Câu hỏi 41

Theo bài ra ta có

$\Delta U = I.R = \frac{P}{U}.\rho .\frac{l}{s} = \frac{P}{U}.\rho .\frac{{2d}}{s}$

Ta có

$\eqalign{
& \Delta U \le 1\% U \Leftrightarrow {P \over U}.\rho .{{2d} \over s} \le 1\% U \cr
& \Leftrightarrow s \ge {1 \over {1\% U}}.{P \over U}.\rho .2d \Leftrightarrow s \ge {1 \over {1000}}.{{{{5.10}^6}} \over {{{100.10}^3}}}.1,{7.10^{ - 8}}{.2.5.10^3} \cr
& \Leftrightarrow s \ge 8,{5.10^{ - 6}}{m^2} = 8,5m{m^2} \cr} $