An electron beam carries a total current of -500 μA in the σ_x direction and has a current density J_z in the region 0 ≤ r ≤ 10^-4 m and zero in the region r > 10^-4 m. Electron beam velocity is given by V_z = (8 × 10⁷ z)m/s. Calculate the volume charge density at z = 2 cm.