How do magnetic fields make an electric starter motor turn?

An electric starter motor operates based on the interaction between magnetic flux and electric current, a principle governed by the Lorentz force.12

When you activate the starter, a direct current from the battery flows into the motor's armature coils. These coils are positioned within a stationary magnetic field produced by the stator's poles, which may contain permanent magnets or field coils. According to the Lorentz force, when a current-carrying conductor is placed in a magnetic field, it experiences a force perpendicular to both the direction of the current and the magnetic field.123

This force creates a torque that accelerates the rotor. To ensure continuous rotation, the motor uses a commutator—a switch that reverses the direction of the current in the armature coils as they rotate. This reversal ensures that the torque consistently acts in the same rotational direction while the coils are under the magnetic poles, allowing the motor to maintain its motion.1