## 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.m ^{2}/s^{2}**

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.

## Electronvolt (unit: eV)

**Electronvolt (unit: eV)**. Electronvolts are a traditional unit of energy particularly in atomic and nuclear physics. 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**. Since this is very small unit, it is more convenient to use multiples of electronvolts: kilo-electronvolts (keV), mega-electronvolts (MeV), giga-electronvolts (GeV) and so on. Since Albert Einstein showed that mass and energy are

**equivalent and convertible**one into the other, the electronvolt is also a unit of mass. It is common in particle physics, where units of mass and energy are often interchanged, to express mass in units of eV/c

^{2}, where c is the speed of light in vacuum (from E = mc

^{2}). For example, it can be said the

**proton**has mass of

**938.3 MeV**, although strictly speaking it should be

**938.3 MeV/c**. For another example, an electron–positron annihilation occurs when a negatively charged electron and a positively charged positron (each with a mass of 0.511 MeV/c

^{2}^{2}) collide. When an electron and a positron collide, they annihilate resulting in the complete conversion of their rest mass to pure energy (according to the E=mc

^{2}formula) in the form of two oppositely directed 0.511 MeV gamma rays (photons).

**e**^{−}** + e**^{+}** → γ + γ (2x 0.511 MeV)**

- 1 eV = 1.603 x 10
^{-19}J - 1 eV = 3.83 x 10
^{-20}cal - 1 eV = 1.52 x 10
^{-22}BTU

**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 +
^{238}U reaction is**at 6.67 eV**(energy of incident neutron), which corresponds to the first**virtual level in**^{239}**U**, has a total width of only 0.027 eV, and the mean life of this state is 2.4×10^{-14}s. - 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 thedecay chain,^{232}U**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^{235}U 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**.