Answer
2.0 volts
Work Step by Step
Use equation 18–3 to find the resistance. The cross-sectional area $A=\pi r^2=\frac{\pi d^2}{4}$.
$$R=\rho\frac{\mathcal{l}}{A}=\rho\frac{4 \mathcal{l}}{\pi d^2}$$
Calculate the voltage drop by combining this with Ohm’s law, equation 18–2.
$$V=IR=I\rho\frac{4 \mathcal{l}}{\pi d^2}$$
$$ =(12A)(1.68\times10^{-8}\Omega \cdot m)\frac{4(21m)}{\pi (1.628\times10^{-3}m)^2}$$
$$ =2.0 volts$$
