HSC Physics — Module 6
Motor Effect — Flashcards & Quiz
The motor effect is the force experienced by a current-carrying conductor in a magnetic field, F = BIL sinθ, and it underpins HSC Physics Module 6 electric motor theory. You need to apply the right-hand rule to predict force direction, calculate the force between parallel wires, and explain how a DC motor converts electrical energy into mechanical rotation — including the role of the split-ring commutator in reversing current at each half-turn.
Key Points
- A current-carrying conductor in a magnetic field experiences a force F = BIL sin θ, where θ is the angle between I and B.
- Direction: use the right-hand rule — fingers point along conventional current, curl toward B, thumb gives the force direction.
- Force between parallel wires: parallel currents attract, antiparallel currents repel. This defines the ampere.
- A DC motor uses the motor effect to rotate a coil: the split-ring commutator reverses the current each half-turn so torque keeps rotating the coil the same way.
- Torque on a coil: τ = nBIA sin θ where n is the number of turns, A is the cross-sectional area, and θ is the angle between the coil normal and B.
- Exam diagram skill: draw current direction, B field direction, and use the right-hand rule to correctly label the resulting force.
Common Mistakes to Avoid
- Using the wrong hand for the right-hand rule — it's the right hand for conventional current.
- Forgetting the sin θ factor — force is maximum when current is perpendicular to B, zero when parallel.
- Confusing motor effect (force on current-carrying conductor) with generator effect (EMF from motion).
- Missing the torque on a coil formula τ = nBIA sin θ.
- Not explaining the role of the split-ring commutator in DC motors.
Exam Strategy
HSC Module 6 motor effect questions ask you to (1) calculate force on a conductor, (2) predict force direction, or (3) explain a DC motor's operation. Method: apply F = BIL sin θ, use the right-hand rule for direction, for motors explain how the commutator reverses current each half-turn to keep torque in the same rotational direction.
Sample Flashcards
Q1: What is the motor effect?
A current-carrying conductor in a magnetic field experiences a force: F = BIl sinθ, where B = magnetic field strength (T), I = current (A), l = length of conductor in field (m), θ = angle between conductor and field. Direction given by the right-hand push rule (or left-hand rule).
Q2: What is the force between two parallel current-carrying conductors?
Parallel currents in the same direction attract each other. Parallel currents in opposite directions repel. Each conductor's magnetic field exerts a force on the other conductor. Force per unit length: F/l = μ₀I₁I₂/(2πd), where μ₀ = 4π × 10⁻⁷ T·m/A and d is the separation distance.
Sample Quiz Questions
Q1: The force on a current-carrying conductor in a magnetic field is perpendicular to both the current and the field.
Answer: TRUE
The motor effect force F = BIlsinθ acts perpendicular to both B and I, as given by the right-hand push rule. This perpendicular force is what causes the conductor to move.
Q2: Parallel wires carrying currents in the same direction attract each other.
Answer: TRUE
The magnetic field from one wire exerts a force on the other. For same-direction currents, the force is attractive. For opposite-direction currents, it is repulsive. F/l = μ₀I₁I₂/(2πd).
Revision Tip
Force direction with the right-hand rule is spatial reasoning — drill Revizi flashcards with diagrams where you're asked to predict the force direction.
Related Concepts
Last updated: March 2026 · 2 flashcards · 2 quiz questions