The ac generator

Alternating current is easily generated by means of a rotating magnet in a coil of wire (Fig. 1A), or by a rotating coil of wire inside a powerful magnet (Fig. 1B). In either case, the ac appears between the ends of the length of wire.
The ac voltage that a generator can develop depends on the strength of the magnet, the number of turns in the wire coil, and the speed at which the magnet or coil rotates. The ac frequency depends only on the speed of rotation. Normally, for utility ac, this speed is 3,600 revolutions per minute (rpm), or 60 complete revolutions per second (rps), so that the frequency is 60 Hz.

Fig. 1: Two forms of ac generator. At A, the magnet rotates; at B, the coil rotates.

When a load, such as a light bulb or heater, is connected to an ac generator, it becomes more difficult to turn the generator. The more power needed from a generator, the greater the amount of power required to drive it. This is why it is not possible to connect a generator to, for instance, your stationary bicycle, and pedal an entire city into electrification. There’s no way to get something for nothing. The electrical power that comes out of a generator can never be more than the mechanical power driving it. In fact, there is always some energy lost, mainly as heat in the generator. Your legs might generate 50 W of power to run a small radio, but nowhere near enough to provide electricity for a household.
The efficiency of a generator is the ratio of the power output to the driving power, both measured in the same units (such as watts or kilowatts), multiplied by 100 to get a percentage. No generator is 100 percent efficient. But a good one can come fairly close to this ideal.
At power plants, the generators are huge. Each one is as big as a house. The generators are driven by massive turbines. The turbines are turned by various natural sources of energy. Often, steam drives the turbines, and the steam is obtained via heat derived from the natural energy source.