Energy is a physical quantity representing the capacity for a system to undergo change or, more precisely in modern physics, a conserved quantity associated with the dynamics and symmetries of a physical system. It is measured in joules in the SI system. Energy appears in many forms or descriptions, including kinetic, potential, thermal, chemical, electrical, nuclear and radiant energy.
These categories are often useful for tracking energy transfers and transformations, but energy itself is not a material substance that is consumed in the ordinary sense. The law of conservation of energy states that the total energy of an isolated system remains constant, although energy can move between systems or change from one form to another. Energy is therefore central to mechanics, thermodynamics, electromagnetism, chemistry, nuclear physics, astrophysics and engineering.
The concept of energy developed historically from the study of mechanics, heat, work and machines. The term ultimately derives from the Greek energeia, associated with activity or operation, through later scientific usage. During the nineteenth century, investigations into heat, mechanical work and physical transformations led to the development of the modern conservation principle and the concept of mechanical equivalent of heat. Scientists including James Prescott Joule demonstrated experimentally the quantitative relationship between mechanical work and thermal effects. Hermann von Helmholtz and others contributed to broader formulations of conservation. The modern concept extends beyond nineteenth-century mechanical energy and is embedded in field theories and quantum physics, where energy is associated with particles, fields, radiation and interactions.
Energy can be transferred by mechanisms including mechanical work, heat transfer and radiation, depending on the physical framework and system definition. Work transfers energy through organised macroscopic forces and displacements, while heat is energy transferred because of a temperature difference in thermodynamic descriptions. Electrical circuits transfer energy through electromagnetic processes, and electromagnetic radiation transports energy through space.
Chemical reactions redistribute energy among chemical bonds and other degrees of freedom, while nuclear reactions can release substantial energy through changes in nuclear binding and mass-energy relationships. Distinguishing energy storage from energy transfer is useful: a battery can store energy in chemical form, whereas electrical work can transfer energy from the battery to an external circuit.
Kinetic energy is the energy associated with motion and, in classical mechanics, is given by K = ½mv². Potential energy describes energy associated with configuration or position within a conservative interaction, such as gravitational or elastic potential energy. Internal energy encompasses microscopic kinetic and potential contributions associated with the internal state of a system. Thermal energy is commonly used to describe energy associated with microscopic degrees of freedom, although precise thermodynamic terminology distinguishes internal energy, heat and temperature. Chemical and nuclear energy are convenient descriptions of energy changes associated with molecular and nuclear configurations. These categories should not be interpreted as mutually exclusive substances; they are accounting descriptions that identify where energy resides or how it is represented within a system.
The conservation of energy does not mean that every individual form of energy remains constant. A falling object can lose gravitational potential energy while gaining kinetic energy. A moving vehicle can lose mechanical energy through friction while transferring energy to its surroundings as internal energy and sound. A battery can undergo chemical changes while delivering electrical energy to a circuit. In a power station, energy transformations can involve chemical, thermal, mechanical and electrical stages. The total energy remains accounted for when the system boundary includes all relevant transfers. Apparent energy losses usually represent energy transferred into forms that are more difficult to recover for the intended purpose, rather than energy disappearing from physical existence.
Energy has a particularly important relationship with power, efficiency and work. Power is the rate of energy transfer or conversion, measured in watts. Efficiency is the ratio of useful output energy to input energy, often expressed as a percentage, with real processes generally producing less useful output than total input because energy is distributed among other forms.
A machine that converts 1,000 joules of input energy into 700 joules of desired output has an efficiency of 70 percent, with the remaining energy transferred into other forms within the chosen system boundary. High efficiency therefore means achieving a desired transformation with a smaller fraction of input energy diverted into unwanted outputs; it does not mean violating conservation of energy.
In modern physics, energy is linked to time-translation symmetry through Noether's theorem in systems where the relevant physical laws possess that symmetry. Special relativity establishes mass-energy equivalence through E = mc², meaning that mass contributes to the energy of a system and can participate in energy transformations. A massive object's rest energy is enormous compared with ordinary chemical energy scales. Quantum mechanics also assigns discrete energy levels to bound systems and establishes relationships between energy and frequency for photons through E = hf. In general relativity and cosmology, the definition and global conservation of energy can become more subtle because curved spacetime may lack a global time-translation symmetry.
Misconceptions about energy include treating it as a substance that is literally used up, assuming that energy conservation guarantees that all energy remains equally useful, and confusing energy with power. Energy describes an amount associated with a physical system or transfer, whereas power describes a rate. Energy can become dispersed into microscopic degrees of freedom, making complete recovery impractical even though conservation still holds. The usefulness of energy therefore depends on its form, concentration, temperature difference, accessibility and conversion pathway. Understanding system boundaries is also essential because statements about where energy went depend on whether the surroundings are included. Energy is consequently both a precise physical quantity and a powerful accounting framework for analysing transformations across many areas of science and technology.
