# Energy [physics]

## How to measure energy?

The SI unit of energy is the joule [J], which is the energy transferred to an object by the work of moving it a distance of 1 meter against a force of 1 newton.

$1\;\textrm{J}=\dfrac{1\;\textrm{kg}\cdot\;\textrm{m}^2}{\textrm{s}^2}$

## The law of conservation of energy

In physics, the law of conservation of energy is one of the most important conservation laws observed in nature. The conservation principle has guided the discovery of new forms of energy and has allowed us to discover new types of physical processes and even new particles.

The principle of conservation of energy reflects the temporal symmetry of the physical laws with respect to time translations; that is, that these do not change over time.

The law of conservation of energy states that the total energy of an isolated system remains constant, it is said to be conserved over time.

This law means that energy cannot be created or destroyed, but is merely changed from one form into another or transferred from one object to another at different stages. So we can conclude that in the entire system, the total energy remains the same, but only the transformation takes place.

For example, the electricity available in an electric oven is converted to a thermal form that goes into the object in the oven.

At the beginning of the 20th century, some nuclear decays were discovered with the emission of electrons that did not seem to satisfy the principle of energy conservation. To solve the problem in 1924, Niels Bohr put forward the idea that at the atomic level energy was not strictly conserved, proposing a theory that turned out to be wrong.

Wolfgang Pauli in 1930 and Enrico Fermi in 1934 postulated the existence of new interactions and a new particle never observed before, which was able to transport energy and which was missing in the experiments. In this way, guided by the principle of conservation of energy, they were able to discover the neutrino, a particle with no electric charge, actually observed experimentally in 1959.

Classically, conservation of energy was distinct from conservation of mass; however, special relativity showed that mass could be converted to energy and vice versa by E = mc2, and science now takes the view that mass-energy is conserved.

## Energy sources

In prehistoric times and for most of human history, sources of energy used by man were: human and animal muscle energy to produce work, the combustion of wood or, more generally, biomass, to produce heat. Later, sailing and water and windmills, introduced a first diversification regarding inanimate energy sources. The energy available per capita, before the nineteenth century, in the West, was reduced: this resulted, in pre-industrial societies, in a low mobility of people as a whole, reduced movement of goods, reduced health care, discontinuous availability of food resources, with periodic famines.

During the twentieth century there has been a significant increase in energy consumption, which has practically doubled from 1973 to 2004. This poses problems, both from an environmental point of view (for example, the greenhouse effect or waste disposal) and from a geopolitical point of view. The choice of an energy source has become a complex and important socio-political fact, which depends on the availability of resources, the cost of a source in relation to the particular conditions of a nation, the reliability of energy production plants and the protection of the environment.

The sources used today for the production of electricity are essentially the combustion of fossil fuels (coal or hydrocarbons), hydroelectricity, atomic energy from fission, wind, geothermal and solar energy.

World energy resources are the estimated maximum capacity for energy production given all available resources on Earth. Energy sources can be categorized as renewable and non-renewable.

renewable resource is a resource that can be used repeatedly and replaced naturally (that can replenish itself at a similar rate to its use by people). Renewable and non-renewable energy sources can be used as primary energy sources to produce useful energy such as heat or used to produce secondary energy sources such as electricity.

non-renewable resource is a natural resource that is used up faster than it can be made by nature. It cannot be produced, grown, or generated on a scale which can sustain how quickly it is being consumed. Once it is used up, there is no more available for the future.

Fossil fuels (such as coal, petroleum, and natural gas), types of nuclear power (uranium), and certain examples. Resources such as timber (when harvested sustainably) or metals (which can be recycled) are considered renewable resources. Non-renewable resources are also called exhaustible resources.

## 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
• Rest energy
• Soundwave energy
• Thermal energy

## Related keywords

• Energy harvesting
• Unconventional energy: this sector includes geothermal energy, wind power, and tidal power. Sustainable energy sources is a growing area.