Power plants burn coal and other fossil fuels to produce the heat and steam necessary to run generators that produce electrical power. When electricity became widely and cheaply available, it meant that industries could convert from steam to electrical power to run their machinery. One central power plant could supply the electrical needs of a relatively large geographic area. However, increasing demands for electricity, by both consumer and commercial customers, led to a rise in the number of power plants. This, in turn, led to problems with air pollution. As more plants went on line, more fossil fuels were burned to generate power. Because modern society depends so heavily on electricity, researchers are constantly searching for alternate forms of energy to satisfy ever-increasing demands.