figure 14-6, and was confused by the torque vs. position graph of the single-loop motor. Under the assumption of a radial magnetic field all around the rotor, the torque must be the same for every angle when neglecting the gravity force on the loop wire. Why has he showed the curve like that?
Because in position 2 (fig 14-4) the electromagnetic forces pulls only in up/down direction (in respect to drawing viewport) and therefore it don't provide any torque to the rotation axe.
On figure 14-5, the situation is oposite, and virtually all electromagnetic forces do contribute to the torque.
Important thing here is: All wires between the two magnets can only be pulled either in up or down directions, assuming there is no other magnet fields other than the magnets and magnet field from the windings.
So this is only in his case as I assumed, that the magnetic field is not radial in his model, thus the force in fig 14-4 will be a centripetal force in that case. But in a standard DC-motor, isn't the torque almost constant, when looking at just one single loop (radial magnetic field)?