Lightning is the electric breakdown of air by strong electric fields, which produce a force on charges. When these charges move through a distance, a flow of energy occurs. The electric potential energy in the atmosphere then is transformed into thermal energy, light, and sound, which are other forms of energy.

In physics, energy (Greek: ἐνέργεια energeia "activity, operation"^[1]) is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems.^[2][3] Since work is defined as a force acting through a distance (a length of space), energy is always equivalent to the ability to exert pulls or pushes against the basic forces of nature, along a path of a certain length.

The total energy contained in an object is identified with its mass, and energy (like mass), cannot be created or destroyed. When matter (ordinary material particles) is changed into energy (such as energy of motion, or into radiation), the mass of the system does not change through the transformation process. However, there may be mechanistic limits as to how much of the matter in an object may be changed into other types of energy and thus into work, on other systems. Energy, like mass, is a scalar physical quantity. In the International System of Units (SI), energy is measured in joules, but in many fields other units, such as kilowatt-hours and kilocalories, are customary. All of these units translate to units of work, which is always defined in terms of forces and the distances that the forces act through.

A system can transfer energy to another system by simply transferring matter to it (since matter is equivalent to energy, in accordance with its mass). However, when energy is transferred by means other than matter-transfer, the transfer produces changes in the second system, as a result of work done on it. This work manifests itself as the effect of force(s) applied through distances within the target system. For example, a system can emit energy to another by transferring (radiating) electromagnetic energy, but this creates forces upon the particles that absorb the radiation. Similarly, a system may transfer energy to another by physically impacting it, but in that case the energy of motion in an object, called kinetic energy, results in forces acting over distances (new energy) to appear in another object that is struck. Transfer of thermal energy by heat occurs by both of these mechanisms: heat can be transferred by electromagnetic radiation, or by physical contact in which direct particle-particle impacts transfer kinetic energy.

Energy may be stored in systems without being present as matter, or as kinetic or electromagnetic energy. Stored energy is created whenever a particle has been moved through a field it interacts with (requiring a force to do so), but the energy to accomplish this is stored as a new position of the particles in the field—a configuration that must be "held" or fixed by a different type of force (otherwise, the new configuration would resolve itself by the field pushing or pulling the particle back toward its previous position). This type of energy "stored" by force-fields and particles that have been forced into a new physical configuration in the field by doing work on them by another system, is referred to as potential energy. A simple example of potential energy is the work needed to lift an object in a gravity field, up to a support. Each of the basic forces of nature is associated with a different type of potential energy, and all types of potential energy (like all other types of energy) appears as system mass, whenever present. For example, a compressed spring will be slightly more massive than before it was compressed. Likewise, whenever energy is transferred between systems by any mechanism, an associated mass is transferred with it.

Any form of energy may be transformed into another form. For example, all types of potential energy are converted into kinetic energy when the objects are given freedom to move to different position (as for example, when an object falls off a support). When energy is in a form other than thermal energy, it may be transformed with good or even perfect efficiency, to any other type of energy, including electricity or production of new particles of matter. With thermal energy, however, there are often limits to the efficiency of the conversion to other forms of energy, as described by the second law of thermodynamics.

In all such energy transformation processes, the total energy remains the same, and a transfer of energy from one system to another, results in a loss to compensate for any gain. This principle, the 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.^[4]

Although the total energy of a system does not change with time, its value may depend on the frame of reference. For example, a seated passenger in a moving airplane has zero kinetic energy relative to the airplane, but non-zero kinetic energy (and higher total energy) relative to the Earth.

History[link]

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.

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

The concept of energy emerged out of 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.^[5] Gustave-Gaspard Coriolis described "kinetic energy" in 1829 in its modern sense, and in 1853, William Rankine coined the term "potential energy". It was argued for some years whether energy was a substance (the caloric) or merely a physical quantity, such as momentum.

William Thomson (Lord Kelvin) amalgamated all of these laws into the laws of thermodynamics, which aided in 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^[6] 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.

—The Feynman Lectures on Physics

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. That is, energy is conserved because the laws of physics do not distinguish between different instants of time (see Noether's theorem).

Energy in various contexts[link]

The concept of energy and its transformations is useful in explaining and predicting most natural phenomena. The direction of transformations in energy (what kind of energy is transformed to what other kind) is often described by entropy (equal energy spread among all available degrees of freedom) considerations, as in practice all energy transformations are permitted on a small scale, but certain larger transformations are not permitted because it is statistically unlikely that energy or matter will randomly move into more concentrated forms or smaller spaces.

The concept of energy is widespread in all sciences.

• In the context of chemistry, energy is an attribute of a substance as a consequence of its atomic, molecular or aggregate structure. Since a chemical transformation is accompanied by a change in one or more of these kinds of structure, it is invariably accompanied by an increase or decrease of energy of the substances involved. Some energy is transferred between the surroundings and the reactants of the reaction in the form of heat or light; thus the products of a reaction may have more or less energy than the reactants. A reaction is said to be exergonic if the final state is lower on the energy scale than the initial state; in the case of endergonic reactions the situation is the reverse. Chemical reactions are invariably not possible unless the reactants surmount an energy barrier known as the activation energy. The speed of a chemical reaction (at given temperature T) is related to the activation energy E, by the Boltzmann's population factor e^−E/kT – that is the probability of molecule to have energy greater than or equal to E at the given temperature T. This exponential dependence of a reaction rate on temperature is known as the Arrhenius equation.The activation energy necessary for a chemical reaction can be in the form of thermal energy.
• In biology, energy is an attribute of all biological systems from the biosphere to the smallest living organism. Within an organism it is responsible for growth and development of a biological cell or an organelle of a biological organism. Energy is thus often said to be stored by cells in the structures of molecules of substances such as carbohydrates (including sugars), lipids, and proteins, which release energy when reacted with oxygen in respiration. In human terms, the human equivalent (H-e) (Human energy conversion) indicates, for a given amount of energy expenditure, the relative quantity of energy needed for human metabolism, assuming an average human energy expenditure of 12,500kJ per day and a basal metabolic rate of 80 watts. For example, if our bodies run (on average) at 80 watts, then a light bulb running at 100 watts is running at 1.25 human equivalents (100 ÷ 80) i.e. 1.25 H-e. For a difficult task of only a few seconds' duration, a person can put out thousands of watts, many times the 746 watts in one official horsepower. For tasks lasting a few minutes, a fit human can generate perhaps 1,000 watts. For an activity that must be sustained for an hour, output drops to around 300; for an activity kept up all day, 150 watts is about the maximum.^[7] The human equivalent assists understanding of energy flows in physical and biological systems by expressing energy units in human terms: it provides a “feel” for the use of a given amount of energy^[8]
• In geology, continental drift, mountain ranges, volcanoes, and earthquakes are phenomena that can be explained in terms of energy transformations in the Earth's interior.,^[9] while meteorological phenomena like wind, rain, hail, snow, lightning, tornadoes and hurricanes, are all a result of energy transformations brought about by solar energy on the atmosphere of the planet Earth.
• In cosmology and astronomy the phenomena of stars, nova, supernova, quasars and gamma ray bursts are the universe's highest-output energy transformations of matter. All stellar phenomena (including solar activity) are driven by various kinds of energy transformations. Energy in such transformations is either from gravitational collapse of matter (usually molecular hydrogen) into various classes of astronomical objects (stars, black holes, etc.), or from nuclear fusion (of lighter elements, primarily hydrogen).

