Energy

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In general, the concept energy refers to "the potential for causing a change". The word is used in several different contexts. The scientific use has a precise, well-defined meaning, whilst the many non-scientific uses often do not.

In physics the energy of a system in a certain state is defined as the work needed to bring the system to that state from some reference state. Because work is defined via force involved, forms of energy are usually classified according to that force (elastic, gravitational, nuclear, electric, etc). Energy is a conserved quantity: it is neither created nor destroyed, but only transfered from place to place or from one form to another. Ultimately, this is because the laws of nature do not change with time.

Etymology

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".

Historical perspective

Energy, in the distant past, was discussed in terms of easily observable effects it has on the properties of objects or changes in state of various systems. It was generally construed that behind all changes, some sort of energy was involved. 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. While in spiritualism they were reflected in changes in a person, in physical sciences it is reflected in different forms of energy. For example, 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. Or you can visit the website at http://www.energy-force.com/

Energy in Natural Sciences

The concept energy is useful to explain almost all kind of phenomena and processes in natural sciences. However, the context varies from one natural science to another. Thus in

Physics: Energy is the ability to do work (work is, simplistically, a force applied through a distance), and has several different forms. However, no matter what the form, physical energy uses the same units as work: a force applied through a distance. 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.

Chemistry: Energy is the cause and effect of all transformations a substance can undergo. These transformations can be a decomposition, synthesis or a reaction of molecules or atoms. A transformation is possible only if the free energy considerations are fulfilled. The speed of a chemical reaction is determined by the activation energy that reactant molecules have to surmount in order to produce product molecules.

Biology: Energy is essential for sustanance of life. Thus, in molecular biology or biochemistry, energy is the source of all biological processes and is due to the making and breaking of certain chemical bonds in the molecules found in biological organisms. These bonds are most often bonds in carbohydrates or parts. Glucose, fructose, ATP and Pyruvate are the main sources of energy for most biological processes.

Geology: a volcano, earthquake, or tsunami are the result of energy flow in the crust of earth.

Cosmology all stellar phenomena are deemed to be related to various forms of energy.

Forms of energy

Kinetic energy is the energy of motion (an object which has speed can perform work on another object by colliding with it).
Potential energy or unreleased kinetic energy. This sort of energy arises when work is done on an object to move it somewhere against an opposing force. For instance, stretching a rubber band increases the elastic potential energy stored within the band. When the band is released, this energy is converted into kinetic energy, and work is performed. Other forms of potential energy include gravitational potential energy (from moving masses apart), electrical potential energy (from moving charges against a field), and chemical potential energy (energy stored within chemical bonds).
Thermal energy the kinetic energy associated with the various motions of microscopic particles. The average thermal energy within a sample of matter is referred to as the sample's temperature (work is required to accelerate the particles and raise the temperature).
Light energy the energy that composes photons and is responsible for the various sorts of electromagnetic radiation (work is required to create photons).
Nuclear energy, the energy stored within the nuclei of atoms.
Mass is also considered as a form of energy, (or in lay terms, the manifestation of energy,) because during annihilation or other mass change, the equivalent amount of energy (E = mc²) is always released.

Conservation of energy

Energy is subject to the law of conservation of energy (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%.

The first law of thermodynamics states 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. In other words, energy is neither created nor destroyed, only converted between forms. 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 the translational symmetry of time, a property of most phenomena below the cosmic scale that makes them independent of their locations on the time coordinate. Put differently, yesterday, today, and tomorrow are physically indistinguishable. The fact that energy is not always conserved in quantum mechanics is a property of the the uncertainty principle, which relates the mutual uncertainty of time and energy as follows:

\Delta E\Delta t\geq h

As such, quantum mechanical 'violations' of the conservation of energy are not really violations at all, but rather an example of the priority the uncertainty principle takes over more classical laws. Since there is always a degree of mutual uncertainty between time and energy, it follows that the more accurately time is measured, the less accurate measurements of energy can be. When the time scales become small enough that this quantum uncertainty becomes significant, energy may not be conserved. However. within the limits set by the uncertainty principle, conservation of energy holds.

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. In all cases, as long as no energy is allowed to escape from the system, the sum of all the different energies in the system remains constant no matter how many changes take place.

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; some energy will escape the system and be converted into 'useless' energies such as sound and heat. If this were not so, 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.

Relations between different forms

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.

Work

Because energy is defined in terms of work, a definition of work is crucial to the understanding of energy.

