An ordinary electric generator produces electric power by spinning a strong magnet inside a set of wire coils. As the magnet spins, its magnetic field sweeps across the coils and gives rise to electric fields in those coils. This effect, in which changing or moving magnetic fields produce electric fields, is a fundamental piece of electromagnetic theory, and allows the spinning magnet to push on electric charges that exist within the wire coils.
But if the generator's power isn't being used, its coils carry no electric current. The fields from the spinning magnet are pushing charges around, but those charges have nowhere to go. It isn't until you begin using the generated power that current can flow through the coils. When that happens, something new appears: magnetic fields around the coils themselves. As current flows through the coils, the coils become electromagnets.
In accordance with a property known as Lenz's law, each coil's magnetism pushes against the spinning magnet's motion, first to slow the magnet's approach and then to delay its departure. By fighting the spinning magnet's rotation, the coils are making it do work on them and extracting energy from its spinning motion. To keep the magnet spinning, something must continuously restore this rotational energy and that something is usually an engine. You can hear the engine begin working hard when you start using power from the generator. And if you're turning the generator by hand, you have no problem noticing when its power is being consumed: its crank suddenly becomes much harder to turn. Generating just 100 watts of power is quite a challenge and you'll find yourself sweating after a minute or two. Many exercise machines use this phenomenon to provide variable resistances. As you exercise, you power a generator and the machine's computer varies your workout by adjusting how much of your electric power it consumes. Unfortunately, a typical workout generates less than 1¢ worth of electricity, so you'd better not plan on earning your way through college as a power company.
A coil rotating with constant angular frequency in a magnetic field produces an electromotive force.
Answered by Lou A. Bloomfield of the University of Virginia.