What is actually happening inside the generator, and where is it drawing the electrons that make up the current from?
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If you spin a magnet next to a loop of copper wire, electrical current will flow around the loop. This happens because a moving magnetic field creates an electric field, and an electric field can push electrons through a circuit. Note that you can spin the magnet next to stationary wire, or spin the wire next to a stationary magnet; the effect is equivalent. If you connect the wire to equipment that uses electricity (such as a lightbulb), then the current generated by the spinning magnet will power the equipment. If you connect a rechargeable battery, the battery can be charged by the spinning magnet. In fact, if you use an external source of voltage to force current through the wire, you can cause the wire to spin the magnet! This is an oversimplification, of course, because many technical details are involved in how to arrange the wire and the connections with respect to the magnet’s orientation.
If you scale this up with big magnets and many loops of copper wire, you can connect to a power grid and supply electricity to buildings. Big magnets need a lot of force to rotate, so usually you need something very strong to turn them. You can use a turbine (basically a fan that gets turned by moving fluid) to get power from air or from water (such as in a dam). You can use big fossil-fuel-burning motors to turn the magnets.
For example, your car has something called an alternator. This is a part of the engine that spins when the engine is running, powered by gasoline combustion. The alternator has a magnet and it has coils of copper wire. The spinning motion of the alternator causes current to flow through the wire, and that current can be used to power electrical components of your car – even recharging the car’s battery.
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