The current density in a cylindrical wire of radius 4mm is 4 × 10 ^...

Current density is defined as current per unit cross-section area. Therefore,

$i = JA = J \times \pi\left({R}^{2} - \dfrac{{R}^{2}}{4}\right)$

$⇒ i = 4 \times {10}^{6} \times \pi \times 16 \times {10}^{- 6} \times \dfrac{3}{4}$

$⇒ i = 48\piA$