conductor

An electrical conductor is a substance in which electrical charge carriers, usually electrons, move easily from atom to atom with the application of voltage. Conductivity, in general, is the capacity to transmit something, such as electricity or heat. 

Pure elemental silver is the best electrical conductor encountered in everyday life. Copper, steel, gold, aluminum, and brass are also good conductors. In electrical and electronic systems, all conductors comprise solid metals molded into wires or etched onto circuit boards.

Some liquids are good electrical conductors. Mercury is an excellent example. A saturated salt-water solution acts as a fair conductor. Gases are normally poor conductors because the atoms are too far apart to allow a free exchange of electrons. However, if a sample of gas contains a significant number of ions, it can act as a fair conductor.

A substance that does not conduct electricity is called an insulator or dielectric material. Common examples include most gases, porcelain, glass, plastic, and distilled water. A material that conducts fairly well, but not very well, is known as a resistor. The most common example is a combination of carbon and clay, mixed together in a specific ratio to produce a constant and predictable opposition to electric current. 

Substances called semiconductors act as good conductors under some conditions and poor conductors under other conditions. Silicon, germanium, and various metal oxides are examples of semiconductor materials. In a semiconductor, both electrons and so-called holes (electron absences) act as charge carriers.

At extremely low temperatures, some metals will conduct electricity better than any known substance at room temperature. This phenomenon is called superconductivity, and a substance that behaves that way is called a superconductor.

ion

An ion is an atom or group of atoms in which the number of electron s is different from the number of proton s. If the number of electrons is less than the number of protons, the particle is a positive ion, also called a cation. If the number of electrons is greater than the number of protons, the particle is a negative ion, also called an anion.

When an atom of an element is short an electron, a plus sign is placed after its chemical symbol as a superscript to indicate that fact. For example, a carbon atom with 5 electrons (the nucleus has 6 protons) is symbolized C + . If the element is short 2 or more electrons, a numeral is also placed in the superscript, directly before the plus sign, to indicate the extent of the electron deficiency. A carbon atom with 4 electrons is therefore symbolized C 2+ , and a carbon atom with 3 electrons is symbolized C 3+ .

If an atom of an element has an excess of an electron, a minus sign is placed after its chemical symbol as a superscript. If there are 2 or more extra electrons, a numeral is included to indicate the extent of the electron surplus. An oxygen atom with 9 electrons (the nucleus has 8 protons) is symbolized O - . An oxygen atom with 10 electrons is symbolized O 2- , and an oxygen atom with 11 electrons is symbolized O 3- .

A compound , as well as individual atom, can be ionized. A common example is nitrate, which consists of a nitrogen atom and 3 oxygen atoms (NO 3 ) in the form of an anion; this is symbolized NO 3 - because it normally has a surplus of a single electron. Another example is sulfate, which consists of a sulfur atom and 4 oxygen atoms (SO 4 ), which occurs with an excess of 2 electrons and is symbolized SO 4 2- .

Ionized substances often behave differently than when they are not ionized. A common phenomenon is for an electrical insulator (non-conductor) to become electrically conductive when it is ionized. In the Earth's upper atmosphere, ultraviolet radiation from the Sun causes ionization of certain gases. As a result, electromagnetic waves can refracted, and their polarization shifted, at certain frequencies when the waves pass through the gases. This makes long-distance radio communication possible, without the aid of satellites, at some frequencies. The ionization occurs in layers which, taken together, form the Earth's ionosphere .

dielectric material

A dielectric material is a substance that is a poor conductor of electricity, but an efficient supporter of electrostatic field s. If the flow of current between opposite electric charge poles is kept to a minimum while the electrostatic lines of flux are not impeded or interrupted, an electrostatic field can store energy. This property is useful in capacitor s, especially at radio frequencies. Dielectric materials are also used in the construction of radio-frequency transmission lines.

