a, ta có \(R_1=\rho.\dfrac{l}{S}\Leftrightarrow2=2.10^{-8}.\dfrac{l}{1.10^{-7}}\Rightarrow l=10\left(m\right)\)
b, R1ntR2 \(\Rightarrow R_{tđ}=2+6=8\left(\Omega\right)\Rightarrow I=\dfrac{U}{R_{tđ}}=\dfrac{12}{8}=1,5\left(A\right)\)
\(\Rightarrow U_2=1,5.6=9\left(V\right)\)\(\Rightarrow P_2=I^2.R_2=1,5^2.6=13,5\left(W\right)\)
c, với P2'=16W ta có gọi R2=x
\(\dfrac{P_2'}{x}=I^2\left(1\right)\)
mà \(I=\dfrac{U}{R_1+R_2}=\dfrac{12}{x+2}\left(2\right)\)
từ (1) và (2) \(\Rightarrow16x^2-80x+64=0\Rightarrow\left\{{}\begin{matrix}x=1\left(loại\right)\\x=4\end{matrix}\right.\)
vậy \(R_2=4\Omega\Rightarrow I=\dfrac{12}{4+2}=2\left(A\right)\)
