Hydroelectric power plant. In a hydroelectric power plant electricity is produced from water. A dam is built across a large river to hold back the river water and raise its level to former reservoir. At the bottom of the dam there is a flood gate. Water flows through the penstock and rotates the turbine. The flowing water rotates the turbine which is placed at the end of the penstock. The turbine in turn rotates the shaft that is connected to the coils of a generator. These coils are placed in between strong magnetic pulls. The rotating shaft turns the coils. The change in magnetic field due to the movement.