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Electrothermal Instability
__NOTOC__ The electrothermal instability (also known as ionization instability, non-equilibrium instability or Velikhov instability in the literature) is a magnetohydrodynamic (MHD) instability appearing in magnetized non-thermal plasmas used in MHD converters. It was first theoretically discovered in 1962 and experimentally measured into a MHD generator in 1963 by Evgeny Velikhov. Physical explanation and characteristics This instability is a turbulence of the electron gas in a non-equilibrium plasma (i.e. where the electron temperature Te is greatly higher than the overall gas temperature Tg). It arises when a magnetic field powerful enough is applied in such a plasma, reaching a critical Hall parameter βcr. Locally, the number of electrons and their temperature fluctuate (electron density and thermal velocity) as the electric current and the electric field. The Velikhov instability is a kind of ionization wave system, almost frozen in the two temperature gas. T ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Magnetoacoustic
A magnetosonic wave, also called a magnetoacoustic wave, is a linear magnetohydrodynamic (MHD) wave that is driven by thermal pressure, magnetic pressure, and magnetic tension. There are two types of magnetosonic waves, the ''fast'' magnetosonic wave and the ''slow'' magnetosonic wave. Both fast and slow magnetosonic waves are present in the solar corona providing an observational foundation for the technique for coronal plasma diagnostics, coronal seismology. Homogeneous plasma In an ideal homogeneous plasma of infinite extent, and in the absence of gravity, the fast and slow magnetosonic waves form, together with the Alfvén wave, the three basic linear MHD waves. Under the assumption of normal modes, namely that the linear perturbations of the physical quantities are of the form :f_1=\tilde_1 e^ (with the constant amplitude), a dispersion relation of the magnetosonic waves can be derived from the system of ideal MHD equations: :\omega^4 - k^2 \left(v_\mathrm^2+v_\mathrm^2\ri ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Electric Current
An electric current is a stream of charged particles, such as electrons or ions, moving through an electrical conductor or space. It is measured as the net rate of flow of electric charge through a surface or into a control volume. The moving particles are called charge carriers, which may be one of several types of particles, depending on the conductor. In electric circuits the charge carriers are often electrons moving through a wire. In semiconductors they can be electrons or holes. In an electrolyte the charge carriers are ions, while in plasma, an ionized gas, they are ions and electrons. The SI unit of electric current is the ampere, or ''amp'', which is the flow of electric charge across a surface at the rate of one coulomb per second. The ampere (symbol: A) is an SI base unit. Electric current is measured using a device called an ammeter. Electric currents create magnetic fields, which are used in motors, generators, inductors, and transformers. In ordinary con ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Coulomb
The coulomb (symbol: C) is the unit of electric charge in the International System of Units (SI). In the present version of the SI it is equal to the electric charge delivered by a 1 ampere constant current in 1 second and to elementary charges, , (about ). Name and history By 1878, the British Association for the Advancement of Science had defined the volt, ohm, and farad, but not the coulomb. In 1881, the International Electrical Congress, now the International Electrotechnical Commission (IEC), approved the volt as the unit for electromotive force, the ampere as the unit for electric current, and the coulomb as the unit of electric charge. At that time, the volt was defined as the potential difference .e., what is nowadays called the "voltage (difference)"across a conductor when a current of one ampere dissipates one watt of power. The coulomb (later "absolute coulomb" or "abcoulomb" for disambiguation) was part of the EMU system of units. The "international coulo ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Elementary Charge
The elementary charge, usually denoted by is the electric charge carried by a single proton or, equivalently, the magnitude of the negative electric charge carried by a single electron, which has charge −1 . This elementary charge is a fundamental physical constant. In the SI system of units, the value of the elementary charge is exactly defined as e = coulombs, or 160.2176634 zeptocoulombs (zC). Since the 2019 redefinition of SI base units, the seven SI base units are defined by seven fundamental physical constants, of which the elementary charge is one. In the centimetre–gram–second system of units (CGS), the corresponding quantity is . Robert A. Millikan and Harvey Fletcher's oil drop experiment first directly measured the magnitude of the elementary charge in 1909, differing from the modern accepted value by just 0.6%. Under assumptions of the then-disputed atomic theory, the elementary charge had also been indirectly inferred to ~3% accuracy from bla ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Gyroradius
The gyroradius (also known as radius of gyration, Larmor radius or cyclotron radius) is the radius of the circular motion of a charged particle in the presence of a uniform magnetic field. In SI units, the non-relativistic gyroradius is given by :r_ = \frac where m is the mass of the particle, v_ is the component of the velocity perpendicular to the direction of the magnetic field, q is the electric charge of the particle, and B is the strength of the magnetic field. The angular frequency of this circular motion is known as the gyrofrequency, or cyclotron frequency, and can be expressed as :\omega_ = \frac in units of radians/second. Variants It is often useful to give the gyrofrequency a sign with the definition :\omega_ = \frac or express it in units of hertz with :f_ = \frac. For electrons, this frequency can be reduced to :f_ = (2.8\times10^\,\mathrm/\mathrm)\times B. In cgs-units the gyroradius :r_ = \frac and the corresponding gyrofrequency :\omega_ = \frac include a factor ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Hall Effect
