a) La resistència R1 . [0,5 punt] \begin{align} &\cssId{Step1}{R_1= 16 * 10^-9 Ω m * 20 m * \frac {1}{ \pi * r^2}}\\[3px] &\cssId{Step2}{R_1= 16 * 10^-9 Ω m * 20 m * \frac {1}{ \pi * (\frac{d}{2})^2}=16 * 10^-9 Ω m * 20 m * \frac {1}{ \pi * (\frac{0.0001}{2}})^2=40,74 Ω }\\[3px] \end{align}