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.

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.


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

sun energy
The Sun’s energy warms the planet’s surface, powering titanic transfers of heat and pressure in weather patterns and ocean currents. The resulting air currents drive wind turbines. Solar energy also evaporates water that falls as rain and builds up behind dams, where its motion is used to generate electricity via hydropower.

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.

Renewable energy

Renewable energy is a kind of energy from sources that are naturally replenishing but flow-limited, such as sunlight, wind, rain, tides, waves, and geothermal heat. They are virtually inexhaustible in duration but limited in the amount of energy that is available per unit of time. Renewable energy often supplies energy in four important areas: electricity production, air/water heating/cooling, transport, and rural energy services (off-network).

Some are considered “inexhaustible,” in the sense that they regenerate at least at the same speed with which they are consumed or are not “exhaustible” in the scale of “geological eras times.” Exceptions are some energy resources which, although renewable, are exhaustible; for example, forests are considered renewable but can be depleted due to excessive exploitation by humans.

Renewable resources, whether they are material or energy, are natural resources which, due to natural characteristics or due to the production of man, are renewed over time (at a higher, or equal, renewal rate than the rate of consumption/use) and can be considered inexhaustible, or may be available for use by humans almost indefinitely. A renewable resource is also said to be “sustainable” if its rate of regeneration is equal to or higher than the rate of use.

There are many forms of renewable energy. Wind and hydroelectric power are the direct results of differential heating of the Earth’s surface which leads to air moving about (wind) and precipitation forming as the air is lifted. Solar energy is the direct conversion of sunlight using panels or collectors. Biomass energy is stored sunlight contained in plants.

Other renewable energies that do not depend on sunlight are geothermal energy, which is a result of radioactive decay in the crust combined with the original heat of accreting the Earth, and tidal energy, which is a conversion of gravitational energy. It is useful to highlight how the forms of energy present on our planet (except nuclear energy, geothermal energy, and tidal energy) almost all originate from solar radiation, in fact:

  • without the Sun there would be no wind, which is caused by the irregular heating of air masses, and with it wind energy;
  • biomass energy can be considered chemically stored solar energy, through the process of chlorophyll photosynthesis;
  • hydroelectric energy, which uses waterfalls, would not exist without the water cycle from evaporation to rain, triggered by the Sun;
  • fossil fuels (coal, oil, and natural gas) derive from the sun’s energy stored in the biomass millions of years ago through the process of chlorophyll photosynthesis.

Non-renewable energy

Non-renewable energies are energy sources that tend to run out over time and therefore the environmental impact associated with their exploitation is generally more significant than that of renewable energy sources, which are instead reintegrated naturally in a relatively short period.

Non-renewable energy sources are often exploited by humanity because they can produce the highest amounts of energy with technologically simple and tested systems. Often, the use of such sources is associated with environmental pollution problems such as the production of greenhouse gases or radioactive waste. The four major nonrenewable energy sources are:

  • Crude oil (petroleum)
  • Natural gas
  • Coal
  • Uranium or plutonium (nuclear energy)

All fossil fuels are nonrenewable, but not all nonrenewable energy sources are fossil fuels; coal, crude oil, and natural gas are all considered fossil fuels because they were formed from the buried remains of plants and animals that lived millions of years ago. Uranium ore, a solid, is mined and converted to a fuel used at nuclear power plants. Uranium is not a fossil fuel, but it is classified as a nonrenewable fuel.

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.

  • Chemical energy
  • Dark energy
  • Electric energy
  • Gravitational energy
  • Internal energy
  • Magnetic energy
  • Mechanical energy
  • Kinetic energy
  • Potential energy
  • Nuclear potential energy
  • Elastic energy
  • Mechanical wave energy
  • Wind wave energy
  • Quantum chromodynamics binding energy
  • Radiant energy
  • Rest energy
  • Soundwave energy
  • Thermal energy

Energy harvesting

Energy harvesting (also known as power harvesting or energy scavenging or ambient power) is a method of generating electrical energy from normally unused energy sources available in the surrounding environment. In other words, it is the process by which energy, coming from alternative energy sources (commonly available in the environment: thermal energy, kinetic energy, chemical energy, potential or solar energy, etc.) is captured and accumulated. This process converts energy into a directly usable electrical current.

Energy harvesting holds great promise for both low-voltage and low-power applications in a wide range of portable or mobile markets such as medical equipment, consumer devices, transportation, industrial controls, and military. Energy conversion takes place in different ways depending on the environmental source. The energy can be captured from a variety of sources deemed wasted or otherwise unusable for any practical purpose.

  • Mechanical sources: translational and rotational kinetic and potential energies, inertia, gravitational field, vibrations, elastic energy, piezoelectricity, triboelectric effect, acoustic waves, sea/ocean waves, and wind energy. For example, the conversion of mechanical motion can take place through piezoelectric crystals or particular polymers, which subjected to mechanical deformation stresses, generate small electrical potentials.
  • Electromagnetic sources: radio waves, magnetic induction, electromagnetic radiation, photovoltaic, potential energy due to electric fields or magnetic fields. For example, energy from broadcasting or theoretically from any electromagnetic emission can be collected. A typical use of this technique is used to power the RFID (Radio Frequency Identification) identifiers.
  • Thermal sources: temperature gradients, pyroelectricity, thermoelectrics. In the presence of thermal gradients, thermoelectric generators can be used.
  • Chemical and biological sources: exothermic reactions, potential energy due to chemical bonds, ionization energy, levels of glucose in the blood, salinity gradients, tree-based metabolic energy.

Unconventional energy

This sector includes geothermal energy, wind power, and tidal power. Sustainable energy sources is a growing area.