Copper and aluminum are being considered for a high-voltage transmission line that must carry a current of 48.1 A. The resistance per unit length is to be 0.156 O/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 l for a copper cable and (c)J and (d)l for an aluminum cable.