Specific Energy

Specific energy is defined as the energy per unit mass. Common metric units are J/kg. It is an intensive property. Contrast this with energy, which is an extensive property. There are two main types of specific energy: potential energy and specific kinetic energy. Others are the gray and sievert, measures for the absorption of radiation. The concept of specific energy applies to a particular or theoretical way of extracting useful energy from the material considered that is usually implied by context.

Thermodynamic properties related to specific energy include specific internal energy, specific enthalpy, specific Gibbs free energy, and specific Helmholtz free energy, all of which use units of energy per mass such as J/kg. These intensive properties are each symbolized by using the lower case letter of the symbol for the corresponding extensive property, which is symbolized by a capital letter. For example, the extensive thermodynamic property enthalpy is symbolized by H; specific enthalpy is symbolized by h.

If a defined chemical compound is used which has a definite molar mass, such intensive thermodynamic properties can be expressed on a per mole basis instead of a per mass basis. Such quantities can be described as molar quantities; for example, molar enthalpy meaning enthalpy per mole. These intensive quantities will use units of energy per mole, such as J/mol (mol is short for mole) or in older chemistry literature kcal/mol.

By dividing by 3.6 the figures for megajoules per kilogram can be converted to kilowatt-hours per kilogram. Unfortunately, the useful energy available by extraction from an energy store is always less than the energy put into the energy store, as explained by the laws of thermodynamics.