Energy
Energy, capacity of matter to perform work as the result of its motion or its position in relation to forces acting on it. Energy associated with motion is known as kinetic energy, and energy related to position is called potential energy. Thus, a swinging pendulum has maximum potential energy at the terminal points; at all intermediate positions it has both kinetic and potential energy in varying proportions. Energy exists in various forms, including mechanical (see Mechanics), thermal (see Thermodynamics), chemical (see Chemical Reaction), electrical (see Electricity), radiant (see Radiation), and atomic (see Nuclear Energy). All forms of energy are interconvertible by appropriate processes. In the process of transformation either kinetic or potential energy may be lost or gained, but the sum total of the two remains always the same.
A weight suspended from a cord has potential energy due to its position, inasmuch as it can perform work in the process of falling. An electric battery has potential energy in chemical form. A piece of magnesium has potential energy stored in chemical form that is expended in the form of heat and light if the magnesium is ignited. If a gun is fired, the potential energy of the gunpowder is transformed into the kinetic energy of the moving projectile. The kinetic mechanical energy of the moving rotor of a dynamo is changed into kinetic electrical energy by electromagnetic induction. All forms of energy tend to be transformed into heat, which is the most transient form of energy. In mechanical devices energy not expended in useful work is dissipated in frictional heat, and losses in electrical circuits are largely heat losses.
Empirical observation in the 19th century led to the conclusion that although energy can be transformed, it cannot be created or destroyed. This concept, known as the conservation of energy, constitutes one of the basic principles of classical mechanics. The principle, along with the parallel principle of conservation of matter, holds true only for phenomena involving velocities that are small compared with the velocity of light. At higher velocities close to that of light, as in nuclear reactions, energy and matter are interconvertible (see Relativity). In modern physics the two concepts, the conservation of energy and of mass, are thus unified.