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Townsend (unit)
The townsend (symbol Td) is a physical unit of the reduced electric field ( ratio ''E''/''N''), where E is electric field and N is concentration of neutral particles. It is named after John Sealy Townsend, who conducted early research into gas ionisation. Definition It is defined by the relation 1\ \text = 10^\ \text \cdot \text^2 = 10^\ \text \cdot \text^2. For example, an electric field of E = 2.5 \cdot 10^4\ \text/\text in a medium with the density of an ideal gas at 1 atm and 0 °C, the Loschmidt constant n_0 = 2.6867811 \cdot 10^\ \text^ gives E/n_0 \approx 10^\ \text \cdot \text^2, which corresponds to 1 Td. Uses This unit is important in gas-discharge physics, where it serves as scaling parameter because the mean energy of electrons (and therefore many other properties of discharge) is typically a function of E/N over broad range of E and N. The concentration N, which is in ideal gas simply related to pressure and temperature, controls the mean fre ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
E/N Ratio
Electric discharge in gases occurs when electric current flows through a gaseous medium due to ionization of the gas. Depending on several factors, the discharge may radiate visible light. The properties of electric discharges in gases are studied in connection with design of lighting sources and in the design of high voltage electrical equipment. Discharge types In cold cathode tubes, the electric discharge in gas has three regions, with distinct current–voltage characteristics:Reference Data for Engineers: Radio, Electronics, Computers and Communications By Wendy Middleton, Mac E. Van Valkenburg, p. 16-42, Newnes, 2002 *I: |
Electric Field
An electric field (sometimes called E-field) is a field (physics), physical field that surrounds electrically charged particles such as electrons. In classical electromagnetism, the electric field of a single charge (or group of charges) describes their capacity to exert attractive or repulsive forces on another charged object. Charged particles exert attractive forces on each other when the sign of their charges are opposite, one being positive while the other is negative, and repel each other when the signs of the charges are the same. Because these forces are exerted mutually, two charges must be present for the forces to take place. These forces are described by Coulomb's law, which says that the greater the magnitude of the charges, the greater the force, and the greater the distance between them, the weaker the force. Informally, the greater the charge of an object, the stronger its electric field. Similarly, an electric field is stronger nearer charged objects and weaker f ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
John Sealy Townsend
Sir John Sealy Edward Townsend (; 7 June 1868 – 16 February 1957) was an Irish physicist who conducted various studies concerning the electrical conduction of gases (concerning the kinetics of electrons and ions) and directly measured the electric charge. He was a Wykeham Professor of Physics at Oxford University. The phenomenon of the electron avalanche was discovered by him, and is known as the Townsend discharge. Career John Townsend was born at Galway in County Galway, Ireland, son of Edward Townsend, a Professor of Civil Engineering at Queen's College Galway and Judith Townsend. In 1885, he entered Trinity College Dublin, was elected a Scholar of the College in 1888, and came top of the class in mathematics with a BA in 1890. He became a Clerk Maxwell Scholar and entered Trinity College, Cambridge, where he became a research student at the same time as Ernest Rutherford. At the Cavendish laboratory, he studied under J. J. Thomson. He developed the "''Townsend's colli ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Loschmidt Constant
The Loschmidt constant or Loschmidt's number (symbol: ''n''0) is the number of particles (atoms or molecules) of an ideal gas per volume (the number density), and usually quoted at standard temperature and pressure. The 2018 CODATA recommended value is at 0 °C and 1 atm. It is named after the Austrian physicist Johann Josef Loschmidt, who was the first to estimate the physical size of molecules in 1865. The term Loschmidt constant is also sometimes used to refer to the Avogadro constant, particularly in German texts. By ideal gas law, p_0V = Nk_\textT_0, and since N = n_0 V, the Loschmidt constant is given by the relationship : n_0 = \frac, where ''k''B is the Boltzmann constant, ''p''0 is the standard pressure, and ''T''0 is the standard thermodynamic temperature. Since the Avogadro constant ''N''A satisfies R = N_\text k, the Loschmidt constant satisfies : n_0 = \frac, where ''R'' is the ideal gas constant. Being a measure of number density, the Loschmidt const ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Plasma Scaling
The parameters of plasmas, including their spatial and temporal extent, vary by many orders of magnitude. Nevertheless, there are significant similarities in the behaviors of apparently disparate plasmas. Understanding the scaling of plasma behavior is of more than theoretical value. It allows the results of laboratory experiments to be applied to larger natural or artificial plasmas of interest. The situation is similar to testing aircraft or studying natural turbulent flow in wind tunnels with smaller-scale models. Similarity transformations (also called similarity laws) help us work out how plasma properties change in order to retain the same characteristics. A necessary first step is to express the laws governing the system in a nondimensional form. The choice of nondimensional parameters is never unique, and it is usually only possible to achieve by choosing to ignore certain aspects of the system. One dimensionless parameter characterizing a plasma is the ratio of ion to ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Ideal Gas
