Electric motor converts electrical energy to kinetic energy. Diagram above shows the structure of a simple direct current motor (DC motor). It consist a rectangular coil of wire placed between 2 permanent magnets. The coil are soldered to a copper split ring known as commutator. 2 carbon brushes are held against the commutator.
The function of the brush is to conduct electricity from the external circuit to the coil and allow the commutator to rotate continuously.
The function of the commutator is to change the direction of the current in the coil and hence change the direction of the couple (the 2 forces in opposite direction) in every half revolution. This is to make sure that the coil can rotate continuously.
The operation principle of a direct current motor are explained in detail th the second Youtube video below.Physics Animation
Direct Current Motor - Walter Fendt
Operation of a Direct Current Motor - sun.ylojarvi,fl
Electric Motor and Generator
This video shows how the commutator changes the direction of the current in the coil, and hence change the direction of the force in every half revolution. Pay attention on the change of direction of the force in every half revolution.
This is a very long video, discussing how the force is produced by the interaction between a current carrying conductor in a magnetic field and its application in direct current motor.
Here is another cool DC motor. I haven't try it yet. Maybe you can try to make one and tell me whether it works.