User:Anthony Argyriou/sandbox: Difference between revisions

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In science, '''energy''' is a measurable physical quantity of a system which can be expressed in [[joule]]s (the [[SI unit]] for a quantity  of energy) or other measurement units such as [[erg]]s, [[calorie]]s, [[watt-hour]]s or [[Btu]].
'''Energy''' is a scalar property of physical systems, measured in units with dimensions M&sdot;L<sup>2</sup>&sdot;T<sup>-2</sup>.  Energy occurs in several forms; these are [[potential energy]], [[kinetic energy]], [[electromagnetic radiation]], and, according to Einstein's [[special relativity]], [[mass-energy]].  Some forms of kinetic energy (such as [[thermal energy]]) are often treated separately, because of the historic circumstances of their discovery, and for convenience. One fundamental law of physics is that [[Law of conservation of energy|energy is conserved]] - it can change in form, but the total energy of a closed system cannot change, and the energy coming into an open system must equal the energy leaving the system.


Energy is commonly defined as the amount of work that a system is capable of performing. However, the [[Laws of thermodynamics|second law of thermodynamics]] states that some kinds of energy cannot be converted into work. Hence, this definition is not completely general and should be used with care.
The dimensions of energy correspond to the application of a force over a distance. '''Potential energy''' represents the potential for this to occur, as in the potential of an object to fall in a gravitational field, where the potential is for the force of gravity to be applied to the object over the distance which in can fall. '''Kinetic energy''' is the energy of motion, and is equal to ½mv², where ''m'' is the mass of the object, and ''v'' is the velocity. [[Special relativity]] shows that mass can be converted to energy, and that the energy of mass at rest is equal to mc², where ''c'' is the [[speed of light]]. Special relativity also shows that kinetic energy increases with velocity faster than predicted by Newtonian physics; that additional energy increment is considered to increase the mass of the moving object, as described by the [[Lorentz equations]].


An additional difficulty in defining energy is that energy has many  seemingly very different forms such as the [[kinetic energy]] of a moving cannon ball, the [[potential energy]] of water stored in an elevated tank, the [[chemical energy]] stored in [[gasoline]] or other [[fuel]]s, the [[heat]] stored in [[steam]], the [[electrical energy]] in a [[battery]], the [[nuclear fusion energy]] in a [[hydrogen]] bomb, etc. While each form of energy may be transformed into another, the total amount of energy is conserved (remains constant). In other words, energy cannot be created or destroyed. This conservation of energy is expressed by the [[Laws of thermodynamics|first law of thermodynamics]] that pervades all of science and is probably science's most important principle.
[[Electromagnetic radiation]] is also a form of energy ... (There needs to be more here, but I'm running out of steam to write this section.)


The concept of energy is best explained by means of examples. To that end, assume that we use a gasoline-fueled [[engine]] driving a pump to pump water up to an elevated reservoir, and when the reservoir is filled, we let the water flow down to drive an [[electrical generator]]. In doing this, we convert the chemical energy of the gasoline to (a) the mechanical energy of the pump to (b) the potential energy of the water in the reservoir to (c) the kinetic energy of the falling water, and finally to (d) the electric energy generated by the generator. If we use the generated electric current for lighting, then the light bulbs convert the electric current to yet another form of energy, namely (e) light ([[electromagnetic radiation]]). During these energy conversion processes, the law of conservation of energy assures us that no energy is lost. To non-scientists the contrary may seem the case sometimes, because heat is generated (especially in burning the gasoline to drive the pump), and the heat will escape to the environment without any useful, or directly noticeable, effect. However, since heat is also a form of energy, it must be included in the energy balance required by the first law of thermodynamics.
The [[SI]] unit of energy is the [[joule (unit)]].

Revision as of 20:13, 25 July 2008

Energy is a scalar property of physical systems, measured in units with dimensions M⋅L2⋅T-2. Energy occurs in several forms; these are potential energy, kinetic energy, electromagnetic radiation, and, according to Einstein's special relativity, mass-energy. Some forms of kinetic energy (such as thermal energy) are often treated separately, because of the historic circumstances of their discovery, and for convenience. One fundamental law of physics is that energy is conserved - it can change in form, but the total energy of a closed system cannot change, and the energy coming into an open system must equal the energy leaving the system.

The dimensions of energy correspond to the application of a force over a distance. Potential energy represents the potential for this to occur, as in the potential of an object to fall in a gravitational field, where the potential is for the force of gravity to be applied to the object over the distance which in can fall. Kinetic energy is the energy of motion, and is equal to ½mv², where m is the mass of the object, and v is the velocity. Special relativity shows that mass can be converted to energy, and that the energy of mass at rest is equal to mc², where c is the speed of light. Special relativity also shows that kinetic energy increases with velocity faster than predicted by Newtonian physics; that additional energy increment is considered to increase the mass of the moving object, as described by the Lorentz equations.

Electromagnetic radiation is also a form of energy ... (There needs to be more here, but I'm running out of steam to write this section.)

The SI unit of energy is the joule (unit).

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