Example of Energies in Electronvolts

Energy Units

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

Electronvolt - definition
Electronvolt is equal to energy gained by a single electron when it is accelerated through 1 volt of electric potential difference. The work done on the charge is given by the charge times the voltage difference, therefore the work W on electron is: W = qV = (1.6 x 10-19 C) x (1 J/C) = 1.6 x 10-19 J.
  • 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.

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