A new era of the use of energy began with the application of nuclear energy. People find many ways to release energy to do work. They change the energy in a waterfall into electrical energy. They can turn this electrical current into radio waves that carry ideas thousands of miles or kilometers. They can release the energy in gasoline by burning it to power automobiles.
People burn coal to turn water into steam and, in turn, use steam to generate electrical energy.
The nucleus of some atoms can produce millions of times more energy per pound or kilogram of material than can be made available by chemical means. For more information on energy sources, see Energy supply . Potential and kinetic energy Potential energy is often called stored energy. It represents work that has already been done. A rock lying on the top of a cliff has potential energy and so does the cartridge in a loaded rifle.