Energy is generally defined as the potential to do work or produce heat. This definition causes the SI unit for energy is the same as the unit of work – the joule (J). Joule is a derived unit of energy and it is named in honor of James Prescott Joule and his experiments on the mechanical equivalent of heat. In more fundamental terms, 1 joule is equal to:
1 J = 1 kg.m2/s2
Since energy is a fundamental physical quantity and it is used in various physical and engineering branches, there are many energy units in physics and engineering.
Example of Energies in Electronvolts
- Thermal neutrons are neutrons in thermal equilibrium with a surrounding medium of temperature 290K (17 °C or 62 °F). Most probable energy at 17°C (62°F) for Maxwellian distribution is 0.025 eV (~2 km/s).
- Thermal energy of a molecule is at room temperature about 0.04 eV.
- Approximately 1 eV corresponds to an infrared photon of wavelength 1240 nm.
- Visible light photons have energies in range 1.65 eV (red) – 3.26 eV (violet).
- The first resonance in n + 238U reaction is at 6.67 eV (energy of incident neutron), which corresponds to the first virtual level in 239U, has a total width of only 0.027 eV, and the mean life of this state is 2.4×10-14s.
- Ionization energy of atomic hydrogen is 13.6 eV.
- Carbon-14 decays into nitrogen-14 through beta decay (pure beta decay). The emitted beta particles have a maximum energy of 156 keV, while their weighted mean energy is 49 keV.
- High energy diagnostic medical x-ray photons have kinetic energies of about 200 keV.
- Thallium 208, which is one of nuclides in the 232U decay chain, emits gamma rays of 2.6 MeV which are very energetic and highly penetrating.
- Typical kinetic energy of alpha particle from radioactive decay is about 5 MeV. It is caused by the mechanism of their production.
- The total energy released in a reactor is about 210 MeV per 235U fission, distributed as shown in the table. In a reactor, the average recoverable energy per fission is about 200 MeV, being the total energy minus the energy of the energy of antineutrinos that are radiated away.
- Cosmic ray can have energies of 1 MeV – 1000 TeV.