\(C=2\pi R=2\pi.0,1=0,2\pi\left(m\right)\Rightarrow N=\dfrac{l}{C}=\dfrac{50}{0,2\pi}=\dfrac{250}{\pi}\left(vong\right)\)
\(B=\dfrac{N.2\pi.10^{-7}.I}{r}\Leftrightarrow9.10^{-4}=\dfrac{\dfrac{250}{\pi}.2\pi.10^{-7}.I}{0,1}\Leftrightarrow I=...\left(A\right)\)