Ions in a solution are subject to a uniform electric field. If each io

Ions in a solution are subject to a uniform electric field. If each ion carries charge q and has radius R, then the ionic current I due to the applied field will depend on q and R as :

I ∝ qR<sup>2</sup>
I ∝ q<sup>2</sup>R
I ∝ qR<sup>0</sup>
I ∝ q<sup>2</sup>R<sup>-1</sup>
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UPSC Geoscientist – 2024
Ions in a solution move under the influence of an electric field (drift velocity), and this motion constitutes an ionic current. The drift velocity is proportional to the electric force and inversely proportional to the viscous drag force.
The electric force on an ion is F_e = qE. Assuming Stokes’ law for the viscous drag force in a solution, F_d = 6πηRv, where η is viscosity, R is the ion’s radius, and v is its drift velocity. In steady state, F_e = F_d, so qE = 6πηRv. The drift velocity is v = qE / (6πηR). The ionic current I is proportional to the charge of each ion (q), the number density of ions (n), their drift velocity (v), and the cross-sectional area (A): I = nqvA.
Substituting the expression for drift velocity, I ∝ q * (qE/R) * A (assuming n, E, η, A are constant). Thus, I ∝ q²/R = q²R⁻¹.
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