In practice, most dielectric materials are solid. Examples include porcelain (ceramic), mica, glass, plastics, and the oxides of various metals. Some liquids and gases can serve as good dielectric materials. Dry air is an excellent dielectric, and is used in variable capacitors and some types of transmission lines. Distilled water is a fair dielectric. A vacuum is an exceptionally efficient dielectric.

An important property of a dielectric is its ability to support an electrostatic field while dissipating minimal energy in the form of heat. The lower the dielectric loss (the proportion of energy lost as heat), the more effective is a dielectric material. Another consideration is the dielectric constant , the extent to which a substance concentrates the electrostatic lines of flux. Substances with a low dielectric constant include a perfect vacuum, dry air, and most pure, dry gases such as helium and nitrogen. Materials with moderate dielectric constants include ceramics, distilled water, paper, mica, polyethylene, and glass. Metal oxides, in general, have high dielectric constants.

The prime asset of high-dielectric-constant substances, such as aluminum oxide, is the fact that they make possible the manufacture of high-value capacitors with small physical volume. But these materials are generally not able to withstand electrostatic fields as intense as low-dielectric-constant substances such as air. If the voltage across a dielectric material becomes too great -- that is, if the electrostatic field becomes too intense -- the material will suddenly begin to conduct current. This phenomenon is called dielectric breakdown . In components that use gases or liquids as the dielectric medium, this condition reverses itself if the voltage decreases below the critical point. But in components containing solid dielectrics, dielectric breakdown usually results in permanent damage.

semiconductor

A semiconductor is a substance, usually a solid chemical element or compound, that can conduct electricity under some conditions but not others, making it a good medium for the control of electrical current. Its conductance varies depending on the current or voltageapplied to a control electrode, or on the intensity of irradiation by infrared (IR), visible light, ultraviolet (UV), or X rays.

The specific properties of a semiconductor depend on the impurities, or dopants, added to it. An N-type semiconductor carries current mainly in the form of negatively-charged electrons, in a manner similar to the conduction of current in a wire. A P-type semiconductor carries current predominantly as electron deficiencies called holes. A hole has a positive electric charge, equal and opposite to the charge on an electron. In a semiconductor material, the flow of holes occurs in a direction opposite to the flow of electrons.

Elemental semiconductors include antimony, arsenic, boron, carbon, germanium, selenium, silicon, sulfur, and tellurium. Silicon is the best-known of these, forming the basis of most integrated circuits (ICs). Common semiconductor compounds include gallium arsenide, indium antimonide, and the oxides of most metals. Of these, gallium arsenide (GaAs) is widely used in low-noise, high-gain, weak-signal amplifying devices.

A semiconductor device can perform the function of a vacuum tube having hundreds of times its volume. A single integrated circuit (IC), such as a microprocessor chip, can do the work of a set of vacuum tubes that would fill a large building and require its own electric generating plant.

superconductivity

Superconductivity is the ability of certain materials to conduct electric current with practically zero resistance. This capacity produces interesting and potentially useful effects. For a material to behave as a superconductor, low temperatures are required.

Superconductivity was first observed in 1911 by H. K. Onnes, a Dutch physicist. His experiment was conducted with elemental mercury at 4 degrees kelvin (approximately -452 degrees Fahrenheit), the temperature of liquid helium. Since then, some substances have been made to act as superconductors at higher temperatures, although the ideal -- a material that can superconduct at room temperature -- remains elusive.

Superconductors have been employed in, or proposed for use in, an enormous variety of applications. Examples include high-speed magnetic-levitation trains, magnetic-resonance-imaging (MRI) equipment, ultra-high-speed computer chips, high-capacity digital memory chips, alternative energy storage systems, radio-frequency (RF) filters, radio-frequency amplifiers, sensitive visible-light and infrared detectors, miniaturized wireless transmitting antennas, systems to detect submarines and underwater mines, and gyroscopes for earth-orbiting satellites. The Josephson junction and the superconducting quantum interference device use superconductors.