What is the source of mechanical energy in an electric generator?

there are a lot of source of mechanical in an electric generator. most common of these is the hydro plant which uses the potential energy of the water as it falls from high place. the water hits the paddle of the generator and gives the needed kinetic energy.

another is the use of wind turbines. which uses almost the same concepts. the blades of the turbine is powered by air. we also have geothermal. it uses the steam from the earth,.

Detailed, nice pictures on this page: http://www.dieselserviceandsupply.com/How_Generators_Work.aspx