The Electron-Volt

The electron volt is a unit of energy useful in the physics of elementary charges and electricity.

Learning Objective

Convert between electron volts and SI units of energy

Key Points

The electron volt is defined as the amount of energy gained or lost by the charge of an electron moved across a one-volt electric potential difference. Its value is approximately equal to 1.602×10^-19 J.
The electron volt became useful through experimentation. Scientists working with electrostatic particle accelerators commonly used the relationship between energy (E), charge (q), and potential difference (V) in their work. This relationship is: E=qV.
As an energy, the electron volt can be used in many calculations, including momentum, mass, wavelength, and temperature.

Terms

particle accelerator
A device that accelerates electrically charged particles to extremely high speeds, for the purpose of inducing high-energy reactions or producing high-energy radiation.
potential difference
The difference in potential energy between two points in an electric field; the difference in charge between two points in an electrical circuit; voltage.
electron volt
A unit for measuring the energy of subatomic particles; the energy equal to that attained by an electron moving through a potential difference of one volt. Equivalent to 1.6022 x 10-19 joules.

Full Text

Overview

The electron volt, symbolized as eV and sometimes written as electronvolt, is a unit of energy useful in the physics of elementary charges and electricity.

The electron volt is defined as the amount of energy gained or lost by the charge of an electron moved across a one-volt electric potential difference. As such, it is equal to the product of one volt (1 J/C) and one elementary charge, giving it a value in joules approximately equal to 1.602×10^-19 J.

Not an SI unit in itself, the electron volt became useful through experimentation. Scientists working with electrostatic particle accelerators commonly used the relationship between energy (E), charge (q), and potential difference (V) in their work:

$E=qV$

All calculations of energy from the above equation were quantized as multiples of the elementary charge, q, for a given voltage, and thus arose the common usage of the electron volt as a unit of measurement.

Momentum

Both electron volts and momentum are measures of energy, and the two are related in high-energy physics. Applying a potential difference to an electron gives it energy, which manifests itself in motion of the electron through it. Given that the electron has both mass and velocity, it has momentum. Dividing electron volts by a constant with units of velocity results in a momentum.

Mass

Given that mass is equivalent to energy, the electron volt can measure mass. In particle physics, the equation E=mc² can be rearranged to solve for mass:

$m=\frac {E}{c^2}$

Wavelength

The energy E, frequency v, and wavelength λ of a photon are related by

$E (eV) = hv = \frac{hc}{\lambda }$

where h is the Planck constant and c is the speed of light. Thus, a photon with a wavelength of 532 nm (green light) would have an energy of approximately 2.33 eV. Similarly, 1 eV would correspond to an infrared photon of wavelength 1240 nm, and so on .

Energy of Photons in the Visible Spectrum

Relationship between wavelength and energy expressed in electron volts.

Temperature

In plasma physics, the electron volt can be used as a unit of temperature. To convert to Kelvins, simply divide the value of 1 eV (in Joules) by the Boltzmann constant (1.3806505(24)×10^-23 J/K).

[ edit ]

Prev Concept

Energy Conservation

Dipole Moments

Next Concept