Energy transformations in the universe over time are characterized by various kinds of potential energy that has been available since the Big Bang, later being "released" (transformed to more active types of energy such as kinetic or radiant energy), when a triggering mechanism is available.

Familiar examples of such processes include nuclear decay, in which energy is released that was originally "stored" in heavy isotopes (such as uranium and thorium), by nucleosynthesis, a process ultimately using the gravitational potential energy released from the gravitational collapse of supernovae, to store energy in the creation of these heavy elements before they were incorporated into the solar system and the Earth. This energy is triggered and released in nuclear fission bombs. In a slower process, radioactive decay of these atoms in the core of the Earth releases heat. This thermal energy drives plate tectonics and may lift mountains, via orogenesis. This slow lifting represents a kind of gravitational potential energy storage of the thermal energy, which may be later released to active kinetic energy in landslides, after a triggering event. Earthquakes also release stored elastic potential energy in rocks, a store that has been produced ultimately from the same radioactive heat sources. Thus, according to present understanding, familiar events such as landslides and earthquakes release energy that has been stored as potential energy in the Earth's gravitational field or elastic strain (mechanical potential energy) in rocks. Prior to this, they represent release of energy that has been stored in heavy atoms since the collapse of long-destroyed supernova stars created these atoms.

In another similar chain of transformations beginning at the dawn of the universe, nuclear fusion of hydrogen in the Sun also releases another store of potential energy which was created at the time of the Big Bang. At that time, according to theory, space expanded and the universe cooled too rapidly for hydrogen to completely fuse into heavier elements. This meant that hydrogen represents a store of potential energy that can be released by fusion. Such a fusion process is triggered by heat and pressure generated from gravitational collapse of hydrogen clouds when they produce stars, and some of the fusion energy is then transformed into sunlight. Such sunlight from our Sun may again be stored as gravitational potential energy after it strikes the Earth, as (for example) water evaporates from oceans and is deposited upon mountains (where, after being released at a hydroelectric dam, it can be used to drive turbines or generators to produce electricity). Sunlight also drives many weather phenomena, save those generated by volcanic events. An example of a solar-mediated weather event is a hurricane, which occurs when large unstable areas of warm ocean, heated over months, give up some of their thermal energy suddenly to power a few days of violent air movement. Sunlight is also captured by plants as chemical potential energy in photosynthesis, when carbon dioxide and water (two low-energy compounds) are converted into the high-energy compounds carbohydrates, lipids, and proteins. Plants also release oxygen during photosynthesis, which is utilized by living organisms as an electron acceptor, to release the energy of carbohydrates, lipids, and proteins. Release of the energy stored during photosynthesis as heat or light may be triggered suddenly by a spark, in a forest fire, or it may be made available more slowly for animal or human metabolism, when these molecules are ingested, and catabolism is triggered by enzyme action.

Through all of these transformation chains, potential energy stored at the time of the Big Bang is later released by intermediate events, sometimes being stored in a number of ways over time between releases, as more active energy. In all these events, one kind of energy is converted to other types of energy, including heat.

Distinction between energy and power[link]

Although in everyday usage the terms energy and power are essentially synonyms, scientists and engineers distinguish between them. In its technical sense, power is not at all the same as energy, but is the rate at which energy is converted (or, equivalently, at which work is performed). Thus a hydroelectric plant, by allowing the water above the dam to pass through turbines, converts the water's potential energy into kinetic energy and ultimately into electric energy, whereas the amount of electric energy that is generated per unit of time is the electric power generated. The same amount of energy converted through a shorter period of time is more power over that shorter time.

Conservation of energy[link]

Energy is subject to the law of conservation of energy. According to this law, energy can neither be created (produced) nor destroyed by itself. It can only be transformed.

Most kinds of energy (with gravitational energy being a notable exception)^[10] are subject to strict local conservation laws as well. In this case, energy can only be exchanged between adjacent regions of space, and all observers agree as to the volumetric density of energy in any given space. There is also a global law of conservation of energy, stating that the total energy of the universe cannot change; this is a corollary of the local law, but not vice versa.^[6][11] Conservation of energy is the mathematical consequence of translational symmetry of time (that is, the indistinguishability of time intervals taken at different time)^[12] - see Noether's theorem.

According to Conservation of energy the total inflow of energy into a system must equal the total outflow of energy from the system, plus the change in the energy contained within the system.

This law is a fundamental principle of physics. It follows from the translational symmetry of time, a property of most phenomena below the cosmic scale that makes them independent of their locations on the time coordinate. Put differently, yesterday, today, and tomorrow are physically indistinguishable.

This is because energy is the quantity which is canonical conjugate to time. This mathematical entanglement of energy and time also results in the uncertainty principle - it is impossible to define the exact amount of energy during any definite time interval. The uncertainty principle should not be confused with energy conservation - rather it provides mathematical limits to which energy can in principle be defined and measured.

In quantum mechanics energy is expressed using the Hamiltonian operator. On any time scales, the uncertainty in the energy is by

Failed to parse (Missing texvc executable; please see math/README to configure.): \Delta E \Delta t \ge \frac { \hbar } {2 }

which is similar in form to the Heisenberg uncertainty principle (but not really mathematically equivalent thereto, since H and t are not dynamically conjugate variables, neither in classical nor in quantum mechanics).

