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In physics, energy is an indirectly observed quantity which comes in many forms, such as kinetic energy, potential energy, radiant energy, and many others; which are listed in this summary article. This is a major topic in science and technology and this article gives an overview of its major aspects, and provides links to the many specific articles about energy in its different forms andcontexts.
The question "what is energy?" is difficult to answer in a simple, intuitive way, although energy can be rigorously defined in theoretical physics. In the words ofRichard Feynman, "It is important to realize that in physics today, we have no knowledge what energy is. We do not have a picture that energy comes in little blobs of a definite amount.".[1]
However, it is clear that energy isalways an indispensable prerequisite for performing mechanical work, and the concept has great importance in natural science. The natural basic units in which energy is measured are those used for mechanical work; they always are equivalent to a unit of force multiplied by a unit of length. Other equivalent units for energy are mass units multiplied by velocity units squared.
Contents  [hide]  *1 Forms of energy * 2 History of understanding * 3 Units of measure * 4 Energy in various contexts * 5 Energy transformation * 6 Conservation of energy * 7 Applications of the concept of energy * 8 Energy transfer * 9 Energy and the laws of motion * 10 Energy and thermodynamics * 11 Measurement * 12 See also * 13 Notes and references * 14 Further reading * 15 Externallinks |
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Forms of energy
Main article: Forms of energy
Energy exists in many forms:

Heat, a form of energy, is partly potential energy and partly kinetic energy.
In the context of physical sciences, several forms of energy have been defined. These include[need quotation to verify]:
* Thermal energy
* Chemical energy
* Electric energy* Radiant energy, the energy ofelectromagnetic radiation
* Nuclear energy
* Magnetic energy
* Elastic energy
* Sound energy
* Mechanical energy
* Luminous energy
* Mass (E=mc²)
These forms of energy may be divided into two main groups; kinetic energy and potential energy. Other familiar types of energy are a varying mix of both potential and kinetic energy[needquotation to verify].
Energy may be transformed between different forms at various efficiencies. Items that transform between these forms are calledtransducers.
The above list of the known possible forms of energy is not necessarily complete. Whenever physical scientists discover that a certain phenomenon appears to violate the law of energy conservation, new forms may be added, as is the casewith dark energy, a hypothetical form of energy that permeates all of space and tends to increase the rate of expansion of the universe.[need quotation to verify]
Classical mechanics distinguishes between potential energy, which is a function of the position of an object within a field, and kinetic energy, which is a function of its movement. Both position and movement are relative to a frame ofreference, which must be specified: this is often (and originally) an arbitrary fixed point on the surface of the Earth, the terrestrial frame of reference[need quotation to verify]. It has been attempted to categorize all forms of energy as either kinetic or potential, but as Richard Feynman points out:
These notions of potential and kinetic energy depend on a notion of length scale. For example,one can speak ofmacroscopic potential and kinetic energy, which do not include thermal potential and kinetic energy. Also what is called chemical potential energy is a macroscopic notion, and closer examination shows that it is really the sum of the potential and kinetic energy on the atomic and subatomic scale. Similar remarks apply to nuclear "potential" energy and most other forms of energy....