Space charge is the electrical current that results when a metal object is heated to incandescence in a vacuum. This effect was first observed by Thomas Edison in light bulb filaments, where it is sometimes called the Edison Effect. In electricity, current is the rate of flow of charges, usually through a metal wire or some other electrical conductor. ... Hot metal work from a blacksmith Look up Metal in Wiktionary, the free dictionary In chemistry, a metal (Greek: Metallon) is an element that readily forms ions (cations) and has metallic bonds, and metals are sometimes described as a lattice of positive ions (cations) surrounded by a sea of delocolised... Molten glassy material glows orange with incandescence in a vitrification experiment. ... Look up Vacuum in Wiktionary, the free dictionary For other uses, see vacuum (disambiguation) A vacuum is a volume of space that is empty of matter, including air, so that gaseous pressure is much less than standard atmospheric pressure. ... Thomas Alva Edison (February 11, 1847–October 18, 1931) was an inventor and businessman who developed many devices which greatly influenced life in the 20th century. ... The Electric Filament is a thread of metal which is used to convert heat into electricity for the incandescent light bulb made in 1878 by Joseph Wilson Swan ... Thermionic emission is the flow of electrons from a metal or metal oxide surface, caused by thermal vibrational energy overcoming the electrostatic forces holding electrons to the surface. ...

Cause

Simply

When a metal object is placed in a vacuum and is heated to incandescence, the energy is sufficient to cause electrons to "boil" away from the surface atoms and surround the metal object in a cloud of free electrons. This is called thermionic emission. The resulting cloud is negatively charged, and can be attracted to any nearby positively charged object, thus producing an electrical current which passes through the vacuum. Properties The electron is a fundamental subatomic particle that carries a negative electric charge. ... Properties Mass: || ≈ 1. ... In physics, the free electron model is a possible model for the behaviour of electrons in a crystal structure. ... Thermionic emission (archaically known as the Edison effect) is the flow of electrons from a metal or metal oxide surface, caused by thermal vibrational energy overcoming the electrostatic forces holding electrons to the surface. ...

Theoretical

If the vacuum is 10^-6mm pressure or less, the main vehicle of conduction is electrons. Equation A/cm² gives the current per cm² of cathode as a function of cathode temperature. The emitted electrons do not all have the same but have a Maxwellian distribution of speeds, viz., proportional to where v is the speed. Look up Vacuum in Wiktionary, the free dictionary For other uses, see vacuum (disambiguation) A vacuum is a volume of space that is empty of matter, including air, so that gaseous pressure is much less than standard atmospheric pressure. ... Pressure (symbol: p) is the force per unit area acting on a surface in a direction perpendicular to that surface. ... Vehicles are non-living means of transportation. ... Properties The electron is a fundamental subatomic particle that carries a negative electric charge. ... Diagram of a copper cathode in a Daniells cell. ... Look up Function in Wiktionary, the free dictionary The word function may mean: In common parlance, a role of a component in an assembly, or of an element in a systemic aggregate (such as a person within a group). ... Temperature is the physical property of a system which underlies the common notions of hot and cold; the material with the higher temperature is said to be hotter. ... The compound derived CGS unit, the maxwell, abbreviated as Mx, is the unit for the magnetic flux. ... Speed (symbol: v) is the rate of motion, or equivalently the rate of change of position, expressed as distance d moved per unit of time t. ... Viz is a method of introducing a list or a series. ...

A₀ = = 120 A/cm²dyne²
k = Boltzmann's constant = 1.38 x 10^-23J/K
h = Planck's constant = 6.55 x 10^-34J sec
φ = work function of the cathode (electron-volt)
T = absolute temperature
ř = a reflection coefficient The Boltzmann constant (k or kB) is the physical constant relating temperature to energy. ... A commemoration plaque for Max Planck on his discovery of Plancks constant, in front of Humboldt University, Berlin. ...

The reflection coefficient can be as low as 0.105 but is usually near 0.5. For Tungsten, (1 - ř)A₀ = 60 to 100 and φ = 4.52. At 2500°C, the emission is 300mA/cm² General Name, Symbol, Number tungsten, W, 74 Chemical series transition metals Group, Period, Block 6, 6, d Appearance grayish white, lustrous Atomic mass 183. ...

The emission current is many times greater than that normally collected by the electrodes, except in some pulsed valves such as the cavity magnetron used in radar: the cathode would suffer rapid exhaustion if the full emission were drawn off. Most of the electrons emitted by the cathode are driven back to it by the repulsion of the cloud of electrons in its neighbourhood. This is called space charge effect, and it determines the current-voltage characteristic of tube. In medicine, a persons pulse is the throbbing of their arteries as an effect of the heart beat. ... A valve is a mechanical device that regulates the flow of fluids (either gases, fluidised solids, slurries or liquids) by opening, closing, or partially obstructing various passageways. ... A cavity magnetron is a high-powered vacuum tube that generates coherent microwaves. ... M*A*S*H , see Corporal Walter (Radar) OReilly. ... For the grindcore band see Repulsion (band) Repulsion is a film made in 1965 directed by Roman Polański. ... Cumulus of fair weather A cloud is a visible mass of condensed droplets or ice crystals suspended in the atmosphere above the surface of the Earth or another planetary body. ... // Electric current is the flow of electric charge. ... This article may be too technical for most readers to understand. ... In electronics, a vacuum tube (U.S. and Canadian English) or (thermionic) valve (outside North America) is a device generally used to amplify, or otherwise modify, a signal by controlling the movement of electrons in an evacuated space. ...

