Copper and aluminum are being considered for a high-voltage transmission line that must carry a current of 56.4 a. the resistance per unit length is to be 0.181 ω/km. the densities of copper and aluminum are 8960 and 2600 kg/m3, respectively. compute (a) the magnitude j of the current density and (b) the mass per unit length λ for a copper cable and (c) j and (d) λ for an aluminum cable.