The relation between the electric field E inside a conductor and the current density j, written in terms of conductivity $\sigma$, is

1. The microscopic form of Ohm's law is $\mathbf{E} = \rho \mathbf{j}$. 2. Conductivity is defined as $\sigma = 1/\rho$. 3. Substituting gives $\mathbf{j} = \sigma \mathbf{E}$. 4. Option B uses $\sigma$ where $\rho$ belongs; the others are dimensionally inconsistent. _Source: syllabus Physics Ch 3 "Current Electricity", p.4_