Lorentz forces create voltage gradients in all conducting liquids moving through magnetic fields as a result of nonhomogeneous distributions of charge carriers. In metallic liquids, the charge carriers are electrons that move through the liquids, pass over the liquid-solid electrode interfaces, and continue on through the external metering circuits. In nonmetallic liquids, the charge carriers are ions. From an electric charge standpoint, the ionic distribution of fig.10.8 is the same as the electron distribution of fig.10.6.
Ions, however, cannot pass over liquid-solid interfaces. As a result, the passage of even the minute currents needed to service metering circuits quickly accumulate a concentration of ions at each electrode. Positive ions accumulate at one electrode and negative ions at the other. This ionic accumulation is called polarization. Polarization creates voltage gradients at the interfaces that block the flow of measuring currents. It also offsets Lorentz forces thereby effectively eliminating signal voltages.
The blocking effect of polarization is reduced if alternating magnetic fields are used instead of nonvarying fields. Such oscillating fields must have half cycles that change direction fast enough to limit the accumulation of ions. Alternating fluxes of high frequency, however, are likely to induce voltages of considerable magnitude into various parts of metering circuits.
Any type of conducting fluid can be used to generate the Lorentz voltages described in the previous sections. Gases, on the other hand, are relatively nonconducting media with high source impedances, making if difficult to extract useful signals. If a gas is heated until it becomes a plasma, however, the situation becomes quite different. In a plasma, electrons are stripped from the molecules, leaving a seething mass of free electrons and positive ions. As shown in fig.10.9, these carriers can be separated by Lorentz forces in the conventional manner.
The conductivity of a plasma is much higher than that of any metal. As a result, the transfer function of velocity to voltage is very efficient, and this technique offers a promising approach to power generation. A major problem is that of containing the very-high-temperature plasmas.
The extraction of heavy currents from such a system affects the placement of voltage- collecting electrodes, resulting in two different types of magnetohydrodynamic generators.
In a Faraday Magnetohydrodynamic Generator, current flows in response to the Lorentz voltages generated at right angles to plasma flow. Several pairs of electrodes are placed at right angles to plasma flow, and each pair is used to drive an individual load.
In a Hall Magnetohydrodynamic Generator, current flows in response to the Hall voltages generated at right angles to the Lorentz currents. In such generators, the generated voltage is the vector sum of the Hall and Lorentz voltages. Voltage-collecting electrodes are placed at oblique angles to plasma flow, and the collected voltages are operated in series to drive one load.
When a plasma moves at high speed, the electrons tend to move faster than the ions. The velocity difference is called Ion Slip. Ion slip creates a voltage that is axial to the direction of plasma flow.
[All above taken from "Handbook of Magnetic Phenomena" by Harry E. Burke 1986]
Le CHATELIER-BROWN PRINCIPLE
The Le Chatelier-Brown principle should also be considered in regard to liquid flow, as it describes an electrokinetic phenomenon: if motion occurs under the influence of an electric field, then an electric field must be formed by motion (in the presence of an electrokinetic potential).
[Taken from "Principles of Electrochemistry" (2nd ed) by Jiri Koryta et al (1993) p243]