Alternating current electric field

In what follows, the laser damage characteristics are discussed when both alternating current electric fields and ultra-short pulse lasers are simultaneously applied to the dielectric materials. It is true that very high electric fields may allow the electron to reach an energy level sufficient to ionize, causing the impact ionization. As discussed earlier, the damage... 

The field of dielectrophoresis is relatively recent. Herbert Pohl did dielectrophoretic research in the 1950s although this area did not gain wide visibility until his groundbreaking book Dielectrophoresis The behavior of neutral matter in nonuniform electric fields was published in 1978 [1]. In this book, the two main components of the electric field in alternating current (AC) dielectrophoresis were extensively discussed, namely, (i) a spatially nonuniform electric field and (ii) an alternating current electric field, both of which contribute to the nonlinear DEP force. Within the last 10 years. 

Dukhin AS, Dukhin SS (2005) Aperiodic capillary electrophoresis method using an alternating current electric field for separation of macromolecules. Electrophor 26 2149-2153... 

Chen, X. Q., Saito, T., Yamada, H. and Matsushige, K., Aligning single-wall carbon nanotubes with an alternating-current electric field . Applied Physics Letters, 2001, 78 (23), 3714-3716. 

Bi R, Schlack M, Siefert E. Lord R. Conelly H (2010) Alternating current electrical field effects on lettuce (lactuca sativa) growing in hydroponic culture with and without cadmium contamination. J Appl Electrochem 40(6) 1217-1233... 

Dukhin AS, Dukhin SS (2005) Aperiodic capillary electrt hore-sis method using an alternating current electric field foi separation of macromolecules. Electrr hor 26 2149-2153 Dukhin AS (1993) Biospecific mechanism of double laya- formation and peculiarities of cell electrophoresis. Colloid Surf A Physicochem Eng Asp 73 29-48... 

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. 

The function that plastics serve in most applications is that of a dielectric or insulator that separates two conductors with an electrical field between them. The field can be a steady direct current (DC) field or an alternating current (AC) field and the frequency range may vary such as from 0 to 1010 Hz. 

Dielectric loss The dielectric loss factor represents energy that is lost to the insulator as a result of its being subjected to alternating current (AC) fields. The effect is caused by the rotation of dipoles in the plastic structure and by the displacement effects in the plastic chain caused by the electrical fields. The frictional effects cause energy absorption and the effect is analogous to the mechanical hysteresis effects except that the motion of the material is field induced instead of mechanically induced. 

With an alternating current (AC) field, the dielectric constant is virtually independent of frequency, so long as one of the multiple polarization mechanisms usually present is active (see Section 8.8.1). When the dominating polarization mechanism ceases as the frequency of the applied field increases, there is an abmpt drop in the dielectric constant of the material before another mechanism begins to dominate. This gives rise to a characteristic stepwise appearance in the dielectric constant versus frequency curve. For each of the different polarization mechanisms, some minimum dipole reorientation time is required for reahgnment as the AC held reverses polarity. The reciprocal of this time is referred to as the relaxation frequency. If this frequency is exceeded, that mechanism wUl not contribute to the dielectric constant. This absorption of electrical energy by materials subjected to an AC electric held is called dielectric loss. 

An alternative method is electrokinetic focusing, which uses a direct current electric field (at high voltages of 1 kV) to focus particles and Uquids into a narrow stream. Typically, the sample fluid stream is driven along the central channel of a cross-shaped channel. As the sample enters the intersection, three fluid streams meet and the sample stream is focused into narrow stream. Dielectrophoresis (DEP) can also be used for sample focusing. DEP is the movement of polarized particles in a nonuniform electric field. This phenomenon is explained further in the next section. The main advantage of this... 

These effects are classified as either electrophoresis (EP) or dielectrophoresis (DEP). Whereas EP arises from the interaction of a fixed charge and an electric field, DEP results from the interaction of an induced charge and the spatial or temporal gradient of the electric field. In EP, direct current (DC) or low-frequency fields, usually homogeneous, are applied. On the other hand, in DEP, alternating current (AC) fields over a wide range of frequencies are used. 

Ionic Conductivities in Aqueous Solutions The thermodynamic quantities for ions in solution dealt with in the previous sections could be measured only for complete electrolytes (or for charge balanced differences between ions of the same sign) but not for individual ions. On the contrary, this is not the case for ionic conductivities (and diffusion coefficients, see Section 2.3.2.2). These can be determined experimentally for individual ions from the electrolyte conductivities and the transference numbers. The conductivity of an electrolyte solution is accurately measured with an alternating external electric field at a rate of lkHz imposed on the solution with a high impedance instrument in a virtually open circuit (zero current). The molar conductivity, Ag, can then be determined per unit concentration. Ion-ion interactions cause the conductivities of electrolytes to diminish as the concentration... 

The dielectrophoretic force expressions given above are proportional to the square of the field strength and are independent of the direction of the field. Therefore an alternating current (AC) field can be used here, unlike electrophoretic motions induced by a DC field (Pohl and Kaler, 1979). Such a time-varying nonuniform electrical field has been used to separate mixtures of whole cells. 

The source requited for aes is an electron gun similar to that described above for electron microscopy. The most common electron source is thermionic in nature with a W filament which is heated to cause electrons to overcome its work function. The electron flux in these sources is generally proportional to the square of the temperature. Thermionic electron guns are routinely used, because they ate robust and tehable. An alternative choice of electron gun is the field emission source which uses a large electric field to overcome the work function barrier. Field emission sources ate typically of higher brightness than the thermionic sources, because the electron emission is concentrated to the small area of the field emission tip. Focusing in both of these sources is done by electrostatic lenses. Today s thermionic sources typically produce spot sizes on the order of 0.2—0.5 p.m with beam currents of 10 A at 10 keV. If field emission sources ate used, spot sizes down to ca 10—50 nm can be achieved. 

Induction furnaces utilize the phenomena of electromagnetic induction to produce an electric current in the load or workpiece. This current is a result of a varying magnetic field created by an alternating current in a cod that typically surrounds the workpiece. Power to heat the load results from the passage of the electric current through the resistance of the load. Physical contact between the electric system and the material to be heated is not essential and is usually avoided. Nonconducting materials cannot be heated directiy by induction fields. 

There is an important practical distinction between electronic and dipole polarisation whereas the former involves only movement of electrons the latter entails movement of part of or even the whole of the molecule. Molecular movements take a finite time and complete orientation as induced by an alternating current may or may not be possible depending on the frequency of the change of direction of the electric field. Thus at zero frequency the dielectric constant will be at a maximum and this will remain approximately constant until the dipole orientation time is of the same order as the reciprocal of the frequency. Dipole movement will now be limited and the dipole polarisation effect and the dielectric constant will be reduced. As the frequency further increases, the dipole polarisation effect will tend to zero and the dielectric constant will tend to be dependent only on the electronic polarisation Figure 6.3). Where there are two dipole species differing in ease of orientation there will be two points of inflection in the dielectric constant-frequency curve. 

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