In particle physics, this inequality permits a qualitative understanding of virtual particles which carry momentum, exchange by which and with real particles, is responsible for the creation of all known fundamental forces (more accurately known as fundamental interactions). Virtual photons (which are simply lowest quantum mechanical energy state of photons) are also responsible for electrostatic interaction between electric charges (which results in Coulomb law), for spontaneous radiative decay of exited atomic and nuclear states, for the Casimir force, for van der Waals bond forces and some other observable phenomena.

Applications of the concept of energy[link]

Energy is subject to a strict global conservation law; that is, whenever one measures (or calculates) the total energy of a system of particles whose interactions do not depend explicitly on time, it is found that the total energy of the system always remains constant.^[13]

• The total energy of a system can be subdivided and classified in various ways. For example, it is sometimes convenient to distinguish potential energy (which is a function of coordinates only) from kinetic energy (which is a function of coordinate time derivatives only). It may also be convenient to distinguish gravitational energy, electric energy, thermal energy, and other forms. These classifications overlap; for instance, thermal energy usually consists partly of kinetic and partly of potential energy.
• The transfer of energy can take various forms; familiar examples include work, heat flow, and advection, as discussed below.
• The word "energy" is also used outside of physics in many ways, which can lead to ambiguity and inconsistency. The vernacular terminology is not consistent with technical terminology. For example, while energy is always conserved (in the sense that the total energy does not change despite energy transformations), energy can be converted into a form, e.g., thermal energy, that cannot be utilized to perform work. When one talks about "conserving energy by driving less," one talks about conserving fossil fuels and preventing useful energy from being lost as heat. This usage of "conserve" differs from that of the law of conservation of energy.^[11]

In classical physics energy is considered a scalar quantity, the canonical conjugate to time. In special relativity energy is also a scalar (although not a Lorentz scalar but a time component of the energy-momentum 4-vector).^[14] In other words, energy is invariant with respect to rotations of space, but not invariant with respect to rotations of space-time (= boosts).

Energy transfer[link]

Because energy is strictly conserved and is also locally conserved (wherever it can be defined), it is important to remember that by the definition of energy the transfer of energy between the "system" and adjacent regions is work. A familiar example is mechanical work. In simple cases this is written as the following equation:

Failed to parse (Missing texvc executable; please see math/README to configure.): \Delta{}E = W

(1)

if there are no other energy-transfer processes involved. Here Failed to parse (Missing texvc executable; please see math/README to configure.): E

is the amount of energy transferred, and Failed to parse (Missing texvc executable; please see math/README to configure.): W

  represents the work done on the system.

More generally, the energy transfer can be split into two categories:

Failed to parse (Missing texvc executable; please see math/README to configure.): \Delta{}E = W + Q

(2)

where Failed to parse (Missing texvc executable; please see math/README to configure.): Q

represents the heat flow into the system.

There are other ways in which an open system can gain or lose energy. In chemical systems, energy can be added to a system by means of adding substances with different chemical potentials, which potentials are then extracted (both of these process are illustrated by fueling an auto, a system which gains in energy thereby, without addition of either work or heat). Winding a clock would be adding energy to a mechanical system. These terms may be added to the above equation, or they can generally be subsumed into a quantity called "energy addition term Failed to parse (Missing texvc executable; please see math/README to configure.): E " which refers to any type of energy carried over the surface of a control volume or system volume. Examples may be seen above, and many others can be imagined (for example, the kinetic energy of a stream of particles entering a system, or energy from a laser beam adds to system energy, without either being either work-done or heat-added, in the classic senses).

Failed to parse (Missing texvc executable; please see math/README to configure.): \Delta{}E = W + Q + E

(3)

Where E in this general equation represents other additional advected energy terms not covered by work done on a system, or heat added to it.

Energy is also transferred from potential energy (Failed to parse (Missing texvc executable; please see math/README to configure.): E_p ) to kinetic energy (Failed to parse (Missing texvc executable; please see math/README to configure.): E_k ) and then back to potential energy constantly. This is referred to as conservation of energy. In this closed system, energy cannot be created or destroyed; therefore, the initial energy and the final energy will be equal to each other. This can be demonstrated by the following:

Failed to parse (Missing texvc executable; please see math/README to configure.): E_{pi} + E_{ki} = E_{pF} + E_{kF}

(4)

The equation can then be simplified further since Failed to parse (Missing texvc executable; please see math/README to configure.): E_p = mgh

(mass times acceleration due to gravity times the height) and Failed to parse (Missing texvc executable; please see math/README to configure.): E_k = \frac{1}{2} mv^2
(half mass times velocity squared). Then the total amount of energy can be found by adding Failed to parse (Missing texvc executable; please see math/README to configure.): E_p + E_k = E_{total}

.

Energy and the laws of motion[link]

In classical mechanics, energy is a conceptually and mathematically useful property, as it is a conserved quantity. Several formulations of mechanics have been developed using energy as a core concept.

The Hamiltonian[link]

The total energy of a system is sometimes called the Hamiltonian, after William Rowan Hamilton. The classical equations of motion can be written in terms of the Hamiltonian, even for highly complex or abstract systems. These classical equations have remarkably direct analogs in nonrelativistic quantum mechanics.^[15]

The Lagrangian[link]

Another energy-related concept is called the Lagrangian, after Joseph Louis Lagrange. This is even more fundamental than the Hamiltonian, and can be used to derive the equations of motion. It was invented in the context of classical mechanics, but is generally useful in modern physics. The Lagrangian is defined as the kinetic energy minus the potential energy.

Usually, the Lagrange formalism is mathematically more convenient than the Hamiltonian for non-conservative systems (such as systems with friction).

Noether's Theorem[link]

Noether's (first) theorem (1918) states that any differentiable symmetry of the action of a physical system has a corresponding conservation law.

Noether's theorem has become a fundamental tool of modern theoretical physics and the calculus of variations. A generalization of the seminal formulations on constants of motion in Lagrangian and Hamiltonian mechanics (1788 and 1833, respectively), it does not apply to systems that cannot be modeled with a Lagrangian; for example, dissipative systems with continuous symmetries need not have a corresponding conservation law.

