Specific energy

Specific energy is energy per unit mass. (It is also sometimes called 'energy density,' though 'energy density' more precisely means energy per unit volume.) It is used to quantify, for example, stored heat and other thermodynamic properties of substances such as specific internal energy, specific enthalpy, specific Gibbs free energy, and specific Helmholtz free energy. It may also be used for the kinetic energy or potential energy of a body. Specific energy is an intensive property, whereas energy and mass are extensive properties. Specific energy is energy per unit mass. (It is also sometimes called 'energy density,' though 'energy density' more precisely means energy per unit volume.) It is used to quantify, for example, stored heat and other thermodynamic properties of substances such as specific internal energy, specific enthalpy, specific Gibbs free energy, and specific Helmholtz free energy. It may also be used for the kinetic energy or potential energy of a body. Specific energy is an intensive property, whereas energy and mass are extensive properties. The SI unit for specific energy is the joule per kilogram (J/kg). Other units still in use in some contexts are the kilocalorie per gram (Cal/g or kcal/g), mostly in food-related topics, watt hours per kilogram in the field of batteries, and the Imperial unit BTU per pound (BTU/lb), in some engineering and applied technical fields. The concept of specific energy is related to but distinct from the chemical notion of molar energy, that is energy per mole of a substance, which uses units of energy per mole, such as J/mol, kJ/mol, or the older (but still widely used) kcal/mol.