Energy
From Free net encyclopedia
- This is an article related to the use of word Energy in the broadest sense. It is not limited to its usage in physics alone. For information on using energy resources sustainably see Energy conservation. For an album by Operation Ivy see Energy (album).
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Broadly speaking, energy refers to "the potential for causing a change". There are different forms of energy. Many forms of energy refer to changes in movement of objects or their potential for causing other kinds of change. Hence, in the real world, energy can be viewed as the ability to perform an action.
The etymology of the term is from Greek ενεργεια, εν- means "in" and έργον means "work"; the -ια suffix forms an abstract noun. The compound εν-εργεια in Epic Greek meant "divine action" or "magical operation"; it is later used by Aristotle in a meaning of "activity, operation" or "vigour", and by Diodorus Siculus for "force of an engine".
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Nature of Energy
In physics energy is defined via work. For example, kinetic energy is the amount of work to accelerate body, gravitational potential energy is the amount of work to elevate mass, etc. Because work is frame dependent (= can only be defined relative to certain initial state or reference state of the system), energy also becomes frame dependent. For example, a speeding bullet has plenty of kinetic energy in the reference frame of non-moving observer, but it has zero kinetic energy in proper (co-moving) reference frame - because it takes zero work to accelerate a bullet from zero speed to zero speed. Of course, the selection of a reference state (or reference frame) is completely arbitrary - and usually is dictated to maximally simplify the problem to be dealt with.
In mechanics very often frame of center of mass is used (because net momentum is zero in such frame, so less equations are used).
Mass is also considered as a form of energy, because during annihilation or other mass change the equivalent amount of energy (E = mc²) is always released.
Energy, in the context of natural sciences, is subject to conservation (law, which is a mathematical restatement of shift symmetry of time). Thus, energy cannot be made or destroyed, it can only be converted from one form to another, that is, transformed.
In practice, during any energy transformation in (macroscopic) system, some energy is converted into incoherent microscopic motion of parts of the system (which is usually called heat or thermal motion), and the entropy of the system increases. Due to mathematical impossibility to invert this process (see statistical mechanics), the efficiency of energy conversion in a macroscopic system is always less than 100%.
Forms of Energy
Scientific forms
Energy, in the context of natural sciences: physics, chemistry, biology etc., can be in several forms: mechanical potential—due to possible physical interactions with other objects (for example, gravitational potential energy); kinetic—contained in macroscopic motion; chemical—potential stored in chemical bonds between atoms; electrical—potential due to possible charge interactions; thermal—contained in the kinetic energy of individual molecules; nuclear—potential stored between constituents of nuclei. Light can be viewed as energy in the form of photons or waves, depending on the context. The theory of special relativity results in equivalence of mass and energy.
Another form of energy, a form of energy said to permeate the universe, is called dark energy.
Non-scientific forms
The word Energy is often used in contexts outside the natural sciences. To mathematicians, engineers, physicists and scientists, the word has a strict and quantifiable definition. The mixing of the non-scientific and scientific definitions of the word is deprecated because it leads to confusion.
In the context of economics, the term energy is used in discussions related to resources, such petroleum products and electric power generation that enable us to use machines.
In the context of psychology, sociology, politics etc., energy can be in in the form of emotional energy, embodied energy, and perhaps psychic energy.
In the context of common speech, the word energy is used to describe the behaviour of individuals. This may be similar to the physical use of the term work (force x distance), although this form is in fact quite different. Energy can be used to describe someone with a vigorous, enterprising, hard working or ambitious drive, or to describe someone’s physical and mental capacity when applied to a particular activity, or to describe someone with an vivid imagination implying vitality and intensity of expression.
In the context of spiritual the word cannot be quantified or even defined. The term energy, in such contexts, is used in traditional and New age mysticism and in fields such as parapsychology, acupuncture. The word energy is often used as reiki (in Japanese culture); Qi (traditional Chinese culture) and prana, kundalini and shakti (in traditional Indian spiritual culture).
Paranormal researchers will often refer to "psychokinetic energy" when attempting to explain paranormal phenomena or the concept of a spirit or soul.
The psychiatrist and psychoanalyst Wilhelm Reich coined the term orgone energy for the non physical energy of conciousness.
Conservation and conversion 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, but frequently violated by quantum mechanics (see off shell). Noether's theorem relates the conservation of energy to the time invariance of physical laws.
The law of conservation of energy, a fundamental principle of physics, follows from translational symmetry of time. By other words, most phenomena below the cosmic scale do not depend on location on time coordinate. Yet by other words: yesterday, today, and tomorrow are physically indistinguishable. The mutual uncertainty of energy and time in quantum mechanics is related through the uncertainty principle:
- <math>\Delta E \Delta t \ge h </math>
which thus shall not be considered as a violation of energy conservation law but rather as a mathematical (=logical) prohibition to define energy over an arbitrary small time interval with arbitrary high certainty.
As a consequence of energy conservation law, one form of energy can be readily transformed into another - for instance, a battery converts chemical energy into electrical energy. Similarly, gravitational potential energy is converted into the kinetic energy of moving water (and a turbine) in a dam, which in turn is transformed into electric energy by a generator.