An ideal gas is a theoretical gas composed of many randomly moving point particles that are not subject to interparticle interactions. The ideal gas concept is useful because it obeys the ideal gas law, a simplified equation of state, and is amenable to analysis under statistical mechanics. The requirement of zero interaction can often be relaxed if, for example, the interaction is perfectly elastic or regarded as point-like collisions. Under various conditions of temperature and pressure, many real gases behave qualitatively like an ideal gas where the gas molecules (or atoms for monatomic gas) play the role of the ideal particles. Many gases such as nitrogen, oxygen, hydrogen, noble gases, some heavier gases like carbon dioxide and mixtures such as air, can be treated as ideal gases within reasonable tolerances over a considerable parameter range around standard temperature and pressure. Generally, a gas behaves more like an ideal gas at higher temperature and lower ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Mean Free Path
In physics, mean free path is the average distance over which a moving particle (such as an atom, a molecule, or a photon) travels before substantially changing its direction or energy (or, in a specific context, other properties), typically as a result of one or more successive collisions with other particles. Scattering theory Imagine a beam of particles being shot through a target, and consider an infinitesimally thin slab of the target (see the figure). The atoms (or particles) that might stop a beam particle are shown in red. The magnitude of the mean free path depends on the characteristics of the system. Assuming that all the target particles are at rest but only the beam particle is moving, that gives an expression for the mean free path: :\ell = (\sigma n)^, where is the mean free path, is the number of target particles per unit volume, and is the effective cross-sectional area for collision. The area of the slab is , and its volume is . The typical number of s ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Collision Frequency
Collision frequency describes the rate of collisions between two atomic or molecular species in a given volume, per unit time. In an ideal gas, assuming that the species behave like hard spheres, the collision frequency between entities of species A and species B is: : Z = N_\text N_\text \sigma_\text \sqrt\frac, which has units of olumetime]−1. Here, * N_\text is the number of A molecules in the gas, * N_\text is the number of B molecules in the gas, * \sigma_\text is the collision cross section (physics), cross section, the "effective area" seen by two colliding molecules, simplified to \sigma_\text = \pi(r_\text+r_\text)^2 , where r_\text the radius of A and r_\text the radius of B. * k_\text is the Boltzmann constant, * T is the temperature, * \mu_\text is the reduced mass of the reactants A and B, \mu_\text = \frac Collision in diluted solution In the case of equal-size particles at a concentration n in a solution of viscosity Viscosity is a mea ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Electric Glow Discharge
A glow discharge is a plasma formed by the passage of electric current through a gas. It is often created by applying a voltage between two electrodes in a glass tube containing a low-pressure gas. When the voltage exceeds a value called the striking voltage, the gas ionization becomes self-sustaining, and the tube glows with a colored light. The color depends on the gas used. Glow discharges are used as a source of light in devices such as neon lights, cold cathode fluorescent lamps and plasma-screen televisions. Analyzing the light produced with spectroscopy can reveal information about the atomic interactions in the gas, so glow discharges are used in plasma physics and analytical chemistry. They are also used in the surface treatment technique called sputtering. Electrical conduction in gas Conduction in a gas requires charge carriers, which can be either electrons or ions. Charge carriers come from ionizing some of the gas molecules. In terms of current flow, glow ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Vacuum Arc
A vacuum arc can arise when the surfaces of metal electrodes in contact with a good vacuum begin to emit electrons either through heating ( thermionic emission) or in an electric field that is sufficient to cause field electron emission. Once initiated, a vacuum arc can persist, since the freed particles gain kinetic energy from the electric field, heating the metal surfaces through high-speed particle collisions. This process can create an incandescent cathode spot, which frees more particles, thereby sustaining the arc. At sufficiently high currents an incandescent anode spot may also be formed. Electric discharge in vacuum is important for certain types of vacuum tubes and for high-voltage vacuum switches. The thermionic vacuum arc (TVA) is a new type of plasma source, which generates a plasma containing ions with a directed energy. TVA discharges can be ignited in high-vacuum conditions between a heated cathode (electron gun) and an anode (tungsten crucible) containing the ma ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
