Nuclear Energy, energy released during the splitting or fusing of atomic
nuclei. The energy of any system, whether physical, chemical, or nuclear, is
manifested by the system’s ability to do work or to release heat or
radiation. The total energy in a system is always conserved, but it can be
transferred to another system or changed in form.
Until about 1800 the principal fuel was wood, its energy derived from solar
energy stored in plants during their lifetimes. Since the Industrial
Revolution, people have depended on fossil fuels—coal, petroleum, and
natural gas—also derived from stored solar energy. When a fossil fuel such
as coal is burned, atoms of hydrogen and carbon in the coal combine with
oxygen atoms in air. Water and carbon dioxide are produced and heat is
released, equivalent to about 1.6 kilowatt-hours per kilogram or about 10
electron volts (eV) per atom of carbon. This amount of energy is typical of
chemical reactions resulting from changes in the electronic structure of the
atoms. A part of the energy released as heat keeps the adjacent fuel hot
enough to keep the reaction going.