Answer
See answers.
Work Step by Step
Use P=IV to find the current, I = P/V. The voltage, power, current, and the strength of the magnetic field are rms values, since it is not a DC situation, but AC.
Use the equation for the magnetic field of a long, current-carrying straight wire.
$$B_{wire}=\frac{\mu_oI}{2\pi r}=\frac{\mu_o P}{2\pi r V}$$
$$ =\frac{4\pi\times10^{-7}(46\times 10^6 W)}{2\pi(13m)(240000V)}=2.9\times10^{-6}T $$
Compare the strength of this field to the Earth’s field.
$$\frac{B_{wire}}{B_{Earth}}=\frac{2.9\times10^{-6}T }{0.5\times10^{-4}T }=0.058$$
The strength of the field caused by the power line is about 6 percent of the strength of the Earth’s field.