Occurrence

Space charge is an inherent property of all vacuum tubes. This has at times has made life harder or easier for electrical engineers who used tubes in their designs. For example, space charge significantly limited the practical application of triode amplifiers because it impedes the flow of electrons from cathode to anode, thus reducing the level of gain that could be achieved in such tubes. In electronics, a vacuum tube (U.S. and Canadian English) or (thermionic) valve (outside North America) is a device generally used to amplify, or otherwise modify, a signal by controlling the movement of electrons in an evacuated space. ... An engineers degree is an academic degree which is intermediate in rank between a masters degree and a doctorate; it is occasionally to be encountered in the United States in technical fields. ... Simplified diagram of a triode. ... An amplifier can be considered to be any device that uses a small amount of energy to control a source of a larger amount of energy, although the term today usually refers to an electronic amplifier. ... Diagram of a copper cathode in a Daniells cell. ... Diagram of a zinc anode in a Daniells cell. ... In electronics, gain is usually taken as the mean ratio of the signal output of a system to the signal input of the system. ...

On the other hand, space charge came in quite handy in some tube applications because it generates a negative EMF within the tube's envelope, which could be used to create a negative bias on the tube's grid. This could improve the engineer's control and fidelity of amplification. Electromotive force (emf),[hammid the smelly fucking arab that cant hold his farts] often denoted by , is a measure of the strength of a source of electrical energy. ...

Space charges can also occur within a solid, liquid, or gas dielectric. For example, when gas near a high voltage electrode begins to undergo dielectric breakdown, electrical charges are injected into the region near the electrode, forming space charge regions in the surrounding gas. Space charges can also occur within solid or liquid dielectrics that are stressed by high electric fields. Trapped space charges in solid dielectrics is often a contributing factor for dielectric failures within high voltage capacitors and power cables. A dielectric, or electrical insulator, is a substance that is highly resistant to the flow of electric current and has a relative permittivity greater than unity. ... The term electrical breakdown has several similar but distinctly different meanings. ... In physics, an electric field or E-field is an effect produced by an electric charge that exerts a force on charged objects in its vicinity. ...

Child's Law

Also known as the Child-Langmuir Law or the Three-Halves Power Law, Child's Law states that the space charge-limited current (SCLC) in a plane-parallel diode varies directly as the three-halves power of the anode voltage and inversely as the square of the distance separating the cathode and the anode..

Mathematically: .

This assumes the following:

1. The electrodes are planar, parallel, equipotential surfaces of infinite dimensions.
2. The electrons have zero velocity at the cathode surface.
3. In the interelectrode region, only electrons are present.
4. The current is space-charge limited.
5. The anode voltage remains constant for a sufficiently long time so that the anode current is steady.

Mott's steady-state space-charge-limited conduction model

The steady-state space-charge-limited conduction-current density J in a plane-parallel dielectric sample is proportional to the square of the applied voltage V.

Mathematically:

This assumes the following:

1. There is only one type of charge carrier present.
2. The material has no intrinsic conductivity, but charges are injected into it from one electrode and captured by the other.
3. The carrier mobility μ and the dielectric permittivity ε are constant throughout the sample.
4. The electric field at the charge-injecting cathode is zero.

Shot noise

Space charge tends to reduce shot noise. Electrons (and positive charge carriers) come with their own built-in negative feedback. Shot noise is a type of noise that occurs when the finite number of particles that carry energy, such as electrons in an electronic circuit or photons in an optical device, gives rise to detectable statistical fluctuations in a measurement. ... Charge is a word with many different meanings. ... Feedback is (generally) information about actions. ...

See also

• Thermionic emission
• Vacuum tubes
• Grid leak

Thermionic emission (archaically known as the Edison effect) is the flow of electrons from a metal or metal oxide surface, caused by thermal vibrational energy overcoming the electrostatic forces holding electrons to the surface. ... In electronics, a vacuum tube (U.S. and Canadian English) or (thermionic) valve (outside North America) is a device generally used to amplify, or otherwise modify, a signal by controlling the movement of electrons in an evacuated space. ... Grid leak is an inherent operating characteristic of triodes and other vacuum tubes. ...

References

Telecommunications by A. T. Starr, Second Ed., Sir Isaac Pitman & Sons, Ltd, London, 1958

Physics of Dielectrics for the Engineer by R. Coelho, Elsevier Scientific Pub. Co., Amsterdam, 1979