The following outline is provided as an overview of and topical guide to energy: Energy – in physics, this is an indirectly observed quantity often understood as the ability of a physical system to do work on other physical systems. Since work is defined as a force acting through a distance (a length of space), energy is always equivalent to the ability to exert force (a pull or a push) against an object that is moving along a definite path of certain length. Forms of energy Chemical energy – energy contained in molecules Electrical energy – energy from electric fields Electro-centric energy – energy sustaining the continuous motion of free electrons. Gravitational energy – energy from gravitational fields Ionization energy – energy that binds an electron to its atom or molecule Kinetic energy – (), energy of the motion of a body Magnetic energy – energy from magnetic fields Mechanical energy – The sum of (usually macroscopic) kinetic and potential energies Mechanical wave – (), a form of mechanical energy propagated by a material's oscillations Nuclear binding energy – energy that binds nucleons to form the atomic nucleus Potential energy – energy possessed by a body by virtue of its position relative to others, stresses within itself, electric charge, and other factors. Elastic energy – energy of deformation of a material (or its container) exhibiting a restorative force Gravitational energy – potential energy associated with a gravitational field. Nuclear potential energy Radiant energy – (), energy of electromagnetic radiation including light and of gravitational radiation Rest energy – () given by , where is an object's rest mass Surface energy Thermal energy – a microscopic, disordered equivalent of mechanical energy Heat – an amount of thermal energy being transferred (in a given process) in the direction of decreasing temperature Work (physics) – an amount of energy being energy transferred in a given process due to displacement in the direction of an applied force Measurement Units List of common units for energy. Official or common symbol in brackets after name and exact or approximate value of unit in joule in brackets after description. SI unit Joule (J) – the SI-unit for energy. Also called newton meter, watt second, or coulomb volt. Other metric units Kilowatt-hour (kW·h) – corresponds to one kilowatt of power being used over a period of one hour (). Calorie (cal) – equal to the energy need to