The interaction between moving electrons and magnetic fields can produce electricity. If a metallic wire that is part of a complete circuit is passed through a magnetic field, the electrons in the wire are pushed through the circuit, resulting in an electric current. An electric generator is any machine that is designed to generate electricity by passing metal wires through a magnetic field. All that is required is to spin the axle on the generator. The largest generators rely on turbines, which contain fan blades that spin when fluid—such as steam produced by burning coal or wind blowing through a windmill—is forced through. The generation of hydroelectric power harnesses falling water to spin turbines attached to electric generators. For instance, the Colorado River flows through the turbines of the Hoover Dam, shown above, generating more than 4 billion kilowatt-hours of electricity each year for use in Nevada, Arizona, and California.
Magnets can be temporary or permanent, depending on their ability to retain charge when not in contact with another magnet. Ferromagnetic materials are substances that can be made into magnets. A ferromagnetic core wrapped with a current-carrying wire forms an electromagnet that can be many times stronger than the magnet alone. Magnets emit magnetic fields, and passing a wire connected to a complete circuit through such fields induces current through the circuit. This is the basic premise behind electric generators, which effectively convert the mechanical energy of the motion of the wire into electrical energy.