equalling , sometimes written capitalized as . In the European Union, food energy labeling in joules is mandatory, often with calories as supplementary information. Atom physics and chemistry In physics and chemistry, it is common to measure energy on the atomic scale in the non-SI, but convenient, units electronvolts (eV). 1 eV is equivalent to the kinetic energy acquired by an electron in passing through a potential difference of 1 volt in a vacuum. It is common to use the SI magnitude prefixes (eg milli- , mega- etc) with electronvolts. Because of the relativistic equivalence between mass and energy, the eV is also sometimes used as a unit of mass. The Hartree (the atomic unit of energy) is commonly used in calculations. Historically Rydberg units have been used. Spectroscopy In spectroscopy and related fields it is common to measure energy levels in units of reciprocal centimetres. These units (cm−1) are strictly speaking not energy units but units proportional to energies, with being the proportionality constant. Explosions A gram of TNT releases upon explosion. To define the tonne of TNT, this was standardized to giving a value of for the tonne of TNT. See also Energy consumption Conversion of units of temperature Conversion of units of energy, work, or amount of heat Kilokaiser List of unusual units of measurement Maximum demand indicator Orders of magnitude (energy) erg Foe (unit) References Conversion of units of measurement