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Energy transformation or energy conversion is the process of changing one form of energy to another. In physics, the term energy describes the capacity to produce certain changes within a system, without regard to limitations in transformation imposed. Changes in total energy of systems can only be accomplished by adding or removing energy from them, as energy is a quantity which is conserved (unchanging), as stated by the first law of thermodynamics. Mass-energy equivalence, which arose from special relativity, says that changes in the energy of systems will also coincide with changes (often small in practice) in the system's mass, and the mass of a system is a measure of its energy content.
Energy in many of its forms may be used in natural processes, or to provide some service to society such as heating, refrigeration, light, or performing mechanical work to operate machines. For example, an internal combustion engine converts the potential chemical energy in gasoline and oxygen into thermal energy which, by causing pressure and performing work on the pistons, is transformed into the mechanical energy that accelerates the vehicle (increasing its kinetic energy). A solar cell converts the radiant energy of sunlight into electrical energy that can then be used to light a bulb or power a computer.
Entropy and limitations in conversion of thermal energy to other types
Conversions to thermal energy (thus raising the temperature) from other forms of energy, may occur with essentially 100% efficiency (many types of friction do this). Conversion among non-thermal forms of energy may occur with fairly high efficiency, though there is always some energy dissipated thermally due to friction and similar processes. Sometimes the efficiency is close to 100%, such as when potential energy is converted to kinetic energy as an object falls in vacuum, or when an object orbits nearer or farther from another object, in space.
On the other hand, conversion of thermal energy to other forms, thus reducing the temperature of a system, has strict limitations, often keeping its efficiency much less than 100% (even when energy is not allowed to escape from the system). This is because thermal energy has already been partly spread out among many available states of a collection of microscopic particles constituting the system, which can have enormous numbers of possible combinations of momentum and position (these combinations are said to form a phase space). In such circumstances, a measure called entropy, or evening-out of energy distributions, dictates that future states of an isolated system must be of at least equal evenness in energy distribution. In other words, there is no way to concentrate energy without spreading out energy somewhere else.
Thermal energy in equilibrium at a given temperature already represents the maximal evening-out of energy between all possible states. Such energy is sometimes considered "degraded energy," because it is not entirely convertible a "useful" form, i.e. one that can do more than just affect temperature. The second law of thermodynamics is a way of stating that, for this reason, thermal energy in a system may be converted to other kinds of energy with efficiencies approaching 100%, only if the entropy (even-ness or disorder) of the universe is increased by other means, to compensate for the decrease in entropy associated with the disappearance of the thermal energy and its entropy content. Otherwise, only a part of thermal energy may be converted to other kinds of energy (and thus, useful work), since the remainder of the heat must be reserved to be transferred to a thermal reservoir at a lower temperature, in such a way that the increase in Entropy for this process more than compensates for the entropy decrease associated with transformation of the rest of the heat into other types of energy.
History of energy transformation from the early universe
Energy transformations in the universe over time are (generally) characterized by various kinds of energy which has been available since the Big Bang, later being "released" (that is, transformed to more active types of energy such as kinetic or radiant energy), when a triggering mechanism is available to do it.
Release of energy from gravitational potential: A direct transformation of energy occurs when hydrogen produced in the big bang collects into structures such as planets, in a process during which part of the gravitational potential is to be converted directly into heat. In Jupiter, Saturn, and Neptune, for example, such heat from continued collapse of the planets' large gas atmospheres continues to drive most of the planets' weather systems, with atmospheric bands, winds, and powerful storms which are only partly powered by sunlight, however, on Uranus, little of this process occurs.
On Earth a significant portion of heat output from interior of the planet, estimated at a third to half of the total, is caused by slow collapse of planetary materials to a smaller size, with output of gravitationally driven heat.
