Alternating current electric field effects

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... 

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. 

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. 

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. 

Alternating-current motors are classified as induction motors or synchronous motors. Faraday found that a stationaiy wire in a magnetic field produced no current. However, when the wire continues to move across magnetic lines of force, it produces a continual current. When the motion stops, so does the current. Thus Faraday proved that electric current is only produced from relative motion between the wire and magnetic field. It is called an induced current—an electromagnetic induction effect. 

Anderson (1992) and Davis (1992) have noted that because all ER fluids have some electrical conductivity, then for direct current (dc) or slow alternating current (ac) electric fields, charges will migrate to the particles, eventually completely screening out the dipoles within the particles. If the electrical conductivity of the medium,, differs from that of the particles, Up, then there will still be an effective particle polarizability but its magnitude... 

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