Physics
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What is energy?
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What is energy?
Energy is defined as a measurement of the ability to do work or to heat an object. Energy plays an essential role both in everyday events and in scientific phenomena (is one of the most quantitative properties of physics in nature).
The term energy was introduced by Aristotle in philosophy to distinguish the “power” (δύναμις, dýnamis) proper to the shapeless matter, from the real capacity (Ancient Greek ἐνέργεια, enérgeia); the word is composed of “en” intensive particle, and “ergon” ability to act. After, the term “energy” was used for the first time to indicate a physical quantity by Kepler in his Harmonice Mundi of 1619. However, the term “energy” was introduced systematically in scientific literature only since the late nineteenth century.
A precise definition of energy is not simple to provide; energy is not a concrete reality but rather an abstract mathematical concept that expresses a link between the possible processes and a temporal symmetry of physical laws. There is, therefore, no substance or fluid corresponding to pure energy. As Feynman wrote: it is important to realize that in physics today, we have no knowledge of what energy is [Richard Feynman, The Feynman Lectures on Physics, Vol I, p. 4-1].
Energy is an extensive physical quantity (the energy of two bodies is simply the sum of the energies of the bodies taken individually), which has a central importance in the formulation of many theories, from classical mechanics to thermodynamics, from the theory of relativity to quantum mechanics.
A body can increase or decrease its energy as a result of an interaction with other bodies: the variation of energy then reflects the changes that have occurred in its microscopic properties.
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Types and forms of energy
Essentially the total energy of a system can be subdivided into potential (stored) energy or kinetic (working) energy, or combinations of the two in various ways. While these two categories are sufficient to describe all forms of energy, it is often convenient to refer to particular combinations of potential and kinetic energy as its form.
Energy tends to pass from one form to another, so the various forms of energy do not remain perpetually as they are, but are transformed into each other: for example, chemical energy is often transformed into heat and sometimes (as in the case of the battery) into electrical energy; nuclear energy and mechanical energy are transformed spontaneously into heat.
The conversion of energy from one form to another can occur spontaneously or in an induced manner, through special machines or systems. With an electric generator, mechanical energy can be transformed into electrical energy, while with an electric motor, electrical energy is transformed into mechanical energy. With an internal combustion engine, of the type used in common cars, the chemical energy of the fuel is used, which is transformed, during combustion (which is a chemical reaction), into thermal energy and then into mechanical energy to drive the wheels of the car. In a common neon tube, electrical energy is transformed into electromagnetic energy, emitted by excited neon atoms in the form of luminous radiation or light. In nature, plants, by means of a green pigment present in the leaves, the chlorophyll, intercept the radiant energy coming from the Sun and transform it into chemical energy (photosynthesis).
In general, for each energy transformation it is possible to calculate the efficiency of the transformation, which measures as a percentage how much of the energy input in a form has been converted into the desired final form. In the case of spontaneous transformations the efficiency is always 100%, while in the case of induced transformations it depends on the type of instrument used and the initial and final forms of energy. Among the various forms of energy, thermal energy has an interesting characteristic: all other forms of energy can spontaneously transform into thermal energy, but the opposite is not true. Because it is related to the motion of atomic-molecular agitation, thermal energy is the most disordered form of energy, or, as they say, the most degraded.
- Chemical energy
- Dark energy
- Electric energy
- Gravitational energy
- Internal energy
- Magnetic energy
- Mechanical energy
- Kinetic energy
- Potential energy
- Elastic energy
- Quantum chromodynamics binding energy
- Radiant energy
- Rest energy
- Soundwave energy
- Thermal energy