Ohm’s law states that the current density \( \overrightarrow{j} \) is proportional to the electric field \( \overrightarrow{E} \), with the conductivity \( \sigma \) as the constant of proportionality: \[ \overrightarrow{j} = \sigma \overrightarrow{E} \] This expresses the relationship between the electric field and current density inside a conductor.