What principle do starter motors operate on?

Starter motors operate based on electromagnetic principles, specifically involving the interaction between magnetic fields. The correct choice describes how the armature of the starter motor is influenced by differing magnetic fields. In this context, the armature rotates from an area of strong magnetic field towards a region of weaker magnetic field due to the interaction with the magnetic field created by the field coils.

When the starter motor is energized, the current flows through the field coils, creating a magnetic field. This magnetic field interacts with the armature, causing it to rotate. The rotation continues as long as there is a sufficient difference in magnetic field strength, which facilitates the movement and ultimately engages the engine of the vehicle.

The other options discuss magnetic principles like attraction and repulsion of magnetic poles, which are relevant in general magnetism but do not specifically explain the operational mechanics of a starter motor as accurately as the correct choice does. The primary function of the starter motor is to convert electrical energy into mechanical energy, enabling the engine to crank and start properly, reflecting the importance of the magnetic field differentials as described.