Ubiquitous, manifold, and constant. Energy is everywhere. It comes in many forms. It is conserved. These characteristics of energy make it seem perhaps the most important single concept in science, apart from space and time. Energy is a central idea in physics, chemistry, biology—indeed in every area of natural science, as well as in engineering and practical affairs. Chameleon-like, it changes among a variety of forms as it appears in different scientific and technical environments. One need only think of some of the commonly used units of measurement for energy to be reminded of its many roles in many places: the kilowatt hour, the joule, the BTU (British thermal unit), the calorie, the foot pound, the electron volt, the megaton. Finally, and most important, energy is conserved. Only because of its conservation are the widespread and manifold forms of energy of special significance. The transformation of electrical energy to mechanical energy by a motor, or of mass energy to kinetic energy in a particle decay, takes on importance only because of conservation. The loss of one kind of energy is exactly balanced by the gain of another kind of energy. Without conservation, energy would be not one single concept appearing in many phenomena in many guises; it would be many different concepts. From the nuclear energy of the Sun’s interior to the work performed by human muscles runs a long and elaborate path of energy transformations through the sciences of astronomy, physics, chemistry, geology, and biology. Because of its conservation at every step along the way, energy is an unbroken thread tying these disciplines together. From its humble beginning as a secondary concept in mechanics, energy has grown into the most important unifying idea of natural science.
Momentum and angular momentum may be called purely mechanical quantities, or better, properties of motion. Energy, on the other hand, is not exclusively a property of motion. In one of its manifestations, kinetic energy, it is analogous to momentum and angular momentum. However, there is no law of conservation of kinetic energy, only a law of conservation of total energy. Kinetic energy, more elusive than momentum or angular momentum, can vanish to reappear in a different form.
Despite my emphasis on the manifold forms of energy, I must remark that the differences among some forms of energy is more difference in appearance than difference in essence. Internal energy is accounted for in terms of microscopic kinetic energy and potential energy. Chemical energy is electric in origin. Potential energy is reflected in changes of mass energy. Heat and work are not strictly forms of energy at all, but measures of energy transfer. At the most basic level of fundamental particles, only two forms of energy—kinetic energy and mass energy—are required to describe all energy transformations. Nevertheless, there is good reason for regarding the various forms of energy as distinct. In some domains of nature or for some ways of looking at nature, these energy forms manifest themselves in such completely different ways that it only makes sense to treat them as different kinds of energy. When a satellite reenters the atmosphere, its kinetic energy is dissipated and converted via heat into internal energy. It is satisfying to know that part of this internal energy is attributable to kinetic energy, the same form of energy characterizing the high-speed motion of the satellite before its re-entry. However, the bulk motion of the satellite as a whole bears so little resemblance to the random molecular motion associated with heat and internal energy that the appreciation of their basic similarity is purely an esthetic pleasure, without practical utility. If an astronaut, after riding comfortably in orbit, were burned to a crisp upon reentering the atmosphere, he might well be piqued that some designers had given inadequate attention to the difference between internal energy and kinetic energy.