<Dd> I = ∫ J → ⋅ d A → (\ displaystyle I = \ int (\ vec (J)) \ cdot d (\ vec (A))) </Dd> <P> where I (\ displaystyle I) is current in the conductor, J → (\ displaystyle (\ vec (J))) is the current density, and d A → (\ displaystyle d (\ vec (A))) is the differential cross-sectional area vector . </P> <P> The current density (current per unit area) J → (\ displaystyle (\ vec (J))) in materials with finite resistance is directly proportional to the electric field E → (\ displaystyle (\ vec (E))) in the medium . The proportionality constant is called the conductivity σ (\ displaystyle \ sigma) of the material, whose value depends on the material concerned and, in general, is dependent on the temperature of the material: </P> <Dl> <Dd> J → = σ E → (\ displaystyle (\ vec (J)) = \ sigma (\ vec (E)) \,) </Dd> </Dl>

Where does current flow from positive to negative