Energy and thermodynamics[link]

Internal energy[link]

Internal energy is the sum of all microscopic forms of energy of a system. It is the energy needed to create the system. It is related to the potential energy, e.g., molecular structure, crystal structure, and other geometric aspects, as well as the motion of the particles, in form of kinetic energy. Thermodynamics is chiefly concerned with changes in internal energy and not its absolute value, which is impossible to determine with thermodynamics alone.^[16]

The laws of thermodynamics[link]

According to the second law of thermodynamics, work can be totally converted into heat, but not vice versa. This is a mathematical consequence of statistical mechanics. The first law of thermodynamics simply asserts that energy is conserved,^[17] and that heat is included as a form of energy transfer. A commonly used corollary of the first law is that for a "system" subject only to pressure forces and heat transfer (e.g., a cylinder-full of gas), the differential change in energy of the system (with a gain in energy signified by a positive quantity) is given as the following equation:

Failed to parse (Missing texvc executable; please see math/README to configure.): \mathrm{d}E = T\mathrm{d}S - P\mathrm{d}V\,

,

where the first term on the right is the heat transfer into the system, defined in terms of temperature T and entropy S (in which entropy increases and the change dS is positive when the system is heated), and the last term on the right hand side is identified as "work" done on the system, where pressure is P and volume V (the negative sign results since compression of the system requires work to be done on it and so the volume change, dV, is negative when work is done on the system). Although this equation is the standard textbook example of energy conservation in classical thermodynamics, it is highly specific, ignoring all chemical, electric, nuclear, and gravitational forces, effects such as advection of any form of energy other than heat, and because it contains a term that depends on temperature. The most general statement of the first law (i.e., conservation of energy) is valid even in situations in which temperature is undefinable.

Energy is sometimes expressed as the following equation:

Failed to parse (Missing texvc executable; please see math/README to configure.): \mathrm{d}E=\delta Q+\delta W\,

,

which is unsatisfactory^[11] because there cannot exist any thermodynamic state functions W or Q that are meaningful on the right hand side of this equation, except perhaps in trivial cases.

Equipartition of energy[link]

The energy of a mechanical harmonic oscillator (a mass on a spring) is alternatively kinetic and potential. At two points in the oscillation cycle it is entirely kinetic, and alternatively at two other points it is entirely potential. Over the whole cycle, or over many cycles, net energy is thus equally split between kinetic and potential. This is called equipartition principle; total energy of a system with many degrees of freedom is equally split among all available degrees of freedom.

This principle is vitally important to understanding the behavior of a quantity closely related to energy, called entropy. Entropy is a measure of evenness of a distribution of energy between parts of a system. When an isolated system is given more degrees of freedom (i.e., given new available energy states that are the same as existing states), then total energy spreads over all available degrees equally without distinction between "new" and "old" degrees. This mathematical result is called the second law of thermodynamics.

Oscillators, phonons, and photons[link]

This section may contain original research. Please improve it by verifying the claims made and adding references. Statements consisting only of original research may be removed. More details may be available on the talk page. (August 2009)

In an ensemble (connected collection) of unsynchronized oscillators, the average energy is spread equally between kinetic and potential types.

In a solid, thermal energy (often referred to loosely as heat content) can be accurately described by an ensemble of thermal phonons that act as mechanical oscillators. In this model, thermal energy is equally kinetic and potential.

In an ideal gas, the interaction potential between particles is essentially the delta function which stores no energy: thus, all of the thermal energy is kinetic.

Because an electric oscillator (LC circuit) is analogous to a mechanical oscillator, its energy must be, on average, equally kinetic and potential. It is entirely arbitrary whether the magnetic energy is considered kinetic and whether the electric energy is considered potential, or vice versa. That is, either the inductor is analogous to the mass while the capacitor is analogous to the spring, or vice versa.

1. By extension of the previous line of thought, in free space the electromagnetic field can be considered an ensemble of oscillators, meaning that radiation energy can be considered equally potential and kinetic. This model is useful, for example, when the electromagnetic Lagrangian is of primary interest and is interpreted in terms of potential and kinetic energy.

2. On the other hand, in the key equation Failed to parse (Missing texvc executable; please see math/README to configure.): m^2 c^4 = E^2 - p^2 c^2 , the contribution Failed to parse (Missing texvc executable; please see math/README to configure.): mc^2

is called the rest energy, and all other contributions to the energy are called kinetic energy.  For a particle that has mass, this implies that the kinetic energy is Failed to parse (Missing texvc executable; please see math/README to configure.): 0.5 p^2/m
at speeds much smaller than c, as can be proved by writing Failed to parse (Missing texvc executable; please see math/README to configure.): E = mc^2 

 √Failed to parse (Missing texvc executable; please see math/README to configure.): (1 + p^2 m^{-2}c^{-2})

and expanding the square root to lowest order.  By this line of reasoning, the energy of a photon is entirely kinetic, because the photon is massless and has no rest energy.  This expression is useful, for example, when the energy-versus-momentum relationship is of primary interest.

The two analyses are entirely consistent. The electric and magnetic degrees of freedom in item 1 are transverse to the direction of motion, while the speed in item 2 is along the direction of motion. For non-relativistic particles these two notions of potential versus kinetic energy are numerically equal, so the ambiguity is harmless, but not so for relativistic particles.

Work and virtual work[link]

Work, a form of energy, is force times distance.

Failed to parse (Missing texvc executable; please see math/README to configure.): W = \int_C \mathbf{F} \cdot \mathrm{d} \mathbf{s}

This says that the work (Failed to parse (Missing texvc executable; please see math/README to configure.): W ) is equal to the line integral of the force F along a path C; for details see the mechanical work article.

Work and thus energy is frame dependent. For example, consider a ball being hit by a bat. In the center-of-mass reference frame, the bat does no work on the ball. But, in the reference frame of the person swinging the bat, considerable work is done on the ball.

Quantum mechanics[link]

In quantum mechanics energy is defined in terms of the energy operator as a time derivative of the wave function. The Schrödinger equation equates the energy operator to the full energy of a particle or a system. In results can be considered as a definition of measurement of energy in quantum mechanics. The Schrödinger equation describes the space- and time-dependence of slow changing (non-relativistic) wave function of quantum systems. The solution of this equation for bound system is discrete (a set of permitted states, each characterized by an energy level) which results in the concept of quanta. In the solution of the Schrödinger equation for any oscillator (vibrator) and for electromagnetic waves in a vacuum, the resulting energy states are related to the frequency by the Planck equation Failed to parse (Missing texvc executable; please see math/README to configure.): E = h\nu

(where Failed to parse (Missing texvc executable; please see math/README to configure.): h
is the Planck's constant and Failed to parse (Missing texvc executable; please see math/README to configure.): \nu
the frequency). In the case of electromagnetic wave these energy states are called quanta of light or photons.