Work is a defined as a path integral of force F over distance s:

W=\int \mathbf {F} \cdot \mathrm {d} \mathbf {s}

The equation above says that the work ( $W$ ) is equal to the integral of the dot product of the force ( $\mathbf {F}$ ) on a body and the infinitesimal of the body's translation ( $\mathbf {s}$ ).

Depending on the 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

W=\int \mathbf {F} \cdot \mathrm {d} \mathbf {s} =\int m\mathbf {a} \cdot \mathrm {d} \mathbf {s}

= mv²/2

and we call this work by the term "kinetic energy" K = mv²/2.

Other forms of energy are similarly 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. When two bodies of different temperature come in to thermal contact, they will exchange internal energy until the temperature is equalized. 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.

Energy in the economy

In the context of economics the word energy is synonymous to energy resources, it refers to substances like fuels, petroleum products and electric power installations. This difference vis a vis energy in natural sciences can lead to some confusion, because energy resources are not conserved in nature in the same way as the energy is conserved in the context of say physics. People often talk about energy crisis and the need to conserve energy, something contrary to the spirit of natural sciences. Thus, production and consumption of energy is very important to the global economy. All economic activity therefore require energy, whether to manufacture goods, provide transportation, feed electricity into computers and other machines, or to grow food to feed workers, or even to harvest new fuels.

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. Scarcity of cheap fuels, pollution, and global warming are key concerns in future energy development.

Some attempts have been made to define "embodied energy" - the sum total of energy expended to deliver a good or service as it travels through the economy.

Psychological and non-scientific uses

Many distinct meanings of the word "energy" have come into popular use, either before the modern scientific concept of energy was fully developed, or later by analogy to it.

Psychological

Many humans actions can be scientifically explained in terms of energy. Thus, humans maintain their metabolism when they exercise or do mechanical work. But the word "energy" is also used in a casual sense as a synonym for psychological motivation, creativity, agitation, excitement, or responsiveness. "Fatigue" or a "lack of energy" can be either a result of expending physical energy, or a psychological condition brought on by an excess of intellectual activity, intense emotional experiences, inadequate sleep, or an imbalance or natural fluctuation in hormones and neurotransmitters. Emotional "exhaustion" is thought to be brought on not by a lack of physical energy (in the scientific sense described above), but by a particular chemical state in the brain. Low motivation is associated with both malnutrition (including insufficient calories - a unit of physical energy - in the diet) and elevated blood glucose (sugar) levels (which provides lots of physical energy, but which has other chemical effects that influence mood). Drugs such as depressants, stimulants (present in some energy drinks and dietary supplements), and sedatives directly affect these types of emotional states. Some mental disorders such as mania, bipolar disorder, and depression involve improper regulation of brain chemicals which determine mood.

Sigmund Freud, among others, developed a notion of psychic energy which is similar to, but distinct from, the general scientific notion of energy. This psychological model, while popular, is no longer considered to be a scientifically accurate of how the mind works.

Spiritual and healing

Vitalism is the idea that there is a non-physical "life force" of some kind that animates living things. Various forms of spiritual "energy" have been postulated, including the Christian idea of the soul or spirit, the traditional Chinese qi, the Indian chakra or shakti, the New Age/paranormal aura, and the "orgone energy" of Wilhelm Reich.

Various forms of mysticism often associate some kind of "energy" with disease and healing powers. For example, acupuncture purports to have beneficial effects on the human body by manipulating its natural flow of energy. Reiki is a similar procedure in Japanese culture which involves the qi (ki) and the laying of hands.

These various types of "spiritual energy" are often considered to be of a different type than those known to science. Most scientists dismiss these claims as non-empirical religious beliefs (and thus either outside the realm of science or presumed to be false or non-sensical) or as pseudoscience.

Throughout history, pseudoscientific claims about "spiritual energy" have attempted to gain credibility by associating with real energies that were poorly understood by scientists or by the public. In the 1800s, electricity and magnetism were in the "borderlands" of science and the subject of considerable electrical quackery. In the 2000s, quantum mechanics and grand unification theory provide similar opportunities for empirical claims of spiritual energy being physically manifest.

Sometimes "spiritual energy" is equated with empirically understood forces. For example, some believers in the "aura" equate it with electromagnetism, but sometimes make claims which can be disproven through experiment.

Electromagnetic fields are used in real medical procedures, such as radiation therapy, electroconvulsive therapy, and magnetic resonance imaging. But other proposed treatments, such as magnet therapy are considered ineffective (other than the associated placebo effect).

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