Energy is a measure of the ability to do work – for example, to lift a body against gravity or drag it against friction, or to accelerate an object.


In another sense, energy is an intrinsic property of everything in the universe, including radiation, matter (see mass-energy relationship), and, strangely enough, even empty space (see quantum vacuum). See also conservation of mass and energy.


Energy exists in a number of equivalent forms. The commonest of these is heat – the motion of the molecules of matter. Ultimately all other forms of energy tend to convert into thermal motion. Another form of energy is the motion of electrons as electricity. Moving electrons give rise to electromagnetic fields and these too contain energy. A pure form of electromagnetic energy is electromagnetic radiation (radiant energy) such as light. According to quantum mechanics, the energy of electromagnetic energy is quantized, referable to discrete units called photons.


When macroscopic bodies move, they too have energy by virtue of their motion; this their kinetic energy and is given by ½mv2 where m is the mass and v the velocity of motion. To change the velocity of a moving body, or to set it in motion, a force must be applied to it and work must be done. This work is equivalent to the change in the kinetic energy of the body and gave physicists one of their earliest definitions of energy: the ability to do work. When work is done against a restraining force, potential energy is stored in the system, ready to be released again. The restraining force may be electromagnetic, torsional, electrostatic, tensional or any other type. On Earth when an object of mass m is raised up to a height h, its gravitational potential energy is given by mgh, where g is the acceleration due to gravity. If the object is let go, it falls and will strike the ground with velocity v, its potential energy having been converted into kinetic energy ½mv 2.


Sound energy is kinetic energy of the vibration of air. Chemical energy is the energy released from a chemical system in the course of a reaction. Although all forms of energy are equivalent, not all interconversion processes go with 100% efficiency (the energy deficit always appears as heat).


The SI unit of energy is the joule.