An electric motor uses electrical energy to produce mechanical energy, through the interaction of magnetic fields and current-carrying conductors. They may be powered by direct current (for example a battery powered portable device or motor vehicle), or by alternating current from a central electrical distribution grid.
The physical principle of production of mechanical force by the interactions of an electric current and a magnetic field was known as early as 1821.
The smallest motors may be found in electric wristwatches. Medium-size motors of highly standardized dimensions and characteristics provide convenient mechanical power for industrial uses. The very largest electric motors are used for propulsion of large ships, and for such purposes as pipeline compressors, with ratings in the millions of watts. Electric motors may be classified by the source of electric power, by their internal construction, and by their application.