Key Concepts: A current-carrying conductor in a magnetic field experiences a force. Fleming's left-hand rule: thuMb = Motion (force), First finger = Field, seCond finger = Current. Force increases with greater current, stronger magnetic field, or longer wire. A DC motor uses this effect to produce continuous rotation.
Section A — The Motor Effect
1. Describe the motor effect. State the conditions needed for a force to act on a wire. [3]
2. State three ways to increase the force on a current-carrying wire in a magnetic field. [3]
3. State what happens to the force when the wire is parallel to the magnetic field. [1]
Section B — Fleming's Left-Hand Rule
4. Describe how to use Fleming's left-hand rule to find the direction of the force, field and current. [3]
5. A horizontal wire carries a current flowing to the right. The magnetic field points vertically upward. Use Fleming's left-hand rule to state the direction of the force on the wire. [2]
6. What happens to the direction of the force if the current in the wire is reversed? [1]
Section C — The DC Electric Motor
7. Describe how a simple DC motor produces continuous rotation. Include the role of the split-ring commutator. [4]
8. State two ways to make an electric motor spin faster. [2]
9. Give two practical applications of the motor effect (other than the motor itself). [2]
Total marks: 21
Mark Scheme
1. A force acts on a current-carrying conductor when it is placed in a magnetic field [1]; the wire must carry a current [1]; and must not be parallel to the field [1] [3]
2. Increase the current [1]; use a stronger magnetic field [1]; increase the length of wire in the field [1] [3]
3. The force is zero when the wire is parallel to the field [1]
4. Hold the left hand with thumb, first finger and second finger all at right angles [1]; First finger = direction of magnetic Field [1]; seCond finger = direction of Current; thuMb = direction of Motion (force) [1] [3]
5. Applying the rule: current to the right (second finger), field upward (first finger) → force is directed out of the page (toward the observer) [2]
6. The direction of the force reverses [1]
7. Current flows through the coil in a magnetic field; the forces on opposite sides of the coil act in opposite directions [1]; this creates a turning effect (torque) on the coil [1]; the split-ring commutator reverses the current direction every half turn [1]; so the force always acts in the same rotational direction, maintaining continuous rotation [1] [4]
8. Any two: increase the current; use stronger magnets; increase the number of turns on the coil [2]
9. Any two: loudspeaker; galvanometer (measuring instrument); maglev train [2]