A d.c electric motor consists of a rectangular coil of insulated wire, which makes up the moving part of the motor, N-S poles of a permanent magnet, pairs of commutators and carbon brushes.
When the current is switched on in the coil, it exists in opposite directions along the two opposite sides of the coil, generating equal but opposite motor force, which form a torque that rotate the coil.
The force on the coil carries it past the point where the current is short-circuited, and beyond that point, the current is reversed in the coil, but forces remain in the same directions, ensuring the continuous rotation of the coil while there is current in the coil.
If the direction of the current, or the poles of the magnet, are reversed, rotation will proceed in the
opposite direction. The combination of magnetic field, current and force are shown in the diagram. Note that heat is also produced, so there is never a 100% conversion of electrical energy into kinetic energy.
Commutator act as current-reversing switch after every half-revolution of the coil.
The carbon brushes serve to make contact between the battery and the rotating commutators. They are mounted on an insulated shaft of the motor, not shown in the picture.