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Extensible Markup Language | 1995-08-15 | 7KB | 34 lines

<?xml version="1.0" encoding="utf-8" ?> <!DOCTYPE card PUBLIC "-//Apple, Inc.//DTD card V 2.0//EN" "" > <card> <id>27491</id> <filler1>0</filler1> <cantDelete> <false /> </cantDelete> <showPict> <true /> </showPict> <dontSearch> <false /> </dontSearch> <owner>5472</owner> <link rel="stylesheet" type="text/css" href="stylesheet_3106.css" /> <content> <layer>background</layer> <id>25</id> <text>lectricity in Action (5 of 6)Conductors and semiconductorsA metal consists of an array of positive ions in a `sea' of free electrons. The electrons move randomly with mean speeds of around 10 to the power of 6 m s-1. When a potential difference is applied across a metal a small drift velocity is added. The metal atoms are thought to give up one or more electrons, which can then migrate freely through the material. These electrons move in a zigzag manner along a conductor. As a result their typical velocity, called the drift velocity, is small, in the order of 10-4 m s-1. Thus it would take more than an hour to move one meter. Note that the electric signals that drive the electrons travel with a speed in the order of 108 m s-1 in some circuits.This classical picture of electron conduction explains some but not all conduction phenomena. For these a quantum mechanical model is required. This model explains the basis of semiconductors, which now play such an important part in electronics.Metals are good conductors of electricity because there are always many unoccupied quantum states into which electrons can move. Non-metallic solids and liquids have nearly all their quantum states occupied by electrons, so it is difficult to produce large currents. If the numbers of unoccupied states and of electrons free to move into them are small the material is an insulator. If there are more free electrons and unoccupied states the substance is called a semiconductor.Semiconductors have a charge-carrier density that lies between those of conductors and insulators. Two metal-like elements, silicon and germanium are the two semiconductors used most frequently. These may be `doped' with an impurity to modify their conduction behavior - n-type doping increases the number of free electrons, p-type increases the number of unoccupied states. If the doping results in the charge carriers being negative electrons, then the result is an n-type semiconductor. If electron deficiencies or holes are the charge carriers, then the result is a p-type semiconductor.Most semiconductor devices are made from materials that are partly p-type and partly n-type. The boundary between them is known as a p-n junction. Such a device, called a semiconductor diode, will act as a rectifier, a device used to convert alternating current to direct current. In some materials, such as gallium arsenide, a p-n junction will emit light when ever an electric current passes through it. This device is called a light-emitting diode. These are used in digital displays in clocks and radios. The light is emitted when an electron and hole meet at the junction and annihilate each other - they cancel each other out.Solar cellsThe photovoltaic effect occurs when light is absorbed by a p-n or n-p junction. Electrons are liberated at the junction by an incident photon and diffuse through the n-type region. The hole drifts through the p-type layer until it recombines with an electron flowing round the external circuit. The first practical photovoltaic device - called a solar cell - was made in 1954. In essence a solar cell is a light-emitting diode acting in reverse - it converts light into electric current, which is the basis of solar power.TransistorsA transistor consists of semiconductor material in n-p-n or p-n-p form. The middle part is the base and the ends are called the emitter and collector. An integrated circuit consists of many transistors, rectifiers or other components embedded in a chip of silicon.SuperconductivitySuperconductivity was discovered by the Dutch physicist Kamerlingh Onnes (1853-1926) in 1911. Below a certain critical temperature, various metals show zero resistance to current flow. Once a current is started in a closed circuit, it keeps flowing as long as the circuit is kept cold. The critical temperature for aluminum is 1.19 K(-272 deg C / -457 deg F), and similar values hold for other metals. Some alloys have higher critical temperatures. Up to 1986 the highest transition temperature known was about -248 deg C (-414 deg F). More recently a new class of copper oxide and other materials have shown superconductivity up to at least -148 deg C (-234 deg F). These developments promise enormous savings in energy.AS</text> </content> <content> <layer>background</layer> <id>26</id> <text>. A diode. Electrons emitted by the heated cathode flow through the vacuum to the anode. A diode allows passage of electricity in one direction only.6. A triode. The potential on the grid controls the flow of the electrons between the cathode and anode. It can act as a switch or an amplifier.</text> </content> <content> <layer>background</layer> <id>23</id> <text>ΓÇó QUANTUM THEORY AND RELATIVITYΓÇó ELECTROMAGNETISMΓÇó ATOMS AND SUBATOMIC PARTICLESΓÇó METALSΓÇó MAN-MADE PRODUCTS ΓÇó ENERGYΓÇó RADIO, TELEVISION AND VIDEOΓÇó HIFIΓÇó TELECOMMUNICATIONSΓÇó COMPUTERS</text> </content> <content> <layer>background</layer> <id>36</id> <text>2634385054304326328330334</text> </content> <name>p036-5</name> <script></script> </card>