In a hydroelectric power plant, a large volume of water flows down a slope to turn a turbine, which turns a metal shaft in a generator, consequently producing electricity. A similar principle applies for geothermal power plants, except steam turns the turbines instead of water in these plants.
Most hydroelectric power plants make use of a dam to hold back the water before releasing it, giving it high potential energy. The dam is usually built on a large river to raise the level of the water and to control the flow. When the dam's gates are opened, water builds pressure as it flows down a pipeline leading to the turbine. The kinetic energy in the water converts into mechanical energy when the water strikes the turbine's blades with force, causing it to rotate.
As the turbine rotates, so do some giant magnets inside the generator. The mechanical energy from the previous stage converts into electrical energy as the magnets rotate past copper coils, producing alternating current. A transformer then takes the alternating current, converts it to a current of higher voltage, and transmits it through power transmission lines. Water that has flowed through the turbine is carried through pipes and released to the river.