EASA / CAR Part-66 · Module 3 · 3.12 DC Motor & Generator Theory · Cat B1/B2
A current-carrying coil in a magnetic field feels a force (F = B·I·L) that turns it. More current or a stronger field means more torque and faster spin. Reverse the current and it spins the other way.
The force on each side of the coil is F = B·I·L. The two sides push in opposite directions, creating a couple that rotates the armature. A commutator reverses the current every half-turn so the torque keeps driving the same way. This is the basis of every DC motor on the aircraft.