V=\(\dfrac{m}{D}=\dfrac{0,39}{7800}=5.10^{-5}m^3\)(1)
Mà V=S.l=5.10-5m3
R=\(\dfrac{U}{I}=\dfrac{36}{1,5}=24\Omega\)
Mặt khác Ta có \(R=p.\dfrac{l}{S}=12.10^{-8}.\dfrac{l}{S}=24\Omega\)
=> \(\dfrac{l}{S}=2.10^8\) => l=2.108.S
Từ 1, 2 => V=l.S=2.108.S.S=5.10-5=>S=5.10-7m2
=> l=100m
b) Ta có S=R2.3,14=5.10-7=>R=0,399cm