Relativity[link]

When calculating kinetic energy (work to accelerate a mass from zero speed to some finite speed) relativistically - using Lorentz transformations instead of Newtonian mechanics, Einstein discovered an unexpected by-product of these calculations to be an energy term which does not vanish at zero speed. He called it rest mass energy - energy which every mass must possess even when being at rest. The amount of energy is directly proportional to the mass of body:

Failed to parse (Missing texvc executable; please see math/README to configure.): E = m c^2

, where

m is the mass,

c is the speed of light in vacuum,

E is the rest mass energy.

For example, consider electron-positron annihilation, in which the rest mass of individual particles is destroyed, but the inertia equivalent of the system of the two particles (its invariant mass) remains (since all energy is associated with mass), and this inertia and invariant mass is carried off by photons which individually are massless, but as a system retain their mass. This is a reversible process - the inverse process is called pair creation - in which the rest mass of particles is created from energy of two (or more) annihilating photons. In this system the matter (electrons and positrons) is destroyed and changed to non-matter energy (the photons). However, the total system mass and energy do not change during this interaction.

In general relativity, the stress-energy tensor serves as the source term for the gravitational field, in rough analogy to the way mass serves as the source term in the non-relativistic Newtonian approximation.^[14]

It is not uncommon to hear that energy is "equivalent" to mass. It would be more accurate to state that every energy has an inertia and gravity equivalent, and because mass is a form of energy, then mass too has inertia and gravity associated with it.

Energy and life[link]

Basic overview of energy and human life.

Any living organism relies on an external source of energy—radiation from the Sun in the case of green plants; chemical energy in some form in the case of animals—to be able to grow and reproduce. The daily 1500–2000 Calories (6–8 MJ) recommended for a human adult are taken as a combination of oxygen and food molecules, the latter mostly carbohydrates and fats, of which glucose (C₆H₁₂O₆) and stearin (C₅₇H₁₁₀O₆) are convenient examples. The food molecules are oxidised to carbon dioxide and water in the mitochondria

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

C₅₇H₁₁₀O₆ + 81.5O₂ → 57CO₂ + 55H₂O

and some of the energy is used to convert ADP into ATP

ADP + HPO₄²⁻ → ATP + H₂O

The rest of the chemical energy in the carbohydrate or fat is converted into heat: the ATP is used as a sort of "energy currency", and some of the chemical energy it contains when split and reacted with water, is used for other metabolism (at each stage of a metabolic pathway, some chemical energy is converted into heat). Only a tiny fraction of the original chemical energy is used for work:^[18]

gain in kinetic energy of a sprinter during a 100 m race: 4 kJ

gain in gravitational potential energy of a 150 kg weight lifted through 2 metres: 3kJ

Daily food intake of a normal adult: 6–8 MJ

It would appear that living organisms are remarkably inefficient (in the physical sense) in their use of the energy they receive (chemical energy or radiation), and it is true that most real machines manage higher efficiencies. In growing organisms the energy that is converted to heat serves a vital purpose, as it allows the organism tissue to be highly ordered with regard to the molecules it is built from. The second law of thermodynamics states that energy (and matter) tends to become more evenly spread out across the universe: to concentrate energy (or matter) in one specific place, it is necessary to spread out a greater amount of energy (as heat) across the remainder of the universe ("the surroundings").^[19] Simpler organisms can achieve higher energy efficiencies than more complex ones, but the complex organisms can occupy ecological niches that are not available to their simpler brethren. The conversion of a portion of the chemical energy to heat at each step in a metabolic pathway is the physical reason behind the pyramid of biomass observed in ecology: to take just the first step in the food chain, of the estimated 124.7 Pg/a of carbon that is fixed by photosynthesis, 64.3 Pg/a (52%) are used for the metabolism of green plants,^[20] i.e. reconverted into carbon dioxide and heat.

Measurement[link]

A schematic diagram of a Calorimeter - An instrument used by physicists to measure energy

Because energy is defined as the ability to do work on objects, there is no absolute measure of energy. Only the transition of a system from one state into another can be defined and thus energy is measured in relative terms. The choice of a baseline or zero point is often arbitrary and can be made in whatever way is most convenient for a problem.

Methods[link]

The methods for the measurement of energy often deploy methods for the measurement of still more fundamental concepts of science, namely mass, distance, radiation, temperature, time, electric charge and electric current.

Conventionally the technique most often employed is calorimetry, a thermodynamic technique that relies on the measurement of temperature using a thermometer or of intensity of radiation using a bolometer.

Units[link]

Throughout the history of science, energy has been expressed in several different units such as ergs and calories. At present, the accepted unit of measurement for energy is the SI unit of energy, the joule. In addition to the joule, other units of energy include the kilowatt hour (kWh) and the British thermal unit (Btu). These are both larger units of energy. One kWh is equivalent to exactly 3.6 million joules, and one Btu is equivalent to about 1055 joules.^[21]

Energy density[link]

Energy density is a term used for the amount of useful energy stored in a given system or region of space per unit volume.

For fuels, the energy per unit volume is sometimes a useful parameter. In a few applications, comparing, for example, the effectiveness of hydrogen fuel to gasoline it turns out that hydrogen has a higher specific energy than does gasoline, but, even in liquid form, a much lower energy density.

Forms of energy[link]

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.

Transformations of energy[link]

One form of energy can often be readily transformed into another with the help of a device- for instance, a battery, from chemical energy to electric energy; a dam: gravitational potential energy to kinetic energy of moving water (and the blades of a turbine) and ultimately to electric energy through an electric generator. Similarly, in the case of a chemical explosion, chemical potential energy is transformed to kinetic energy and thermal energy in a very short time. Yet another example is that of a pendulum. At its highest points the kinetic energy is zero and the gravitational potential energy is at maximum. At its lowest point the kinetic energy is at maximum and is equal to the decrease of potential energy. If one (unrealistically) assumes that there is no friction, the conversion of energy between these processes is perfect, and the pendulum will continue swinging forever.

