Energy is a fundamental quantity that every physical system possesses; it allows us to predict how much work the system could be made to do, or how much heat it can exchange. In the past, energy was discussed in terms of easily observable effects it has on the properties of objects or changes in state of various systems. Basically, if something changes, some sort of energy was involved in that change. As it was realized that energy could be stored in objects, the concept of energy came to embrace the idea of the potential for change as well as change itself. Such effects (both potential and realized) come in many different forms; examples are the electrical energy stored in a battery, the chemical energy stored in a piece of food, the thermal energy of a hot water heater, or the kinetic energy of a moving train. To simply say energy is "change or the potential for change", however, misses many important examples of energy as it exists in the physical world.
Energy can be used not only to produce observable change, it also is used to prevent change in which case unaided observation of this kind of energy can be difficult. For example, looking at a statue holding a 50 pound weight, the presence of energy needed to do so may not be observable. However, if you are holding up the fifty pound weight instead of the statue the need for energy to accomplish this becomes apparent. You can feel the gravitational force on you both when you are moving the weight up and when you are not moving it.
Energy can be readily transformed from one form into another; for instance, using a battery to power an electrical heater converts electrical energy into thermal energy. In the previous example of holding the fifty pound weight, the work you perform to raise the weight is observed as kinetic energy of motion which is converted to potential energy. Letting go of the weight once again transforms this stored potential energy back into kinetic energy as the weight falls under the force of gravity. The law of conservation of energy states that the total amount of energy, corresponding to the sum of a system's constituent energy components, remains constant. Scientists have also defined several forms of energy that are not easily measured by the unaided observer.
Units
SI and related units
The SI unit for both energy and work is the joule (J), named in honour of James Prescott Joule and his experiments on the mechanical equivalent of heat . In slightly more fundamental terms, 1 joule is equal to 1 newtonmetre and, in terms of SI base units, 1 J is equal to 1 kg m^{2} s^{−2}.
An energy unit that is used in particle physics is the electronvolt (eV). One eV is equivalent to 1.602176462×10^{−19} J.
(Note that torque, which is typically expressed in newtonmetres, has the same dimension and this is not a simple coincidence: a torque of 1 newtonmetre applied on 1 radian requires exactly 1 newtonmetre=joule of energy.)
Other units of energy
In cgs units, one erg is 1 g cm^{2} s^{−2}, equal to 1.0×10^{−7} J. Another obsolete metric unit is the litreatmosphere (101.325 J).
The imperial/US units for both energy and work include the footpound (1.3558 J), the British thermal unit (Btu) which has various values in the range of 1055 J, and the horsepowerhour (2.6845 MJ).
The energy unit used for everyday electricity, particularly for utility bills, is the kilowatthour (kW h), and one kW h is equivalent to 3.6×10^{6} J (3600 kJ or 3.6 MJ; the metric units usually are selfconsistent, and this particular one may seem arbitrary; it's not, the metric measurement for time is the second, and there are 3,600 seconds in an hour  in other words, 1 kW second = 1 J, but the kW h is a more convenient unit for everyday use).
The calorie is mainly used in nutrition and equals the amount of heat necessary to raise the temperature of one gram of water by 1 degree Celsius, at a pressure of 1 atm. This amount of heat depends somewhat on the initial temperature of the water, which results in various different units sharing the name of "calorie" but having slightly different energy values. It is approximately equal to 4.186 J.
The calories used for food energy in nutrition are the large calories based on the kilogram rather than the gram, often identified as food calories. These are sometimes called kilocalories with that calorie being the small calorie based on the gram, and as a result the prefixes are generally avoided for the large calories (i.e., 1 kcal is 4.184 kJ, never 4.184 MJ, even if "calories" are also used for the other, larger unit in the same document or the same nutrition label). Food calories are sometimes noted as Calories (1000 calories) or simply abbreviated Cal with the capital C, but that convention is more often found in chemistry or physics textbooks—which do not use these large calories—than it is in realworld applications by those who do use these calories. (This convention is also, of course, useless when the word calorie appears in a location where it would ordinarily be capitalized, as at the beginning of a sentence or in the first column of a nutrition label as a substitute for the quantity being measured, which is energy, when all the other quantities such as "Iron" and "Sugars" are also capitalized.)
Transfer of energy
Work
Main article: mechanical work.
Work is a measure of energy expended in applying force over a distance. Performing work requires energy, and thus the amount of energy in a system limits the maximum amount of work that a system could conceivably perform.
The equation above says that the energy used in the process of performing work (E) is equal to the integral of the dot product of the force () on a body and the infinitesimal of the body's position ().
In most simple physics models, this is assumed to be the same quantity as the work that is actually performed on the body in question. In reality, however, not all energy given by the above equation is transferred into a recoverable form: for example, energy may be converted into heat which cannot then be converted into another useful form of energy. Thus, in practice, the amount of energy in a system available for performing work may be much less than the total amount of energy in the system.
Heat
Main article: Heat.
