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Space charge - Cause | | Space charge - Cause: Encyclopedia II - Space charge - Cause | |
Space charge - 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.
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See also:Space charge, Space charge - Cause, Space charge - Simply, Space charge - Theoretical, Space charge - Occurence, Space charge - Child's Law, Space charge - Shot noise | | | Space charge, Space charge - Cause, Space charge - Child's Law, Space charge - Occurence, Space charge - Shot noise, Space charge - Simply, Space charge - Theoretical, Thermionic emission, Vacuum tubes, Grid leak | | |
| | | Space charge: Encyclopedia II - Space charge - Cause
Space charge - Cause
Space charge - 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.
Space charge - 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.
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 reflection coefficient can be as low as 0.1 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
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.
Other related archivesBoltzmann's constant, EMF, Edison Effect, Grid leak, Maxwellian, Planck's constant, SCLC, Shot noise, Thermionic emission, Thomas Edison, Tungsten, Vacuum tubes, amplifiers, anode, atoms, cathode, cavity magnetron, charge, cloud, current, dielectric, dielectric breakdown, electric fields, electrical current, electrical engineers, electrons, feedback, filaments, free electrons, function, gain, incandescence, metal, pressure, pulsed, radar, repulsion, speeds, temperature, thermionic emission, triode, tube, vacuum, vacuum tubes, valves, vehicle, viz, voltage
 Adapted from the Wikipedia article "Cause", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki |
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