hand rule — it's the right hand for conventional current.
3. Mixing up when torque is maximum (coil parallel to B) vs zero (coil perpendicular).
4. Claiming motors work with any magnetic material — they need a strong, fixed field.
5. Ignoring the sinθ factor in the torque equation.

Exam Strategy

VCAA Unit 3 AOS 3 DC motor questions ask you to explain the operation or calculate force/torque. Method: (1) identify the current direction and the magnetic field direction, (2) apply the right-hand rule to determine force direction, (3) use F = BIL sinθ or τ = nBIA sinθ for magnitude, (4) describe how the commutator reverses current at each half turn to maintain rotation. Diagrams of the coil and force arrows add marks.

Revision Tip

Motor effect direction problems are spatial — drill a Revizi deck with 8+ scenarios asking you to predict force direction using the right-hand rule.

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