Electron temperature
Encyclopedia
If the velocities
of a group of electron
s, e.g., in a plasma
, follow a Maxwell-Boltzmann distribution, then the electron temperature is well-defined as the temperature
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 freedom
,
.
The SI
unit of temperature is the kelvin
(K), but using the above relation the electron temperature is often expressed in terms of the energy unit electronvolt
(eV). Each kelvin (1 K) corresponds to 8.617343(15)×10−5 eV; this factor is the ratio of the Boltzmann constant to the elementary charge
.
The electron temperature of a plasma can be several orders of magnitude higher than the temperature of the neutral species or of the ion
s. This is a result of two facts. Firstly, many plasma source
s heat the electrons more strongly than the ions. Secondly, atoms and ions are much heavier than electrons, and energy transfer in a two-body collision
is much more efficient if the masses are similar.
Velocity
In physics, velocity is speed in a given direction. Speed describes only how fast an object is moving, whereas velocity gives both the speed and direction of the object's motion. To have a constant velocity, an object must have a constant speed and motion in a constant direction. Constant ...
of a group of electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s, e.g., in a plasma
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...
, follow a Maxwell-Boltzmann distribution, then the electron temperature is well-defined as the temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
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 freedom
Degrees of freedom (physics and chemistry)
A degree of freedom is an independent physical parameter, often called a dimension, in the formal description of the state of a physical system...
,
.The SI
International System of Units
The International System of Units is the modern form of the metric system and is generally a system of units of measurement devised around seven base units and the convenience of the number ten. The older metric system included several groups of units...
unit of temperature is the kelvin
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
(K), but using the above relation the electron temperature is often expressed in terms of the energy unit electronvolt
Electronvolt
In physics, the electron volt is a unit of energy equal to approximately joule . By definition, it is equal to the amount of kinetic energy gained by a single unbound electron when it accelerates through an electric potential difference of one volt...
(eV). Each kelvin (1 K) corresponds to 8.617343(15)×10−5 eV; this factor is the ratio of the Boltzmann constant to the elementary charge
Elementary charge
The elementary charge, usually denoted as e, is the electric charge carried by a single proton, or equivalently, the absolute value of the electric charge carried by a single electron. This elementary charge is a fundamental physical constant. To avoid confusion over its sign, e is sometimes called...
.
The electron temperature of a plasma can be several orders of magnitude higher than the temperature of the neutral species or of the ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
s. This is a result of two facts. Firstly, many plasma source
Plasma source
Plasma 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. Secondly, atoms and ions are much heavier than electrons, and energy transfer in a two-body collision
Collision
A collision is an isolated event which two or more moving bodies exert forces on each other for a relatively short time.Although the most common colloquial use of the word "collision" refers to accidents in which two or more objects collide, the scientific use of the word "collision" implies...
is much more efficient if the masses are similar.