The Physical Concept of Energy

Man used nuclear energy from the sun long before the physical concept of energy became known. In the form of electromagnetic radiation solar energy travels through interplanetary space, in the form of chemical energy it is stored in organic materials on the surface of the earth. Collected over large areas and through periods of months in plants, solar energy is transformed into muscular power of man and animals. By integration through decades in wood and through millions of years in coal solar energy provided the fuel for steam engines, the last energy technology of the pre-scientific era. Solar energy is transformed into kinetic energy of air and thus may drive windmills, it is also transformed into potential energy of water and thereby may power water mills.