Electrochemical reactions are used by most mobile devices such as laptop computers and mobile phones to release energy from batteries. Liquid hydrocarbons (fuels such as gasoline, diesel and kerosene) are today the densest way known to economically store and transport chemical energy at a large scale (1 kg of diesel fuel burns with the oxygen contained in ≈15 kg of air). Chemical reactions are used by organisms to derive energy from food and by automobiles to derive energy from gasoline. Nuclear reactions take place in stars and nuclear power plants, both of which derive energy from the binding energy of nuclei. In order of the typical magnitude of the energy released, these types of reactions are: nuclear, chemical, electrochemical, and electrical. There are different types of energy stored in materials, and it takes a particular type of reaction to release each type of energy. A pressure gradient describes the potential to perform work on the surroundings by converting internal energy to work until equilibrium is reached. Likewise, the energy required to compress a gas to a certain volume may be determined by multiplying the difference between the gas pressure and the external pressure by the change in volume. For example, the energy density of a magnetic field may be expressed as and behaves like a physical pressure. In cosmological and other general relativistic contexts, however, the energy densities considered are those that correspond to the elements of the stress-energy tensor and therefore do include mass energy as well as energy densities associated with pressure.Įnergy per unit volume has the same physical units as pressure and in many situations is synonymous. Often only the useful or extractable energy is measured, which is to say that inaccessible energy (such as rest mass energy) is ignored. It is sometimes confused with energy per unit mass which is properly called massic energy or gravimetric energy density. In physics, energy density or volumic energy is the amount of energy stored in a given system or region of space per unit volume.

Water density kgl