Release of energy from radioactive potential: Familiar examples of other such processes transforming energy from the Big Bang include nuclear decay, in which energy is released which was originally "stored" in heavy isotopes, such as uranium and thorium. This energy was stored at the time of these elements' nucleosynthesis, a process which ultimately uses the gravitational potential energy released from the gravitational collapse of type IIa supernovae, to store energy in the creation of these heavy elements before they were incorporated into the solar system and the Earth. Such energy locked into uranium is triggered for sudden-release in nuclear fission bombs, and similar stored energies in atomic nuclei are released spontaneously, during most types of radioactive decay. In such processes, heat from decay of these atoms of radioisotope in the core of the Earth is transformed immediately to heat. This heat in turn may lift mountains, via plate tectonics and orogenesis. This slow lifting of terrain thus represents a kind of gravitational potential energy storage of the heat energy. The stored potential energy may be released to active kinetic energy in landslides, after a triggering event. Earthquakes also release stored elastic potential energy in rocks, a kind of mechanical potential energy which has been produced ultimately from the same radioactive heat sources.
Thus, according to present understanding, familiar events such as landslides and earthquakes release energy which has been stored as potential energy in the Earth's gravitational field, or elastic strain (mechanical potential energy) in rocks. Prior to this, the energy represented by these events had been stored in heavy atoms (or in the gravitational potential of the Earth). The energy stored in heat atoms had been stored as potential ever since the time that gravitational potentials transforming energy in the collapse of long-destroyed stars (supernovae) created these atoms, and in doing so, stored the energy within.
Release of energy from hydrogen fusion potential: In other similar chain of transformations beginning at the dawn of the universe, nuclear fusion of hydrogen in the Sun 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 resulted in hydrogen representing a store of potential energy which can be released by nuclear 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 may again be stored as gravitational potential energy after it strikes the Earth, as (for example) snow-avalanches, or when water evaporates from oceans and is deposited high above sea level (where, after being released at a hydroelectric dam, it can be used to drive turbine/generators to produce electricity). Sunlight also drives many weather phenomena on Earth. 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 green plants as chemical potential energy, when carbon dioxide and water are converted into a combustible combination of carbohydrates, lipids, and oxygen. Release of this energy as heat and light may be triggered suddenly by a spark, in a forest fire; or it may be 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.
Examples of sets of energy conversions in machines
For instance, a coal-fired power plant makes lots of energy and involves these energy transformations:
- Chemical energy in the coal converted to thermal energy in the exhaust gases of combustion.
- Thermal energy of the exhaust gases converted into thermal energy of steam through the heat exchanger.
- Thermal energy of steam converted to mechanical energy in the turbine.
- Mechanical energy of the turbine converted to electrical energy by the generator, which is the ultimate output
In such a system, the first and fourth step are highly efficient, but the second and third steps are less efficient. The most efficient gas-fired electrical power stations can achieve 50% conversion efficiency. Oil- and coal-fired stations achieve less.
In a conventional automobile, these energy transformations are involved:
- Chemical energy in the fuel converted to kinetic energy of expanding gas via combustion
- Kinetic energy of expanding gas converted to linear piston movement
- Linear piston movement converted to rotary crankshaft movement
- Rotary crankshaft movement passed into transmission assembly
- Rotary movement passed out of transmission assembly
- Rotary movement passed through differential
- Rotary movement passed out of differential to drive wheels
- Rotary movement of drive wheels converted to linear motion of the vehicle.
Other energy conversions
There are many different machines and transducers that convert one energy form into another. A short list of examples follows:
- Thermoelectric (Heat → Electric energy)
- Geothermal power (Heat→ Electric energy)
- Heat engines, such as the internal combustion engine used in cars, or the steam engine (Heat → Mechanical energy)
- Ocean thermal power (Heat → Electric energy)
- Hydroelectric dams (Gravitational potential energy → Electric energy)
- Electric generator (Kinetic energy or Mechanical work → Electric energy)
- Fuel cells (Chemical energy → Electric energy)
- Battery (electricity) (Chemical energy → Electric energy)
- Fire (Chemical energy → Heat and Light)
- Electric lamp (Electric energy → Heat and Light)
- Microphone (Sound → Electric energy)
- Wave power (Mechanical energy → Electric energy)
- Windmills (Wind energy → Electric energy or Mechanical energy)
- Piezoelectrics (Strain → Electric energy)
- Acoustoelectrics (Sound → Electric energy)
- Friction (Kinetic energy → Heat)
- Heater (Electric energy → Heat)
- Light bulb (electricity → light)