This animation shows a close up view of the commutator in the d.c electric motor.
As the coil spins a current only passes through the coil when the ends of the coil are in contact with the
commutator 'brushes'. When this happens the coil experiences a force which gives it a 'kick' and keeps it rotating.
The direction of the current in the coil changes every half rotation.
The force acts all the time that the ends of the coil are in contact with the brushes.
A torque (Fd) is applied to the coil during this time.
Where F is the force on the two sides of the coil and d is the distance between them.