Energy gives rise to weight when it is trapped in a system with zero momentum, where it can be weighed. It is also equivalent to mass, and this mass is always associated with it. Mass is also equivalent to a certain amount of energy, and likewise always appears associated with it, as described in mass-energy equivalence. The formula E = mc², derived by Albert Einstein (1905) quantifies the relationship between rest-mass and rest-energy within the concept of special relativity. In different theoretical frameworks, similar formulas were derived by J. J. Thomson (1881), Henri Poincaré (1900), Friedrich Hasenöhrl (1904) and others (see Mass-energy equivalence#History for further information).

Matter may be destroyed and converted to energy (and vice versa), but mass cannot ever be destroyed; rather, mass remains a constant for both the matter and the energy, during any process when they are converted into each other. However, since Failed to parse (Missing texvc executable; please see math/README to configure.): c^2

is extremely large relative to ordinary human scales, the conversion of ordinary amount of matter (for example, 1 kg) to other forms of energy (such as heat, light, and other radiation) can liberate tremendous amounts of energy (~Failed to parse (Missing texvc executable; please see math/README to configure.): 9x10^{16}
joules = 21 megatons of TNT), as can be seen in nuclear reactors and nuclear weapons. Conversely, the mass equivalent of a unit of energy is minuscule, which is why a loss of energy (loss of mass) from most systems is difficult to measure by weight, unless the energy loss is very large. Examples of energy transformation into matter (i.e., kinetic energy into particles with rest mass) are found in high-energy nuclear physics.

Transformation of energy into useful work is a core topic of thermodynamics. In nature, transformations of energy can be fundamentally classed into two kinds: those that are thermodynamically reversible, and those that are thermodynamically irreversible. A reversible process in thermodynamics is one in which no energy is dissipated (spread) into empty energy states available in a volume, from which it cannot be recovered into more concentrated forms (fewer quantum states), without degradation of even more energy. A reversible process is one in which this sort of dissipation does not happen. For example, conversion of energy from one type of potential field to another, is reversible, as in the pendulum system described above. In processes where heat is generated, quantum states of lower energy, present as possible excitations in fields between atoms, act as a reservoir for part of the energy, from which it cannot be recovered, in order to be converted with 100% efficiency into other forms of energy. In this case, the energy must partly stay as heat, and cannot be completely recovered as usable energy, except at the price of an increase in some other kind of heat-like increase in disorder in quantum states, in the universe (such as an expansion of matter, or a randomization in a crystal).

As the universe evolves in time, more and more of its energy becomes trapped in irreversible states (i.e., as heat or other kinds of increases in disorder). This has been referred to as the inevitable thermodynamic heat death of the universe. In this heat death the energy of the universe does not change, but the fraction of energy which is available to do work through a heat engine, or be transformed to other usable forms of energy (through the use of generators attached to heat engines), grows less and less.

See also[link]

	Energy portal
	Physics portal

	Book: Energy
Wikipedia books are collections of articles that can be downloaded or ordered in print.

• Index of energy articles
• Index of wave articles

Notes and references[link]

Further reading[link]

External links[link]

Find more about Energy on Wikipedia's sister projects:
	Definitions and translations from Wiktionary
	Images and media from Commons
	Learning resources from Wikiversity
	News stories from Wikinews
	Quotations from Wikiquote
	Source texts from Wikisource
	Textbooks from Wikibooks

• Energy at the Open Directory Project

Keri Hilson
Keri Hilson performing at Supafest in Australia, April 2011
Background information
Birth name	Keri Lynn Hilson
Born	(1982-12-05) December 5, 1982 (age 29) Decatur, Georgia, U.S.
Genres	R&B, pop
Occupations	Singer, songwriter, dancer, actress
Years active	2001–present
Labels	Zone 4, Mosley Music, Interscope
Associated acts	The Clutch, Polow da Don, Timbaland, Chris Brown, Akon, Kanye West
Website	kerihilson.com

Keri Lynn Hilson (born December 5, 1982) is an American singer, songwriter and actress. She was born and raised in Decatur, Georgia, and spent most of her youth working with producer Anthony Dent, as a songwriter and background vocalist for several R&B and hip hop artists. By the age of 14, Hilson had secured a record deal with the girl group D'Signe, who later disbanded. She attended Emory University in Atlanta, while she continued writing songs for artists, including Britney Spears and Mary J. Blige, with the production and songwriting team, The Clutch. In 2006, Hilson signed with American rapper and producer Timbaland's record label, Mosley Music. Her breakthrough came the following year after appearing on Timbaland's single, "The Way I Are", which topped charts around the world.

Hilson's debut studio album, In a Perfect World..., was released in 2009, and debuted at number four on the US Billboard 200 chart, eventually being certified gold by the Recording Industry Association of America (RIAA). The album spawned the hit singles, "Knock You Down", "Turnin Me On" and "I Like". It also earned Hilson two Grammy Award nominations for Best New Artist and Best Rap/Sung Collaboration for "Knock You Down". Her second studio album, No Boys Allowed, was released in 2010 and was considerably less successful than its predecessor; however, it included the hit single, "Pretty Girl Rock". Hilson's achievements include a BET Award, MOBO Award, NAACP Image Award and two Soul Train Music Awards. Apart from her work in music, she has pursued a career in acting. Hilson made her acting debut in the romantic comedy film, Think Like a Man, in 2012.

Biography[link]

1982–2007: Early life and career beginnings[link]

Keri Lynn Hilson was born on December 5, 1982 in Decatur, Georgia.^[1][2] She was raised in an African-American neighborhood and came from a middle class family.^[3] Hilson and her siblings did not attend a school within the neighborhood because her parents cared a lot for their education, stating that "they shipped us off for an hour to get to school every day. It was just that important for us to maintain the life that they had created".^[3] From the age of 12, Hilson wanted to pursue a musical career, after watching television talent shows, Star Search and Showtime at the Apollo.^[1] Her mother then hired a piano teacher to give her piano lessons; however, Hilson wanted to become a singer and therefore "converted those sessions into vocal lessons, accompanying the teacher on piano".^[1] By the age of 14, Hilson had secured a record deal with the girl group D'Signe, who later disbanded.^[4][1]

