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Energy transformation; In a typical lightning strike, 500 megajoules of electric potential energy is converted into the same amount of energy in other forms, most notably light energy, sound energy and thermal energy.

In physics, energy is one of the basic quantitative properties describing a physical system or object's state. Energy can be transformed (converted) among a number of forms that may each manifest and be measurable in differing ways. The law of conservation of energy states that the (total) energy of a system can increase or decrease only by transferring it in or out of the system. The total energy of a system can be calculated by simple addition when it is composed of multiple non-interacting parts or has multiple distinct forms of energy. Common energy forms include the kinetic energy of a moving object, the radiant energy carried by light and other electromagnetic radiation, and various types of potential energy such as gravitational and elastic. Energy is measured in SI units of joules (J). Common types of energy transfer and transformation include processes such as heating a material, performing mechanical work on an object, generating or making use of electric energy, and many chemical reactions.

Units of measurement for energy are usually defined via a work process. The work performed by a given body on another is defined in physics as the force (SI unit: newton) applied by the given body, multiplied by the distance (SI unit: metre) of movement against the opposing force exerted by the other body. Thus, the energy unit is the newton-metre, which is called the joule. The SI unit of power (energy per unit time) is the watt, which is simply a joule per second. Thus, a joule is a watt-second, so 3600 joules equal a watt-hour. The CGS energy unit is the erg, and the imperial and US customary unit is the foot pound. Other energy units such as the electron volt, food calorie or thermodynamic kcal (based on the temperature change of water in a heating process), and BTU are used in specific areas of science and commerce and have unit conversion factors relating them to the joule.

Potential energy is energy stored by virtue of the position of an object in a force field, such as a gravitational, electric or magnetic field. For example, lifting an object against gravity performs work on the object and stores gravitational potential energy; if it falls, gravity does work on the object which transforms the potential energy to kinetic energy associated with its speed. Some specific forms of energy include elastic energy due to the stretching or deformation of solid objects, chemical energy such as is released when a fuel burns, and thermal energy, the microscopic kinetic and potential energies of the disordered motions of the particles making up matter.

Not all of the energy in a system can be transformed or transferred by a work process; the amount that can is called the available energy. In particular the second law of thermodynamics limits the amount thermal energy that can be transformed into other forms of energy. Mechanical and other forms of energy can be transformed in the other direction into thermal energy without such limitations.

Any object that has mass when stationary (thus called rest mass), equivalently has rest energy as can be calculated using Albert Einstein's equation E = mc2. Being a form of energy, rest energy can be transformed to or from other forms of energy, while the total amount of energy does not change. From this perspective, the amount of matter in the universe contributes to its total energy.

Similarly, all energy manifests as a proportionate amount of mass. For example, adding 25 kilowatt-hours (90 megajoules) of any form of energy to an object increases its mass by 1 microgram. If you had a sensitive enough mass balance or scale, this mass increase could be measured. Our Sun (or a nuclear bomb) transforms nuclear potential energy to other forms of energy; its total mass doesn't decrease due to that in itself (since it still contains the same total energy even if in different forms), but its mass does decrease when the energy escapes out to its surroundings, largely as radiant energy.

A new form of energy can't be defined arbitrarily. In order to be valid, it must be shown to be transformable to or from a predictable amount of some known form(s) of energy, thus showing how much energy it represents in the same units used for all other forms. It must obey conservation of energy, so it must never decrease or increase except via such a transformation (or transfer). Also, if an alleged new form of energy can be shown not to change the mass of a system in proportion to its energy, then it is not a form of energy.

Living organisms require available energy to stay alive; humans get such energy from food along with the oxygen needed to metabolize it. Civilization requires a supply of energy to function; energy resources such as fossil fuels are a vital topic in economics and politics. Earth's climate and ecosystem are driven by the radiant energy Earth receives from the sun, and are sensitive to changes in the amount received.

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:

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

Thomas Young – the first to use the term "energy" in the modern sense.

The word energy derives from the Greek ἐνέργεια energeia, which possibly appears for the first time in the work of Aristotle in the 4th century BCE. (Ancient Greek: ἐνέργεια energeia "activity, operation"[1])

The concept of energy emerged from the idea of vis viva (living force), which Gottfried Leibniz defined as the product of the mass of an object and its velocity squared; he believed that total vis viva was conserved. To account for slowing due to friction, Leibniz theorized that thermal energy consisted of the random motion of the constituent parts of matter, a view shared by Isaac Newton, although it would be more than a century until this was generally accepted.

In 1807, Thomas Young was possibly the first to use the term "energy" instead of vis viva, in its modern sense.[2] Gustave-Gaspard Coriolis described "kinetic energy" in 1829 in its modern sense, and in 1853, William Rankine coined the term "potential energy".

The law of conservation of energy, was first postulated in the early 19th century, and applies to any isolated system. According to Noether's theorem, the conservation of energy is a consequence of the fact that the laws of physics do not change over time.[3] Since 1918 it has been known that the law of conservation of energy is the direct mathematical consequence of the translational symmetry of the quantity conjugate to energy, namely time.

It was argued for some years whether energy was a substance (the caloric) or merely a physical quantity, such as momentum. In 1845 James Prescott Joule discovered the link between mechanical work and the generation of heat. This led to the theory of conservation of energy, and development of the first law of thermodynamics.

Finally, William Thomson (Lord Kelvin) amalgamated these many discoveries into the laws of thermodynamics, which aided the rapid development of explanations of chemical processes by Rudolf Clausius, Josiah Willard Gibbs, and Walther Nernst. It also led to a mathematical formulation of the concept of entropy by Clausius and to the introduction of laws of radiant energy by Jožef Stefan.

During a 1961 lecture[4] for undergraduate students at the California Institute of Technology, Richard Feynman, a celebrated physics teacher and Nobel Laureate, said this about the concept of energy:

There is a fact, or if you wish, a law, governing all natural phenomena that are known to date. There is no known exception to this law—it is exact so far as we know. The law is called the conservation of energy. It states that there is a certain quantity, which we call energy, that does not change in manifold changes which nature undergoes. That is a most abstract idea, because it is a mathematical principle; it says that there is a numerical quantity which does not change when something happens. It is not a description of a mechanism, or anything concrete; it is just a strange fact that we can calculate some number and when we finish watching nature go through her tricks and calculate the number again, it is the same.

Units of measure

Energy, like mass, is a scalar physical quantity. The joule is the International System of Units (SI) unit of measurement for energy. It is a derived unit of energy, work, or amount of heat. It is equal to the energy expended (or work done) in applying a force of one newton through a distance of one metre. However energy is also expressed in many other units such as ergs, calories, British Thermal Units, kilowatt-hours and kilocalories for instance. There is always a conversion factor for these to the SI unit; for instance; one kWh is equivalent to 3.6 million joules.[5]

Energy in various contexts

Classical mechanics