Editing Energy (section)

== Transformation ==
{{Main|Energy transformation}}
{| class="wikitable" style="text-align:center;"
|+Some forms of [[Energy transfer|transfer]] of energy ("energy in transit") from one object or system to another
! Type of transfer [[thermodynamic process|process]]!! Description
|-
|[[Heat]]
|equal amount of [[Thermal energy#Differentiation from heat|thermal energy]] in transit spontaneously towards a lower-[[temperature]] object
|-
|[[Work (physics)|Work]]
|equal amount of energy in transit due to a displacement in the direction of an applied [[force]]
|-
|Transfer of material
|equal amount of energy carried by [[matter]] that is moving from one system to another
|-
|}

[[File:Turbogenerator01.jpg|thumb|A [[turbo generator]] transforms the energy of pressurized steam into electrical energy.]]
Energy may be [[energy transformation|transformed]] between different forms at various [[energy conversion efficiency|efficiencies]]. Items that transform between these forms are called [[transducer]]s. Examples of transducers include a [[Battery (electric)|battery]] (from [[chemical energy]] to [[electric energy]]), a dam (from [[gravitational potential energy]] to [[kinetic energy]] of moving water (and the blades of a [[turbine]]) and ultimately to [[electric energy]] through an [[electric generator]]), and a [[heat engine]] (from heat to work).

Examples of energy transformation include generating [[electric energy]] from heat energy via a steam turbine, or lifting an object against gravity using electrical energy driving a crane motor. Lifting against gravity performs mechanical work on the object and stores gravitational potential energy in the object. If the object falls to the ground, gravity does mechanical work on the object which transforms the potential energy in the gravitational field to the kinetic energy released as heat on impact with the ground. The Sun transforms [[nuclear potential energy]] to other forms of energy; its total mass does not decrease due to that itself (since it still contains the same total energy even in different forms) but its mass does decrease when the energy escapes out to its surroundings, largely as [[radiant energy]].

There are strict limits to how efficiently heat can be converted into [[Work (physics)|work]] in a cyclic process, e.g. in a heat engine, as described by [[Carnot's theorem (thermodynamics)|Carnot's theorem]] and the [[second law of thermodynamics]]. However, some energy transformations can be quite efficient. The direction of transformations in energy (what kind of energy is transformed to what other kind) is often determined by [[entropy]] (equal energy spread among all available [[degrees of freedom (physics and chemistry)|degrees of freedom]]) considerations. 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.

Energy transformations in the universe over time are characterized by various kinds of potential energy, that has been available since the [[Big Bang]], 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 [[nucleosynthesis]], a process ultimately using the gravitational potential energy released from the [[gravitational collapse]] of [[supernova]]e to "store" energy in the creation of heavy isotopes (such as [[uranium]] and [[thorium]]), and [[nuclear decay]], a process in which energy is released that was originally stored in these heavy elements, before they were incorporated into the Solar System and the Earth. This energy is triggered and released in nuclear [[fission bomb]]s or in civil nuclear power generation. Similarly, in the case of a [[Chemical explosive|chemical explosion]], [[chemical potential]] energy is transformed to [[kinetic energy|kinetic]] 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 its maximum. At its lowest point the [[kinetic energy]] is at its maximum and is equal to the decrease in [[potential energy]]. If one (unrealistically) assumes that there is no [[friction]] or other losses, the conversion of energy between these processes would be perfect, and the [[pendulum]] would continue swinging forever.

Energy is also transferred from potential energy (<math>E_p</math>) to kinetic energy (<math>E_k</math>) and then back to potential energy constantly. This is referred to as conservation of energy. In this [[isolated 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:
{{NumBlk||<math display="block">E_{pi} + E_{ki} = E_{pF} + E_{kF}</math>|{{EquationRef|4}}}}

The equation can then be simplified further since <math>E_p = mgh</math> (mass times acceleration due to gravity times the height) and <math display="inline">E_k = \frac{1}{2} mv^2</math> (half&nbsp;mass times velocity squared). Then the total amount of energy can be found by adding <math>E_p + E_k = E_\text{total}</math>.

=== Conservation of energy and mass in transformation ===
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''&nbsp;=&nbsp;''mc''<sup>2</sup>, derived by [[Albert Einstein]] (1905) quantifies the relationship between [[relativistic mass]] and 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).

Part of the rest energy (equivalent to rest mass) of [[matter]] may be converted to other forms of energy (still exhibiting mass), but neither energy nor mass can be destroyed; rather, both remain constant during any process. However, since <math>c^2</math> is extremely large relative to ordinary human scales, the conversion of an everyday amount of rest mass (for example, 1&nbsp;kg) from rest energy to other forms of energy (such as kinetic energy, thermal energy, or the radiant energy carried by light and other radiation) can liberate tremendous amounts of energy (~&nbsp;{{val|9|e=16|u=joules}}, equivalent to 21 megatons of TNT), as can be seen in [[nuclear reactor]]s and nuclear weapons.

Conversely, the mass equivalent of an everyday amount energy is minuscule, which is why a loss of energy (loss of mass) from most systems is difficult to measure on a weighing scale, unless the energy loss is very large. Examples of large transformations between rest energy (of matter) and other forms of energy (e.g., kinetic energy into particles with rest mass) are found in [[nuclear physics]] and [[particle physics]]. Often, however, the complete conversion of matter (such as atoms) to non-matter (such as photons) is forbidden by [[conservation law]]s.

=== Reversible and non-reversible transformations ===
Thermodynamics divides energy transformation into two kinds: [[Reversible process (thermodynamics)|reversible processes]] and [[irreversible process]]es. An irreversible process is one in which 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 thermal energy 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 with time, more and more of its energy becomes trapped in irreversible states (i.e., as heat or as other kinds of increases in disorder). This has led to the hypothesis of 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), continues to decrease.