Energy

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n 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 and contexts. 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 of Richard 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 is always an indispensable prerequisite for performing work or causing changes, and the concept has great importance in natural science. 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 External links
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: Thermal energy, thermal energy in transit is called heat

Chemical energy
Electric energy
Radiant energy, the energy of electromagnetic 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. Energy may be transformed between these forms, some with 100% energy conversion efficiency and others with less. Items that transform between these forms are called transducers. 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 case with dark energy, a hypothetical form of energy that permeates all of space and tends to increase the rate of expansion of the universe. Classical mechanics distinguishes between potential energy, which is a function of the position of an object, and kinetic energy, which is a function of its movement. Both position and movement are relative to a frame of reference, which must be specified: this is often (and originally) an arbitrary fixed point on the surface of the Earth, the terrestrial frame of reference. It has been attempted to categorize all forms of energy as either kinetic or potential: this is not incorrect, but neither is it clear that it is a real simplification, as Feynman points out: These notions of potential and kinetic energy depend on a notion of length scale. For example, one can speak of macroscopic 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. This dependence on length scale is non-problematic if the various length scales are decoupled, as is often the case ... but confusion can arise when different length scales are coupled, for instance when friction converts macroscopic work into microscopic thermal energy.

History of understanding

Main articles: History of energy and timeline of thermodynamics,...
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