## Physics for Scientists and Engineers: A Strategic Approach with Modern Physics (4th Edition)

(a) $B = 2.0\times 10^{-6}~T$ (b) 4.0%
(a) We can find the magnetic field at the point: $B = \frac{\mu_0~I}{2~\pi~r}$ $B = \frac{(4\pi\times 10^{-7}~T~m/A)~(200~A)}{(2~\pi)~(20~m)}$ $B = 2.0\times 10^{-6}~T$ (b) We can express this value as a percentage of the Earth's magnetic field: $\frac{2.0\times 10^{-6}~T}{50\times 10^{-6}~T}\times 100\% = 4.0\%$