“Energy transformation”的意思、由来-开放百科全书

Energy transformation, also known as energy conversion, is the process of changing energy from one form into another. In physics, energy is a quantity that provides the capacity to perform work (lifting an object). In addition to being convertible, according to the law of conservation of energy, energy is transferable to a different location or object, but it cannot be created or destroyed.

Energy in many of its forms may be used in natural processes, or to provide some service to society such as heating, refrigeration, lighting or performing mechanical work to operate machines. For example, in order to heat a home, the furnace burns fuel, whose chemical potential energy is thus converted into thermal energy, which is then transferred to the home's air to raise its temperature.

In another example, an internal combustion engine burns gasoline to create pressure that pushes the pistons, thus performing work in order to accelerate your vehicle, ultimately converting the fuel's chemical energy to your vehicle's additional kinetic energy corresponding to its increase in speed.

Limitations in the conversion of thermal energy

Conversions to thermal energy (thus raising the temperature) from other forms of energy, may occur with 100% efficiency{{Citation needed|date=October 2012}}. 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 a vacuum, or vice-versa, as an object in an elliptical orbit around another body moves away from it and converts its kinetic energy (speed) into gravitational potential energy (distance from the other object) - as it reaches the furthest point, it will reverse the process, accelerating and converting potential energy into kinetic. Since space is a near-vacuum, this process has close to 100% efficiency.

Thermal energy is unique because it can not in itself be converted to other forms of energy. Only a difference in the density of thermal energy (temperature) can be used to perform work, and the efficiency of the conversion will be (much) less than 100%. This is because thermal energy represents a particularly disordered form of energy - its spread out randomly among many available states of a collection of microscopic particles constituting the system (these combinations of position and momentum for each of the particles are said to form a phase space). The measure of this disorder or randomness is entropy, and its defining feature is that the entropy of an isolated system never decreases - one cannot take high-entropy state (like a hot substance, with a certain amount of thermal energy) and convert it into a low entropy state (like a low-temperature substance, with the corresponding amount of chemical potential energy), without putting that entropy somewhere else (like the surrounding air). 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 the transformation of the rest of the heat into other types of energy.

Transformation of kinetic energy of charged particles to electric energy

In order to make energy transformation more efficient, it is desirable to avoid thermal conversion. For example, the efficiency of nuclear energy reactors, where the kinetic energy of the nuclei is first converted to thermal energy and then to electric energy, lies at around 35%.^[1]^[2] By direct conversion of kinetic energy to electric energy, i.e. by eliminating the intermediate thermal energy transformation, the efficiency of the energy transformation process can be dramatically improved.^[3]

History of energy transformation

Energy transformations in the universe over time are (generally) characterized by various kinds of energy which have 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 continue to drive most of the planets' weather systems, with atmospheric bands, winds, and powerful storms, which are only partly powered by sunlight (the whole electromagnetic radiations, or starlight, generated by the Sun). However, on Uranus, little of this process occurs.

On Earth, a significant portion of the heat output from the interior of the planet, estimated at a third to half of the total, is caused by the slow collapse of planetary materials to a smaller size, with the 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, which releases energy that was originally "stored" in heavy isotopes, such as uranium and thorium. This energy was stored at the time of the nucleosynthesis of these elements, a process which ultimately uses the gravitational potential energy released from the gravitational collapse of Type II 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 similarly stored energies in atomic nuclei are released spontaneously, during most types of radioactive decay. In such processes, heat from the 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 a 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 the gravitational collapse of hydrogen clouds when they produce stars, and some of the fusion energy is then transformed into starlight. Considering the solar system, starlight, overwhelmingly from the Sun, 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. The 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, the potential energy stored at the time of the Big Bang is later released by intermediate events, sometimes being stored in a number of different ways for long periods of time between releases, as more active energy. All of these events involve the conversion of one kind of energy into others, including heat.

Examples

Examples of sets of energy conversions in machines

In such a system, the first and fourth steps 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.

Other energy conversions

There are many different machines and transducers that convert one energy form into another. A short list of examples follows: