If the
velocitiesIn physics, velocity is the rate of change of position. It is a vector physical quantity; both speed and direction are required to define it. In the SI system, it is measured in meters per second: or ms-1. The scalar absolute value of velocity is speed...
of a group of
electronAn electron is a subatomic particle that carries a negative electric charge. It has no known substructure and is believed to be a point particle. An electron has a mass that is approximately 1836 times less than that of the proton. The intrinsic angular momentum of the electron is a half integer...
s, e.g. in a
plasmaIn physics and chemistry, plasma is a partially ionized gas, in which a certain proportion of electrons are free rather than being bound to an atom or molecule. The ability of the positive and negative charges to move somewhat independently makes the plasma electrically conductive so that it...
, follow a Maxwell-Boltzmann distribution, then the
electron temperature is well-defined as the
temperatureIn physics, temperature is a physical property of a system that underlies the common notions of hot and cold; something that feels hotter generally has the higher temperature. Temperature is one of the principal parameters of thermodynamics...
of that distribution. For other distributions, two-thirds of the average energy is often referred to as the temperature, since for a Maxwell-Boltzmann distribution with three
degrees of freedomDegrees of freedom is a general term used in explaining dependence on parameters, and implying the possibility of counting the number of those parameters...
, .
The
SIThe International System of Units is the modern form of the metric system and is generally a system devised around the convenience of the number ten. It is the world's most widely used system of measurement, both in everyday commerce and in science.The older metric system included several groups...
unit of temperature is the
kelvinThe kelvin is a unit increment of temperature and is one of the seven SI base units. The Kelvin scale is a thermodynamic temperature scale where absolute zero, the theoretical absence of all thermal energy, is zero kelvin...
(K), but using the above relation the electron temperature is often expressed in terms of the energy unit
electronvoltIn physics, the electron volt is a unit of energy. By definition, it is equal to the amount of kinetic energy gained by a single unbound electron when it accelerates through an electrostatic potential difference of one volt...
(eV).
If the
velocitiesIn physics, velocity is the rate of change of position. It is a vector physical quantity; both speed and direction are required to define it. In the SI system, it is measured in meters per second: or ms-1. The scalar absolute value of velocity is speed...
of a group of
electronAn electron is a subatomic particle that carries a negative electric charge. It has no known substructure and is believed to be a point particle. An electron has a mass that is approximately 1836 times less than that of the proton. The intrinsic angular momentum of the electron is a half integer...
s, e.g. in a
plasmaIn physics and chemistry, plasma is a partially ionized gas, in which a certain proportion of electrons are free rather than being bound to an atom or molecule. The ability of the positive and negative charges to move somewhat independently makes the plasma electrically conductive so that it...
, follow a Maxwell-Boltzmann distribution, then the
electron temperature is well-defined as the
temperatureIn physics, temperature is a physical property of a system that underlies the common notions of hot and cold; something that feels hotter generally has the higher temperature. Temperature is one of the principal parameters of thermodynamics...
of that distribution. For other distributions, two-thirds of the average energy is often referred to as the temperature, since for a Maxwell-Boltzmann distribution with three
degrees of freedomDegrees of freedom is a general term used in explaining dependence on parameters, and implying the possibility of counting the number of those parameters...
, .
The
SIThe International System of Units is the modern form of the metric system and is generally a system devised around the convenience of the number ten. It is the world's most widely used system of measurement, both in everyday commerce and in science.The older metric system included several groups...
unit of temperature is the
kelvinThe kelvin is a unit increment of temperature and is one of the seven SI base units. The Kelvin scale is a thermodynamic temperature scale where absolute zero, the theoretical absence of all thermal energy, is zero kelvin...
(K), but using the above relation the electron temperature is often expressed in terms of the energy unit
electronvoltIn physics, the electron volt is a unit of energy. By definition, it is equal to the amount of kinetic energy gained by a single unbound electron when it accelerates through an electrostatic potential difference of one volt...
(eV). 1 K corresponds to 8.617343(15)×10
-5 eV; this factor is the ratio of the
Boltzmann constantThe Boltzmann constant is the physical constant relating energy at the particle level with temperature observed at the bulk level. It is the gas constant R divided by the Avogadro constant N
A:...
to the
elementary chargeThe elementary charge, usually denoted e, is the electric charge carried by a single proton, or equivalently, the negative of the electric charge carried by a single electron. This is a fundamental physical constant. To avoid confusion over its sign, e is sometimes called the "elementary positive...
.
The electron temperature of a plasma can be several orders of magnitude higher than the temperature of the neutral species or of the
ionAn ion is an atom or molecule where the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge...
s. This is a result of two facts. First, many
plasma sourcePlasma sources generate plasmas.Excitation of a plasma requires partial ionization of neutral atoms and/or molecules of a medium.There are several ways to cause ionization:collisions of energetic particles, strong electric fields acting on bond...
s heat the electrons more strongly than the ions. Second, atoms and ions are much heavier than electrons, and energy transfer in a two-body
collisionA collision is an isolated event in which two or more moving bodies exert relatively strong forces on each other for a relatively short time.-Dynamics:Collisions involve forces...
is much more efficient if the masses are similar.