How long does it take electrons to get from the car battery to the starting motor? assume the current is 143 a and the electrons travel through copper wire with cross sectional area 40.2 mm2 and length 84.5 cm . the mass density of copper is 8960 kg/m3 and the molar mass is 63.5 g/mol . avogadro’s number is 6.022 × 1023 . assume that each copper atom contributes one conduction electron.