Hilson spent most of her teenage years working with producer Anthony Dent, as a songwriter and background vocalist for the likes of Usher, Ludacris, Kelly Rowland, Ciara and Polow da Don.^[1] After graduating from high school, Hilson attended Emory University in Atlanta for three years and studied a course in theater.^[3] She continued working as a songwriter and background vocalist during her schooling years.^[1] Hilson eventually stopped working with Dent, and began working more with Polow da Don.^[1] She also became a member of the songwriting and production team, The Clutch.^[1] Together, they wrote Mary J. Blige's "Take Me as I Am" (2006), Omarion's "Ice Box" (2006), Ciara's "Like a Boy" (2006), The Pussycat Dolls' "Wait a Minute" (2007) and Britney Spears' "Gimme More" (2007).^[1][4] In 2006, Polow introduced Hilson to American rapper and producer Timbaland, who then signed her to his record label, Mosley Music Group.^[1] In November 2006, Hilson was featured on American rapper Lloyd Banks' single, "Help", for his album Rotten Apple (2006).^[5]

The following year, Hilson was featured on three singles; in June, on Timbaland's song "The Way I Are", the second single from his album Shock Value (2007); on Rich Boy's "Good Things" with Polow da Don; and thereafter, in December, on Timbaland's song "Scream" with Nicole Scherzinger, the fifth single from Shock Value.^[6] Out of the three singles, "The Way I Are" achieved the most success, topping nine charts around the world.^{[7][8][9][10][11]}

[edit] 2008–09: In a Perfect World...

Hilson in April 2009

In 2008, Hilson appeared in the music videos for Usher's single, "Love in This Club", and Ne-Yo's single, "Miss Independent".^[12][13] Her debut studio album, In a Perfect World..., was released on March 24, 2009.^[14] The album contained pop-oriented R&B songs.^[15] It was initially planned to be released the previous year; however, the album was delayed several times because Hilson wanted to make sure it came out to her liking, stating "I'm just a perfectionist. I have had many [release dates], but it's all for a good cause. The label wants to make sure the album has its proper release, and I'm thankful for that. Very grateful, even though fans look at it like it's a bad thing."^[16] In the US, In a Perfect World... debuted at number four on the Billboard 200 chart, and number one on the Top R&B/Hip-Hop Albums chart, with first-week sales of 94,000 copies.^[17][18] By October 2009, the album was certified gold by the Recording Industry Association of America (RIAA).^[19] Its first single, "Energy", released in May 2008, peaked within the top eighty on the US Billboard Hot 100 chart.^[20] It reached the top fifty in the United Kingdom, and peaked the highest in New Zealand at number seven, and was certified gold in the country.^[10][21][22] Hilson also featured on three singles in 2008; in June, on rapper Nas' song "Hero"; in September, on Kardinal Offishall's "Numba 1 (Tide Is High)"; and thereafter, in October, on Chris Brown's song "Superhuman".^[23][6][24]

"Return the Favor", featuring Timbaland, served as the second international single from Hilson's In a Perfect World... album, while "Turnin Me On" featuring Lil Wayne, was released as the second US single. The latter achieved commercial success, reaching number 15 on the Billboard Hot 100, and number two on the Hot R&B/Hip-Hop Songs chart, eventually being certified platinum in the US.^[20][19] The next single, "Knock You Down" featuring Kanye West and Ne-Yo, achieved more success, reaching number three on the Hot 100 and number one on the Hot R&B/Hip-Hop Songs chart, eventually being certified two times platinum.^[20][19] The song also reached the top ten in Canada, Ireland, The Netherlands, New Zealand and the United Kingdom.^{[20][25][21][10]} "Make Love", "Slow Dance", and "Change Me" featuring Akon, were released as the album's fourth, fifth and sixth US singles, respectively.

Hilson, along with Gym Class Heroes, Gorilla Zoe and T-Pain, were supporting acts on Lil Wayne's I Am Music Tour in North America.^[26] At the 2009 BET Awards, she was nominated in four categories, including Best New Artist, Best Female R&B Artist, as well as Viewer's Choice and Best Collaboration for "Turnin Me On" with Lil Wayne.^[27] She eventually won the Best New Artist category.^[28] In 2009, Hilson was featured on five singles; "Number One" with R. Kelly, "She Don't Wanna" with Asher Roth, "Everything, Everyday, Everywhere" with Fabolous, "Medicine" with Plies, and on the remix of Sean Paul's song, "Hold My Hand".^[29][6] Hilson was nominated for Breakthrough Artist and Favorite Soul/R&B Female Artist at the 2009 American Music Awards, but lost to Beyoncé Knowles.^[30] The re-released edition of In a Perfect World..., in January 2010, included the single "I Like", which reached number one in Germany, Poland and Slovakia, and peaked within the top ten in Austria, Norway and Switzerland.^{[31][32][33][34]} At the 52nd Grammy Awards, Hilson was nominated for Best New Artist and Best Rap/Sung Collaboration for "Knock You Down" with Kanye West and Ne-Yo.^[35]

[edit] 2010–present: No Boys Allowed

Hilson performing at the 2010 VH1 Divas Salute the Troops concert

During the first half of 2010, Hilson guest featured on rapper Trina's single, "Million Dollar Girl", and on T.I.'s single, "Got Your Back".^[6] In April 2010, it was made known that she had replaced Jennifer Hudson as the new face of the cosmetics and personal care products company, Avon.^[36] "Breaking Point" was released as the first single from Hilson's second studio album, No Boys Allowed, in September 2010; it only appeared on the US Hot R&B/Hip-Hop Songs Chart at number 44.^[20] On December 2, 2010, Hilson was among one of the many female artists who performed at the VH1 Divas Salute the Troops concert, where she sang "Turnin Me On", "Knock You Down", "Pretty Girl Rock", and a duet with American country music duo, Sugarland, on the Aretha Franklin song, "Think".^[37] No Boys Allowed was released on December 21, 2010.^[38] Described as a "girl power album", No Boys Allowed was primarily of the R&B and pop genres.^[39][40] Despite the album's title, Hilson stated "it's not about excluding men. It's more about women understanding that there comes a time in your life when you want a man. A real man. A grown up. Not a boy. And that's not a bad thing."^[41] Speaking of the songs on the album, she explained "I write from a female perspective, but I'm also telling men what women are really thinking and feeling about them".^[41] In the US, No Boys Allowed debuted at number 11 on the Billboard 200, and number seven on the Top R&B/Hip-Hop Albums chart, with first-week sales of 102,000 copies.^[42][43] Although it sold 8,000 copies more than her debut album, In a Perfect World..., it failed to match that album's debut chart position of number four.^[42] As of February 2011, No Boys Allowed has sold 205,500 copies in the United States.^[44]

Hilson achieved success with the album's second single, "Pretty Girl Rock", which reached number 24 on the Billboard Hot 100 and number four on the Hot R&B/Hip-Hop Songs Chart, eventually being certified platinum.^[20][19] The single reached the top twenty in Germany and New Zealand, and top thirty in Austria and Slovakia.^{[45][21][46][47]} "One Night Stand" featuring Chris Brown, and "Lose Control (Let Me Down)" featuring Nelly, were released as the album's third and fourth singles, respectively. In 2011, she was featured on British rapper Chipmunk's single "In the Air", for his album Transition (2011).^[48] In April 2011, Hilson, along with many other R&B and hip hop acts, traveled to Australia to be part of its biggest urban music festival, Supafest.^[49] At the 2011 BET Awards, Hilson was nominated for Best Female R&B Artist and Video of the Year for "Pretty Girl Rock".^[50] In July 2011, she was a supporting act on the second leg of Lil Wayne's I Am Still Music Tour in North America, before embarking on her first headlining European tour in October.^[51][52]

2012–present: Acting debut and third studio album[link]

During an interview with 106 & Park on October 25, 2011, Hilson revealed that she has already begun recording her third studio album.^[53] She explained, "There's some people in the world that aren't gon' like this album coming from me. As I was experiencing the world, there were things that I was also experiencing on a personal front, and in my music it's coming out. There's a specific few people who ain't gon' like to listen to this album. Very emotional, I'll say that."^[53] She further explained in an interview with The Boombox that she's working on a new sound for the album, and described it as a mixture of her first two albums with some surprising elements thrown in.^[54] Hilson also said a release date has not been confirmed yet, saying "I just write about my experiences and keep the release dates far, far from me. When I feel that I have [good material] is when I will give it to the label. But I'm not quite there yet. I'm still working."^[54] Hilson made her acting debut in the romantic comedy film, Think Like a Man, released on April 20, 2012.^[55][56] She will also star alongside Vin Diesel in the sci-fi action film, Riddick (2013), but it is unknown what role Hilson will play.^[57]

Artistry[link]

Musical style and influences[link]

Hilson's music is generally R&B and pop. Her debut album, In a Perfect World..., consists of a pop/R&B style, with elements of electro music.^[15][58] Many of the album's themes deal with relationships, physical attraction and lovemaking.^[59] Hilson said, "lyrically it's a very vulnerable album. You know, I definitely didn't want to paint myself as perfect on this project. Instead, I wanted it to be something that was very relatable, especially to women".^[60] Her second album, No Boys Allowed, displays a wide variety of styles, including pop, R&B, hip hop, soul, acoustic, electronic and reggae.^{[40][61][62][63]} Hilson describes the album as "more self assured. It's a lot more aggressive".^[64] Kristin Macfarlane of The Daily Post noted that the album is "pure girl power and about being sexually confident, and confident as a woman; not putting up with crap relationships and getting your way".^[65] Hilson sings with "smooth vocals".^[66] Following the release of her debut album, Sophie Bruce of BBC Music expressed that, "there's no denying Hilson has a great voice, but it lacks the feisty edge of [Nicole] Scherzinger, [Mary J.] Blige or [Beyoncé] Knowles".^[67] Mark Nero of About.com commented, "Keri's voice, while strong, isn't particularly distinctive or memorable".^[68] Hilson has stated that her musical influences come from her father's side of the family.^[60] In addition, she cites Michael Jackson and Lauryn Hill as inspirations.^[69][70]

Philanthropy[link]

Besides her career in music, Hilson has also been involved with several charities. In January 2010, she teamed up with Akon on the charity single, "Oh Africa".^[71] All proceeds of the single benefited underprivileged African youth.^[71] The following month, Hilson joined the extended list of artists during the recording session of the single, "We Are the World 25 for Haiti", to benefit victims of the 2010 Haiti earthquake.^[72] On May 25, 2010, she performed at the Virginia Stand Up! A Call to Action benefit concert, organized by Chris Brown to help with continued relief efforts in Haiti.^[73] On October 21, 2010, Hilson attended Collins Academy High School in Chicago as part of the Get Schooled National Challenge and Tour, a "program aimed at increasing high school and college graduation rates and promoting the importance of education".^[74][75] A year later, she received a Get Schooled Award for her involvement with the program.^[76] In 2011, Hilson contributed in the fight against AIDS, by posing in retail-clothing company H&M's celebrity-driven collection for Fashion Against AIDS.^[77] 25% of all sales from the collection were donated to the Designers Against Aids charity.^[77] Hilson contributed to the It Gets Better Project, a project which aims to prevent suicide among LGBT youth.^[78][79]

In June 2011, Hilson became an ambassador for MTV Staying Alive, a foundation which raises awareness and prevention of HIV and AIDS.^[80] In September 2011, she lent her support to US First Lady Michelle Obama's Let's Move! campaign, which aims to combat childhood obesity.^[81] She made t-shirts for the campaign that read "Pretty Girls Sweat", and exercised with the young women at The Educational Alliance's Sirovich Senior Center in New York on September 24.^[81] During Hilson's visit in Africa in November 2011, she stopped by an orphanage to visit her young fans who had lost their families during the 2010–2011 Ivorian crisis.^[82] She gave each orphan a "care" package, which consisted of bookbags, t-shirts, sunglasses, notebooks and an electric keyboard.^[82] In April 2012, Hilson teamed up with DoSomething.org to encourage young people to join in the nationwide Epic Book Drive by bringing books to local people in need.^[83] In a press release, she stated: "Joining DoSomething.org's newest effort to provide for people in need with Epic Book Drive was the easiest decision ever!. I truly believe providing access to those who want to learn is the key to bettering lives around the country. And helping kids find healthy outlets, such as books, has always been a passion of mine. So my involvement here is a natural fit."^[83]

Discography[link]

• In a Perfect World... (2009)
• No Boys Allowed (2010)

Filmography[link]

Tours[link]

Headlining

• European Tour (2011)

Supporting

• I Am Music Tour (2009) (with Lil Wayne)
• I Am Still Music Tour (2011) (with Lil Wayne)

Awards and nominations[link]

References[link]

External links[link]

	Book: Keri Hilson
Wikipedia books are collections of articles that can be downloaded or ordered in print.

Media related to Keri Hilson at Wikimedia Commons

• Official website
• Keri Hilson at the Internet Movie Database