Heat is an amount of energy which is usually linked with a change in temperature or in a change in phase of matter. In chemistry, heat is the amount of energy which is absorbed or released by a given chemical reaction. The relationship between heat and energy is similar to that between work and energy. Heat flows from areas of high temperature to areas of low temperature. All objects (matter) have a certain amount of internal energy that is related to the random motion of their atoms or molecules. This internal energy is directly proportional to the temperature of the object. When two bodies of different temperature come in to thermal contact, they will exchange internal energy until the temperature is equalised. The amount of energy transferred is the amount of heat exchanged. It is a common misconception to confuse heat with internal energy, but there is a difference: the change of the internal energy is the heat that flows from the surroundings into the system plus the work performed by the surroundings on the system.
Conservation of energy
The first law of thermodynamics says that 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 used in all branches of physics. Noether's theorem relates the conservation of energy to the time invariance of physical laws.
Kinetic energy
Main article: Kinetic energy.
Kinetic energy is the portion of energy related to the motion of a body.
The equation above says that the kinetic energy (E_{k}) is equal to the integral of the dot product of the velocity () of a body and the infinitesimal of the body's momentum ().
For nonrelativistic velocities, that is velocities much smaller than the speed of light, we can use the Newtonian approximation
where
E_{k} is kinetic energy
m is mass of the body
v is velocity of the body
At nearlight velocities, we use the relativistic formula:
where
v is the velocity of the body
m is its rest mass
c is the speed of light in a vacuum, which is approximately 300,000 kilometers per second
is the total energy of the body
is again the rest mass energy.
In the form of a Taylor series, the relativistic formula for can be written as:
Hence, the second and higher terms in the series correspond with the "inaccuracy" of the Newtonian approximation for kinetic energy in relation to the relativistic formula.
Potential energy
Main article: Potential energy.
While kinetic energy is the portion of a system's energy related to motion, potential energy is the energy of a system associated with the spatial configuration of the system's components and their interaction(s) with each other.
In an isolated system consisting of two stationary objects lying on the xaxis that exert a force f(x) on each other, the potential energy is most generally defined as
where the force between the objects varies only with distance x and is integrated along the line connecting the two objects.
To further illustrate the relationship between force and potential energy, consider the same system of two objects situated along the xaxis. If the potential energy due to one of the objects at any point x is U(x), then the force on the that object x is
This relationship demonstrates that the force between the objects is in the direction of decreasing potential energy, and the magnitude of the force is proportional to the extent to which potential energy decreases. A large force is associated with a large decrease in potential energy, while a small force is associated with a small decrease in potential energy. Notice how the force on an object depends entirely on its potential energy.
These two relationships – the definition of potential energy based on force, and the dependence of force on potential energy – show how the concepts of force and potential energy are intimately linked: if two objects do not exert forces on each other, there is no potential energy between them. If two objects do exert forces on each other, then potential energy naturally arises in the system as part of the system's total energy. Since potential energy arises from forces, any change in the system's spatial configuration will either increase or decrease the system's potential energy as the objects are repositioned.
When a system moves to a lower potential energy state, energy is either released in some form or converted into another form of energy, such as kinetic energy. The potential energy can be "stored" as gravitational energy, elastic energy, chemical energy, rest mass energy or electrical energy, but arises in all cases from the spatial positioning and interaction of objects within a system. Unlike kinetic energy, which exists in any moving body, potential energy exists in any body which is interacting with another object.
For example a mass released above the Earth initially has potential energy resulting from the gravitational attraction of the Earth, which is transferred to kinetic energy as the gravitational force acts on the object and its potential energy is decreased as it falls.
Equation:
where m is the mass, h is the height and g is the value of acceleration due to gravity at the Earth's surface (see gee).
Internal energy
Main article: Internal energy.
Internal energy is the kinetic energy associated with the motion of molecules, and the potential energy associated with the rotational, vibrational and electric energy of atoms within molecules. Internal energy, like energy, is a quantifiable state function of a system.
Energy as a function of the state
The energy is a characteristic of the state of the system. If the system is moved to a different configuration and then put back to the previous configuration the system will have the same energy as it was previously. For this to be true all the forces (or fields) should be conservative. In the case there are nonconservative forces, the socalled principle of conservation of energy loses its importance. Usually it is needed to take in consideration some energy channel that was previously neglected (like friction) to know the reason of otherwise unexplainable loss of energy. In practice, available energy is never perfectly conserved when a system changes state; otherwise, the creation of perpetual motion machines would be possible.
Examples
An example of the conversion and conservation of energy is a pendulum. At its highest points the kinetic energy is zero and the potential gravitational energy is at its maximum. At its lowest point the kinetic energy is at its maximum and is equal to the decrease of potential energy. If one unrealistically assumes that there is no friction, the energy will be conserved and the pendulum will continue swinging forever.
Another example is a chemical explosion in which potential chemical energy is converted to kinetic energy and heat in a very short time.
Further on energy (natural sciences)
Energy use by humans
Major topics
Other articles
Energy agencies
External links
Further reading

Feynman, Richard. Six Easy Pieces: Essentials of Physics Explained by Its Most Brilliant Teacher. Helix Book. See the chapter "conservation of energy" for Feynman's explanation of what energy is and how to think about it.
References
Apart from its usage in physics the concept of energy is also widely used in the many movements and beliefs that comprise New Age but in contrast to physics its operationalization is not practical and reliable, or is even left completely undefined.
Last updated: 10242005 16:29:33