Monday, August 24, 2009
The classical example of thermionic emission is the emission of electrons from a hot metal cathode into a vacuum (known as the Edison effect) used in vacuum tubes. However, the term "thermionic emission" is now used to refer to any thermally excited charge emission process, even when the charge is emitted from one solid-state region into another. This process is crucially important in the operation of a variety of electronic devices and can be used for power generation or cooling. The magnitude of the charge flow increases dramatically with increasing temperature. However, vacuum emission from metals tends to become significant only for temperatures over 1000 K. The science dealing with this phenomenon has been known as thermionics, but this name seems to be gradually falling into disuse.
Cold cathode is an element used within some Nixie tubes, gas discharge lamps, gas filled tubes, and vacuum tubes. The term 'cold cathode' refers to the fact that the cathode is not independently heated. In spite of this, the cathode itself may still operate at temperatures as high as if the cathode were heated.
Cold cathode fluorescent lamps (CCFLs) are usually also called cold cathodes. Neon lamps are a very common example of a cold cathode lamp.
Cold Cathodes remain popular for LCD backlighting and enthusiast computer case modders.
A cathode is any electrode that emits electrons. When used in electrical and electronic devices (most fluorescent lamps, vacuum tubes, etc.), the cathode is explicitly heated, creating a hot cathode. By taking advantage of thermionic emission, electrons can overcome the work function of the cathode without an electric field to pull the electrons out. But if sufficient voltage is present, electrons can still be stripped even out of a cathode operating at ambient temperature. Because it is not deliberately heated, such a cathode is referred to as a cold cathode, although several mechanisms may eventually cause the cathode to become quite hot once it is operating. Most cold cathode devices are filled with a gas which can be ionized. A few cold cathode devices contain a vacuum.
The thermoelectric effect is the direct conversion of temperature differences to electric voltage and vice versa. A thermoelectric device creates a voltage when there is a different temperature on each side. Conversely when a voltage is applied to it, it creates a temperature difference (known as the Peltier effect). At atomic scale (specifically, charge carriers), an applied temperature gradient causes charged carriers in the material, whether they are electrons or holes, to diffuse from the hot side to the cold side, similar to a classical gas that expands when heated; hence, the thermally-induced current.
This effect can be used to generate electricity, to measure temperature, to cool objects, or to heat them or cook them. Because the direction of heating and cooling is determined by the sign of the applied voltage, thermoelectric devices make very convenient temperature controllers.
Traditionally, the term thermoelectric effect or thermoelectricity encompasses three separately identified effects, the Seebeck effect, the Peltier effect, and the Thomson effect. In many textbooks, thermoelectric effect may also be called the Peltier–Seebeck effect. This separation derives from the independent discoveries of French physicist Jean Charles Athanase Peltier and Estonian-German physicist Thomas Johann Seebeck. Joule heating, the heat that is generated whenever a voltage is applied across a resistive material, is somewhat related, though it is not generally termed a thermoelectric effect (and it is usually regarded as being a loss mechanism due to non-ideality in thermoelectric devices). The Peltier–Seebeck and Thomson effects can in principle be thermodynamically reversible, whereas Joule heating is not.
Wednesday, August 5, 2009
Electron–positron annihilation occurs when an electron and a positron (the electron's anti-particle) collide. The result of the collision is the conversion of the electron and positron and the creation of gamma ray photons or, less often, other particles. The process must satisfy a number of conservation laws, including:
Conservation of charge. The net charge before and after is zero.
Conservation of linear momentum and total energy. This forbids the creation of a single gamma ray. However, in quantum field theory this process is allowed.
Conservation of angular momentum.
As with any two charged objects, electrons and positrons may also interact with each other without annihilating, in general by elastic scattering.
Low energy case
There are only a very limited set of possibilities for the final state. The most possible is the creation of two or more gamma ray photons. Conservation of energy and linear momentum forbid the creation of only one photon. In the most common case, two photons are created, each with energy equal to the rest energy of the electron or positron (511 keV). A convenient frame of reference is that in which the system has no net linear momentum before the annihilation; thus, after collision, the gamma rays are emitted in opposite directions. It is also common for three to be created, since in some angular momentum states, this is necessary to conserve C parity. It is also possible to create any larger number of photons, but the probability becomes lower with each additional photon because these more complex processes have lower quantum mechanical amplitudes.
Since neutrinos also have a smaller mass than electrons, it is also possible — but exceedingly unlikely — for the annihilation to produce one or more neutrino/antineutrino pairs. The same would be true for any other particles, which are as light, as long as they share at least one fundamental interaction with electrons and no conservation laws forbid it. However, no other such particles are known.
High energy case
If the electron and/or positron have appreciable kinetic energies, other heavier particles can also be produced (e.g. D mesons), since there is enough kinetic energy in the relative velocities to provide the rest energies of those particles. It is still possible to produce photons and other light particles, but they will emerge with higher energies.
At energies near and beyond the mass of the carriers of the weak force, the W and Z bosons, the strength of the weak force becomes comparable with electromagnetism. This means that it becomes much easier to produce particles such as neutrinos that interact only weakly.
The heaviest particle pairs yet produced by electron-positron annihilation in particle accelerators are W+/W− pairs. The heaviest single particle is the Z boson. The driving motivation for constructing the International Linear Collider is to produce Higgs bosons in this way.
This process is the physical phenomenon relied on as the basis of PET imaging and Positron Annihilation Spectroscopy. Also used as a method of measuring the Fermi surface and Band structure in metals.
The reverse reaction is a form of matter creation governed by two-photon physics.