The law of conservation of energy states that in a closed system the total amount of energy, corresponding to the sum of a system's constituent energy components, remains constant. Some works, thus some forms of energy, are not easily measured by the unaided observer.
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. (In practice, available energy is never perfectly conserved when a system changes state; otherwise, the creation of perpetual motion machines would be possible.)
Another example is a chemical explosion in which potential chemical energy is converted to kinetic energy and heat in a very short time.
Transfer of energy
Work
Because energy is defined as a work, then definition of a work is central to understand various kinds of energy.
Work is a defined as a path integral of force F over distance s:
<math> W = \int \mathbf{F} \cdot \mathrm{d}\mathbf{s}</math>
The equation above says that the work (<math>W</math>) is equal to the integral of the dot product of the force (<math>\mathbf{F}</math>) on a body and the infinitesimal of the body's translation (<math>\mathbf{s}</math>).
Depending on kind of force F involved, work of this force results in corresponding kind of energy (gravitational, electrostatic, kinetic, etc).
For example, the gravitational force F=-mg acting on a mass m when the mass is elevated from some height h1 (reference height) to the height h2 is therefore:
W = -mg(h1-h2)= mgh2-mgh1
and we call this work by the term "gravitational potential energy" U = mgh.
Similar, work by the force F = ma to accelerate a bullet from zero velocity to the velocity v is
<math> W = \int \mathbf{F} \cdot \mathrm{d}\mathbf{s} = \int m \mathbf{a} \cdot \mathrm{d}\mathbf{s} </math> = mv2/2
and we call this work by the term "kinetic energy" K = mv2/2.
Other forms of energy are similar defined via work.
Heat
Heat is the common name for thermal energy of an object that is due to the motion of the constituents - usually atoms and molecules. This motion can be translational (motion of molecules or atoms as a whole); vibrational (relative motion of atoms within molecules) or rotational (motion of the atoms of a molecule about a common centre). It is the form 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 when atoms are rearranged between various molecules by a 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. Heat energy is transferred in three different ways: conduction, convection and/or radiation.
Relations between different forms of energy
In the context of natural sciences, all forms of energy: thermal, chemical, electrical, radiant, nuclear etc. can be in fact reduced to kinetic energy or potential energy. For example thermal energy is essentially kinetic energy of atoms and molecules; chemical energy can be visualized to be the potential energy of atoms within molecules; electrical energy can be visualized to be the potential and kinetic energy of electrons; similarly radiant energy can be visualized to be the potential and kinetic energy of photons and nuclear energy as the potential energy of nucleons in atomic nuclei.
History
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 changed, 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 water heater, or the kinetic energy of a moving train.
The concept of energy and work are relatively new additions to the physicist’s toolbox. Neither Galileo nor Newton made any contributions to the theoretical model of energy, and it was not until the middle of the 19th century that these concepts were introduced.
The development of steam engines required engineers to develop concepts and formulas that would allow them to describe the mechanical and thermal efficiencies of their systems. Engineers such as Sadi Carnot and James Prescott Joule, mathematicians such as Émile Claperyon and Hermann von Helmholtz , and amateurs such as Julius Robert von Mayer all contributed to the notions that the ability to perform certain tasks, called work, was somehow related to the amount of energy in the system. The nature of energy was elusive, however, and it was argued for some years whether energy was a substance (the caloric) or merely a physical quantity, such as momentum.
William Thomson (Lord Kelvin) amalgamated all of these laws into his laws of thermodynamics, which aided in the rapid development of energetic descriptions of chemical processes by Rudolf Clausius, Josiah Willard Gibbs, Walther Nernst. In addition, this allowed Ludwig Boltzmann to describe entropy in mathematical terms, and to discuss, along with Jožef Stefan, the laws of radiant energy.
For further information, see the Timeline of thermodynamics.
Energy and Economy
Image:Energy per capita.png Template:Main articles The way in which humans use energy is one of the defining characteristics of an economy. The progression from animal power to steam power, then the internal combustion engine and electricity, are key elements in the development of modern civilization. Future energy development, for example of renewable energy, may be key to avoiding the effects of global warming.
See also
Energy in natural sciences
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- Power (physics)
- Solar radiation
- Specific orbital energy
- Thermodynamic entropy
- Thermodynamics
- Units of energy measurements
Energy in technology
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- Policy
- Quality
- Renewable energy
- Resources
- Storage
- Transmission
- EU Energy Label
- EU Intelligent Energy
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.
- Einstein, Albert (1952). Relativity: The Special and the General Theory (Fifteenth Edition). ISBN 0-517-88441-0
- Alfred J. Lotka (1956). Elements of Mathematical Biology, forerly published as 'Elements of Physical Biology', Dover, New York.
Notes
Template:Note This definition is one of the most common; e.g. Glossary at the NASA homepage
External links
- What does energy really mean? From Physics World
- Glossary of Energy Terms
- International Energy Agency IEA - OECD
- Template:PDFlink 'Actual' (First-Law) Energy in Relation to Free Energy and Entropyar:طاقة
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