Movements under the Influence of an Applied Electric Field

Ionic Movements under the Influence of an Applied Electric Field 

In seeking an atomic view of the process of conduction, one approach is to begin with the picture of ionic movements as described in the treatment of diffusion (Section 4.2.4) and then to consider how these movements are perturbed by an electric field. In the treatment of ionic movements, it was stated that the ions in solution perform a random walk in which all possible directions are equally likely for any particular step. The analysis of such a random walk indicated that the mean displacement of ions is zero (Section 4.2.4), diffusion being the result of the statistical bias in the movement of ions, due to inequalities in their numbers in different regions. 

however, the ions are situated in an electric field, their movements are affected by the fact that they are charged. Hence, the imposition of an electric field singles out one direction in space (the direction parallel to the field) for preferential ionic movement. Positively charged particles will prefer to move toward the negative electrode, and negatively charged particles, in the opposite direction. The walk is no longer quite random. The ions drift. 

Another way of looking at ionic drift is to consider the fate of any particular ion under the field. The electric force field would impart to it an accelaation according to Newton s second law. Were the ion completely isolated (e.g., in vacuum), it would accelerate indefinitely until it collided with the electrode. In an electrolytic solution, however, the ion very soon collides with some other ion or solvent molecule that crosses its path. This collision introduces a discontinuity in its speed and direction. The motion of the ion is not smooth it is as if the medium offers resistance to the motion of the ion. Thus, the ion stops and starts and zigzags. However, the applied electric field imparts to the ion a direction (that of the oppositely charged electrode), and the ion gradually works its way, though erratically, in the direction of this electrode. The ion drifts in a preferred direction. 

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