Energy

Energy is a fundamental physical quantity that expresses the capacity of a system to perform work or transfer heat, and it is conserved in all known physical processes.

First systematically studied in the context of classical mechanics during the seventeenth and eighteenth centuries, energy has since been recognised as the unifying concept connecting mechanics, thermodynamics, electromagnetism, relativity, and quantum theory. The International System of Units (SI) measures energy in joules (J), where one joule equals one newton-metre.

Forms of energy

Energy exists in numerous interconvertible forms. No single form is more fundamental than another; each describes the same conserved quantity expressed in a different physical context.

Kinetic energy

Kinetic energy is the energy possessed by an object by virtue of its motion. For a classical particle of mass m moving at speed v, it equals ½mv². At relativistic speeds, this expression is replaced by the full relativistic energy formula derived from special relativity.

Potential energy

Potential energy is stored energy associated with the configuration or position of a system. Gravitational potential energy, elastic potential energy, and chemical potential energy are common examples. Gravitational potential energy near Earth's surface equals mgh, where g is the local gravitational acceleration and h is height.

Thermal energy

Thermal energy arises from the random microscopic motion of atoms and molecules within a substance. It is closely related to temperature but is an extensive property of the whole system, not an intensive one like temperature.

Electromagnetic energy

Electromagnetic energy is carried by electromagnetic radiation, including visible light, radio waves, X-rays, and gamma rays. Its quantum unit is the photon, whose energy equals Planck's constant multiplied by the radiation's frequency (E = hf).

Chemical energy

Chemical energy is stored in the bonds between atoms within molecules and is released or absorbed during chemical reactions. Combustion and metabolism are everyday processes that convert chemical energy into thermal and mechanical energy.

Nuclear energy

Nuclear energy is released when atomic nuclei undergo fission (splitting) or fusion (combining). It arises from changes in nuclear binding energy and is related to mass change via Albert Einstein's famous equation E = mc², derived from special relativity in 1905.

Conservation of energy

The law of conservation of energy states that the total energy of an isolated system remains constant over time; energy can be transformed from one form to another but cannot be created or destroyed. This principle, sometimes called the first law of thermodynamics in thermal contexts, was consolidated in the mid-nineteenth century through the work of figures including James Prescott Joule, Julius Robert von Mayer, and Hermann von Helmholtz.

Emmy Noether's theorem (1915) provides the deepest mathematical justification: conservation of energy is a direct consequence of the time-translation symmetry of the laws of physics.

Thermodynamics and entropy

The study of energy transformations is formalised in thermodynamics. While the first law ensures energy is conserved, the second law of thermodynamics states that in any real process, the total entropy of an isolated system never decreases. This imposes a directionality on energy conversions: not all energy in a system is available to do useful work. The portion that is available is called free energy (Gibbs or Helmholtz, depending on the constraints).

The concept of energy efficiency — the ratio of useful output energy to total input energy — is central to engineering and environmental science. No real machine achieves 100 percent efficiency because some energy is always dissipated as waste heat.

Energy in modern science and technology

Quantum mechanics

In quantum mechanics, energy is quantised: systems can only occupy discrete energy levels. The energy of a quantum state is given by the expectation value of the Hamiltonian operator. The famous Heisenberg uncertainty principle links uncertainty in energy to uncertainty in time (ΔE·Δt ≥ ℏ/2).

Relativity

Einstein's mass–energy equivalence (E = mc²) demonstrated that mass itself is a form of energy. This insight underpins nuclear physics, particle physics, and the understanding of stellar processes such as nuclear fusion in stars.

Energy sources and generation

Human civilisation depends on the large-scale extraction and conversion of energy. Primary sources include:

• Fossil fuels — coal, oil, and natural gas, formed from ancient organic matter and releasing chemical energy through combustion.
• Nuclear power — exploiting fission of heavy nuclei such as uranium-235 and plutonium-239.
• Renewable energy — solar, wind, hydroelectric, geothermal, and tidal sources that are replenished on human timescales.
• Biomass — organic material burned or converted biochemically to release stored chemical energy.

Global energy consumption and its environmental consequences — particularly climate change driven by greenhouse-gas emissions from fossil-fuel combustion — are among the most pressing policy issues of the early twenty-first century. Figures for global energy consumption change frequently; readers are advised to consult current data from agencies such as the International Energy Agency.

Energy storage

Efficient energy storage is critical to integrating intermittent renewable sources into power grids. Technologies include electrochemical batteries, pumped-storage hydroelectricity, compressed air, hydrogen fuel cells, and flywheels. Each involves trade-offs among energy density, power density, cost, and cycle life.

Units and measurement

The SI unit of energy is the joule (J). Other units in common use include:

• Kilowatt-hour (kWh) — used in electricity billing; 1 kWh = 3.6 × 10⁶ J.
• Calorie (cal) — used in chemistry; 1 cal ≈ 4.184 J.
• Kilocalorie (kcal) — used in nutrition, often labelled "Calorie" with a capital C.
• Electronvolt (eV) — used in atomic and particle physics; 1 eV ≈ 1.602 × 10⁻¹⁹ J.
• British thermal unit (BTU) — used in engineering contexts in the United States.

Frequently asked questions

Is energy the same as power?

No. Energy is the total capacity to do work, measured in joules, while power is the rate at which energy is transferred or converted, measured in watts (joules per second). A high-power device uses energy quickly; a low-power device uses it slowly.

Is energy always conserved?

In all experimentally tested situations, yes. The law of conservation of energy holds universally; any apparent violation has invariably been resolved by identifying a previously unaccounted form of energy, as occurred when the neutrino was postulated to preserve energy conservation in beta decay.

Is dark energy the same as ordinary energy?

Dark energy is a hypothetical form of energy thought to permeate all of space and drive the accelerating expansion of the universe. It does not behave like any known form of ordinary energy and remains one of the major unsolved problems in cosmology as of the early 2020s.

Is it possible to convert mass entirely into energy?

In principle, yes: matter–antimatter annihilation converts rest mass entirely into electromagnetic energy (photons) with 100 percent efficiency, as described by E = mc². Practical production of antimatter in significant quantities is not yet technologically feasible.

Is renewable energy truly inexhaustible?

Renewable energy sources are replenished on human timescales by ongoing natural processes — solar radiation, wind, and water cycles — and are effectively inexhaustible relative to human civilisation's likely lifespan. However, individual installations have finite lifetimes and specific geographic and material constraints.