Direct current , electricity

Electrodialysis. Electro dialysis processes transfer ions of dissolved salts across membranes, leaving purified water behind. Ion movement is induced by direct current electrical fields. A negative electrode (cathode) attracts cations, and a positive electrode (anode) attracts anions. Systems are compartmentalized in stacks by alternating cation and anion transfer membranes. Alternating compartments carry concentrated brine and purified permeate. Typically, 40—60% of dissolved ions are removed or rejected. Further improvement in water quaUty is obtained by staging (operation of stacks in series). ED processes do not remove particulate contaminants or weakly ionized contaminants, such as siUca. 

Superconductivity The physical state in which all resistance to the flow of direct-current electricity disappears is defined as superconductivity. The Bardeen-Cooper-Schriefer (BCS) theoiy has been reasonably successful in accounting for most of the basic features observed of the superconducting state for low-temperature superconductors (LTS) operating below 23 K. The advent of the ceramic high-temperature superconductors (HTS) by Bednorz and Miller (Z. Phys. B64, 189, 1989) has called for modifications to existing theories which have not been finahzed to date. The massive interest in the new superconductors that can be cooled with liquid nitrogen is just now beginning to make its way into new applications. 

Of greater interest in recent years have been the peculiar piezolectric properties"" of polyfvinylidene fluoride). In 1969 it was observed" that stretched film of the polymer heated to 90°C and subsequently cooled to room temperature in a direct current electric field was 3-5 times more piezoelectric than crystalline quartz. It was observed that the piezolectric strain coefficients were higher in the drawn film and in the normal directions than in the direction transverse to the film drawing. 

Commercially available PV systems most often include modules made from single-crystal or poly-ciystalline silicon or from thin layers of amoiphous (non-crystalline) silicon. The thin-filni modules use considerably less semiconductor material but have lower efficiencies for converting sunlight to direct-current electricity. Cells and modules made from other thin-filni PV materials such as coppcr-indiuni-diselenide and cadmium telluride are under active development and are beginning to enter the market. 

Cooper, M.S., Schliwa, M. (1986). Motility of cultured fish epidermal cells in the presence and absence of direct current electric fields. J. Cell Biol. 102, 1384-1399. 

The pAf values of all functional groups of an amino acid dictate its net charge at a given pH. pi is the pH at which an amino acid bears no net charge and thus does not move in a direct current electrical field. 

The simplest analytical method is direct measurement of arsenic in volatile methylated arsenicals by atomic absorption [ 11 ]. A slightly more complicated system, but one that permits differentiation of the various forms of arsenic, uses reduction of the arsenic compounds to their respective arsines by treatment with sodium borohydride. The arsines are collected in a cold trap (liquid nitrogen), then vaporised separately by slow warming, and the arsenic is measured by monitoring the intensity of an arsenic spectral line, as produced by a direct current electrical discharge [1,12,13]. Essentially the same method was proposed by Talmi and Bostick [10] except that they collected the arsines in cold toluene (-5 °C), separated them on a gas chromatography column, and used a mass spectrometer as the detector. Their method had a sensitivity of 0.25 xg/l for water samples. 

Polar Cell Systems for Membrane Transport Studies Direct current electrical measurement in epithelia steady-state and transient analysis, 171, 607 impedance analysis in tight epithelia, 171, 628 electrical impedance analysis of leaky epithelia theory, techniques, and leak artifact problems, 171, 642 patch-clamp experiments in epithelia activation by hormones or neurotransmitters, 171, 663 ionic permeation mechanisms in epithelia biionic potentials, dilution potentials, conductances, and streaming potentials, 171, 678 use of ionophores in epithelia characterizing membrane properties, 171, 715 cultures as epithelial models porous-bottom culture dishes for studying transport and differentiation, 171, 736 volume regulation in epithelia experimental approaches, 171, 744 scanning electrode localization of transport pathways in epithelial tissues, 171, 792. 

Electroremediation is a method for the in situ removal of heavy-metal and organic compounds from low-permeability soils by the application of direct-current electric fields. These fields induce the transport of water and contaminants to wells where they are pumped to the surface. Massachusetts Institute of Technology (MIT) developed the Electroremediation technology, which is commercially available through Corrpro Companies Incorporated. 

In the quadrupole mass analyzer, focusing electrodes direct and accelerate the ionized fragments into a mass filter consisting of four cylindrical electrodes in a vacuum. Tire cylindrical electrodes establish a combination radio-frequency and direct-current electrical field that permits only those ions with a specific, selected mass-to-charge ratio to pass all the way through the filter. The rest of the ions impact die electrodes and do not travel to the exit. Varying the electrical field allows ions with other masses to pass through the filter. 

Figure 26-37 Separation of 2 000- to 23 000-Da DNA fragments by sieving through quartz pillars in a capillary electrophoresis channel on a quartz plate. [From N. Kaji, Y, Tezuka, Y. Takamura, M. Uedo. T. NisNmoto, H. Nakanishi, Y. Horiike. and Y. Baba, Separation of Long DNA Molecules by Quartz Nanopillar CNps Under a Direct Current Electric Field," Anal. Chem 2004, 76, IS.]...

E Electrochromatography. It is defined by Heftmann(Ref 78,p 14) as "a method of analysis in which direct current electrical potential promotes the separation of substances by differential migration from a narrow zone in a stabilized background electrolytic solution . 

Kaji, N., Tezuka, Y., Takamura, Y., Ueda, M., Nishimoto, T., Nakanishi, H., Horiike, Y., Baba, Y., Separation of long DNA molecules by quartz nanopillar chips under a direct current electric field. Anal. Chem. 2004, 76, 15-22. 

A second commonly used mass analyzer is the quadrupole. A quadrupole acts as a mass filter, allowing a certain mass-to-charge ratio of ions to be transmitted while filtering out all others. The quadrupole consists of a set of four electrodes positioned in an array. Superimposed radio frequency and direct current electric fields can be mutually tuned to allow transmission of ions of the selected mass-to-... 

Direct current — Electric current that flows in a constant direction, i.e., a current with constant sign (either + or -). A direct current does not periodically change its direction like an -> AC current. AC and DC currents can be superimposed (see also -> decoupling of AC and DC currents). 

The originality of the process resides in the use of aTiigh-power (3200 kW) eieciric. arc furnace supplied v,ith direct current electricity. The Huls arc furnace consists of a vertical tube surmounted by an injection chamber into which the gas is introduced... 

Photovoltaic devices typicaHy consist of a series of thin semiconductor layers that are designed to convert sunlight to direct-current electricity (see Semiconductors). As long as the device is exposed to sunlight, a photovoltaic (PV) ceH produces an electric current proportional to the amount of light it receives. The photovoltaic effect, first observed in 1839, did not see commercial appHcation until the 1950s when photovoltaic modules were used to power early space sateHites. Many good descriptions of the photovoltaic phenomenon are available (7). 

The arc is an electrical discharge that must be triggered by an auxiliary self-ionising discharge. The direct-current electric arc (300 V, 20 A) is created between two solid electrodes. One of the electrodes contains the sample in powder form. This is compacted in a crater hollowed out in the end of the electrode consisting of a graphite rod (see Fig. 3.2). 

The arsines are collected in a cold trap (liquid nitrogen), then vaporized separately by slow warming, and the arsenic measured by monitoring the intensity of an arsenic spectral line, as produced by a direct-current electrical discharge " -. A similar method was used by Talmi and Bostick who used a mass spectrometer as the detector. Their method had a sensitivity of 0.25 igl for water samples. 

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