R = 20Ω
d = 2mm = 0,2cm = 0,002m
L = 100m
R = ρ * [tex]\frac{L}{S}[/tex] => ρ = [tex]\frac{R*S}{L}[/tex]
S = [tex]\pi[/tex][tex]r^{2}[/tex]
r = [tex]\frac{d}{2}[/tex]
r = [tex]\frac{0,002}{2}[/tex] = 0,001m
S = 3,14*(0,001)² = 0,00000314m²
ρ = [tex]\frac{20*0,00000314}{100}[/tex] = 6,28*[tex]10^{-7}[/tex] Ω*m