Physics
Physics
Problem
Two identical rectangular coils, A and B, are placed between the same pair of magnetic poles and connected separately to identical d.c. batteries. • Coil A is fitted with a split-ring commutator. • Coil B is fitted with two slip rings so that the current direction in each side of the coil remains fixed relative to the battery. What motion is observed when both switches are closed? a) Both coils rotate continuously in the same direction b) Only coil B rotates continuously; coil A vibrates and then stops c) Only coil A rotates continuously; coil B oscillates about its initial position and gradually stops d) Coil A rotates continuously, while coil B rotates in the opposite directionIntuitive explanation
The key difference lies in how each commutator delivers current to the coil sides as they rotate. Coil A's split-ring commutator automatically reverses current direction every half-turn, so the force on each side always pushes in the same rotational direction—it spins continuously. Coil B's slip rings maintain fixed current direction relative to the battery, meaning the force direction flips as the coil rotates, causing it to oscillate back and forth rather than spin. This illustrates why commutators are essential for converting electrical energy into sustained rotational motion in motors.
Intuitive explanation
The key difference lies in how each commutator delivers current to the coil sides as they rotate. Coil A's split-ring commutator automatically reverses current direction every half-turn, so the force on each side always pushes in the same rotational direction—it spins continuously. Coil B's slip rings maintain fixed current direction relative to the battery, meaning the force direction flips as the coil rotates, causing it to oscillate back and forth rather than spin. This illustrates why commutators are essential for converting electrical energy into sustained rotational motion in motors.