The Hall effect is the production of a voltage difference (the Hall voltage) across an electrical conductor that is transverse to an electric current in the conductor and to an applied magnetic field perpendicular to the current. It was discovered by Edwin Hall in 1879. A Hall effect can also occur across a void or hole in a semiconductor or metal plate, when current is injected via contacts that lie on the boundary or edge of the void or hole, and the charge flows outside the void or hole, in the metal or semiconductor. This Hall effect becomes observable in a perpendicular applied magnetic field across voltage contacts that lie on the boundary of the void on either side of a line connecting the current contacts. It exhibits apparent sign reversal in comparison to the standard "ordinary Hall effect" in the simply connected specimen, and depends only on the current injected from within the void. Superposition may also be realized in the Hall effect: first imagine the standard ... [...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, 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 discharge falls b ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Electrode
An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or air). Electrodes are essential parts of batteries that can consist of a variety of materials depending on the type of battery. The electrophore, invented by Johan Wilcke, was an early version of an electrode used to study static electricity. Anode and cathode in electrochemical cells Electrodes are an essential part of any battery. The first electrochemical battery made was devised by Alessandro Volta and was aptly named the Voltaic cell. This battery consisted of a stack of copper and zinc electrodes separated by brine-soaked paper disks. Due to fluctuation in the voltage provided by the voltaic cell it wasn't very practical. The first practical battery was invented in 1839 and named the Daniell cell after John Frederic Daniell. Still making use of the zinc–copper electrode combination. Since then many more batteries have be ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Electric Potential
The electric potential (also called the ''electric field potential'', potential drop, the electrostatic potential) is defined as the amount of work energy needed to move a unit of electric charge from a reference point to the specific point in an electric field. More precisely, it is the energy per unit charge for a test charge that is so small that the disturbance of the field under consideration is negligible. Furthermore, the motion across the field is supposed to proceed with negligible acceleration, so as to avoid the test charge acquiring kinetic energy or producing radiation. By definition, the electric potential at the reference point is zero units. Typically, the reference point is earth or a point at infinity, although any point can be used. In classical electrostatics, the electrostatic field is a vector quantity expressed as the gradient of the electrostatic potential, which is a scalar quantity denoted by or occasionally , equal to the electric potential energy o ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Voltage
Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge to move a test charge between the two points. In the International System of Units, the derived unit for voltage is named ''volt''. The voltage between points can be caused by the build-up of electric charge (e.g., a capacitor), and from an electromotive force (e.g., electromagnetic induction in generator, inductors, and transformers). On a macroscopic scale, a potential difference can be caused by electrochemical processes (e.g., cells and batteries), the pressure-induced piezoelectric effect, and the thermoelectric effect. A voltmeter can be used to measure the voltage between two points in a system. Often a common reference potential such as the ground of the system is used as one of the points. A voltage can represent either a source ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Gas-discharge Lamp
Gas-discharge lamps are a family of artificial light sources that generate light by sending an electric discharge through an ionized gas, a plasma. Typically, such lamps use a noble gas (argon, neon, krypton, and xenon) or a mixture of these gases. Some include additional substances, like mercury, sodium, and metal halides, which are vaporized during startup to become part of the gas mixture. Single ended self-starting lamps are insulated with a mica disc and contained in a borosilicate glass gas discharge tube (arc tube) and a metal cap. They include the sodium-vapor lamp that is the gas-discharge lamp in street lighting. In operation, some of the electrons are forced to leave the atoms of the gas near the anode by the electric field applied between the two electrodes, leaving these atoms positively ionized. The free electrons thus released flow onto the anode, while the cations thus formed are accelerated by the electric field and flow towards the cathode. Typicall ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Geissler Tube
A Geissler tube is an early gas discharge tube used to demonstrate the principles of electrical glow discharge, similar to modern neon lighting. The tube was invented by the German physicist and glassblower Heinrich Geissler in 1857. It consists of a sealed, partially evacuated glass cylinder of various shapes with a metal electrode at each end, containing rarefied gasses such as neon, argon, or air; mercury vapor or other conductive fluids; or ionizable minerals or metals, such as sodium. When a high voltage is applied between the electrodes, an electrical current flows through the tube. The current dissociates electrons from the gas molecules, creating ions, and when the electrons recombine with the ions, the gas emits light by fluorescence. The color of light emitted is characteristic of the material within the tube, and many different colors and lighting effects can be achieved. The first gas-discharge lamps, Geissler tubes were novelty items, made in many artistic shapes a ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
