Energy is a basic quantity in physics. It describes how much a physical system can do work on another system or heat it. Energy shows up in many forms, including motion, position in a force field, chemical bonds, atomic nuclei, radiation and heat. It can pass from one system to another and change from one form to another. In an isolated system, though, the total amount stays the same. This rule, the conservation of energy, is one of the most firmly established principles in science. It connects classical mechanics, thermodynamics, electromagnetism, chemistry and biology. In the International System of Units, energy is measured in joules. One joule is the work done when the point of application of a force of 1 newton is displaced through a distance of 1 metre in the direction of the force.
Etymology and history
The word 'energy' originates from the Greek energhéia, a concept Aristotle linked with the idea of hypothetical entities becoming real. For Aristotle, energeia was a qualitative philosophical concept, broad enough to include ideas such as happiness and pleasure.
The quantitative idea came out of debates in early modern mechanics. Proposed by Gottfried Leibniz over the period 1676–1689, the concept of vis viva ("living force") was defined as the product of the mass of an object and its velocity squared; he believed that total vis viva was conserved. Leibniz's idea was controversial as it seemed to oppose the theory of conservation of quantity of motion advocated by René Descartes. Its modern equivalent, kinetic energy, differs from vis viva only by a factor of two. Thomas Young gave a lecture on collisions in 1807, and in it he linked the word "energy" to this quantity, apparently for the first time. The related term "work" was defined in 1828/29 by Gustave Gaspard de Coriolis and Jean-Victor Poncelet. In 1851 − 1852, William Thomson (Lord Kelvin) and William J. M. Rankine began to use the word "energy" to denote any kind of "force" across all branches of science.
The full conservation principle needed one more step: showing that heat is a form of energy. The older caloric theory had treated heat as a substance that could not be created or destroyed. conservation of energy entails the contrary principle that heat and mechanical work are interchangeable. In James Prescott Joule's best-known experiment, a descending weight attached to a string caused a paddle immersed in water to rotate. He showed that the gravitational potential energy lost by the weight in descending was equal to the thermal energy (heat) gained by the water. Julius Robert Mayer and others reached similar results independently, which makes this a classic case of "simultaneous discovery". A clear general statement came with Über die Erhaltung der Kraft (On the Conservation of Force), a most important memoir of 1847 written by the 26-year-old German physician and physicist Hermann von Helmholtz. Then in 1905, Albert Einstein established the general equivalence of energy and mass with his theory of relativity.
Forms of energy
Physicists divide energy into kinetic energy, which comes from motion, and potential energy, which comes from position or arrangement within a force field. Potential energy includes:
- gravitational potential energy, linked to gravity
- elastic potential energy
- electric potential energy, linked to electricity
Many of the familiar "forms" are combinations of these at the microscopic level:
- Thermal energy is the random kinetic and potential energy of atoms and molecules. Statistical mechanics studies it.
- Chemical energy is stored in chemical bonds and released or absorbed in chemical reactions.
- Nuclear energy comes from the binding of protons and neutrons in the atomic nucleus, as described by nuclear physics.
- Radiant energy is carried by electromagnetic waves such as light. In quantum mechanics, radiant energy comes in discrete packets called photons.
Special relativity adds rest energy, expressed by E = mc². This means mass itself is a form of energy.
Conservation and its limits
As one summary puts it, energy can be converted from one form to another, for example from chemical energy in gasoline to thermal energy and kinetic energy in an automobile engine, but the total amount of energy remains unchanged. No violations of this principle have ever been found. In thermodynamics, the first law is a statement of conservation of energy. The second law adds a limit: in any real conversion, part of the energy ends up as less useful heat, and entropy increases. For this reason no heat engine can turn heat completely into work.
The theory behind conservation was deepened in the 20th century. In 1918 Noether generalized results like Lagrange's and proved a converse: symmetries imply conservation laws and vice versa. In this framework, Noether's theorem links conservation of energy to the fact that physical laws do not change over time. The same framework explains why energy is hard to define globally in some settings. Einstein's General Relativity, which stimulated Noether's work, is another source of difficulty for conservation laws. This difficulty matters in an expanding universe, which cosmology studies.
Energy in living systems
Living things are open systems: they need a constant flow of energy to stay organised. In photosynthesis, plants and other autotrophs capture solar radiation and store it as chemical energy in sugars. Cellular respiration then releases this energy and stores it in ATP, which powers growth, movement and homeostasis. As energy passes up the food chains of an ecosystem, most of it is lost as heat at each step. This loss limits how many levels a food chain can have. In human nutrition, the energy in food is usually given in kilocalories or kilojoules.
Energy resources and society
In everyday and economic language, "energy" often means the resources that supply useful work: fuels, electricity and heat. Mechanised energy use expanded sharply during the Industrial Revolution, when the steam engine turned the chemical energy of coal into mechanical work.
According to the Energy Institute's Statistical Review of World Energy, in 2025, total energy supply, as measured in exajoules (EJ), rose by 1.7% over 2024 to just over 600 EJ. Fossil fuels retained their dominant position, accounting for 86% of global energy supply. Other sources made up the rest. Renewables represented 5.9% of the total primary energy consumption, hydropower 2.7%, and nuclear power 5.2%. These numbers are for primary energy. Analysts point out that much fossil fuel energy never becomes useful work, because roughly two-thirds of their energy is lost during combustion. Electricity is a growing share of energy use. Electricity demand rose 3% in 2025 over 2024 levels, primarily due to growth in electric vehicles and AI data centers.
Burning fossil fuels releases carbon dioxide, which links energy production to climate change. This connection drives investment in renewable energy, nuclear power and energy efficiency. "Energy conservation" in this sense means cutting energy demand. It should not be confused with the physical law of the same name.
References
- JOULE Definition & Meaningdictionary.com
- The history of the concept of energy and workspark.iop.org
- Who was the first to use the word "energy"?quizzclub.com
- Vis vivaen.wikipedia.org
- Energyen.wikipedia.org
- What does energy really mean?physicsworld.com
- Historical Development of the Word "Energy"home.uni-leipzig.de
- Conservation of energy - New World Encyclopedianewworldencyclopedia.org
- The Source of Solar Energy, ca. 1840-1910: From Meteoric Hypothesis to Radioactive Speculationsarxiv.org
- Conservation of Energy - an overviewsciencedirect.com
- Conservation of Energy: Missing Features in Its Nature and Justification and Why They Matterncbi.nlm.nih.gov
- Renewables Supplied Just 6% of Global Primary Energy in 2025clintel.org
- Fossil Fuels Dominate Global Energy Supply in 2025 - IERinstituteforenergyresearch.org
- news: Energy Institute releases 2026 Statistical Review of World Energydieselnet.com
- Six charts show how clean power was world’s largest source of new energy in 2025 - Carbon Briefcarbonbrief.org