Đáp án:

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Giải thích các bước giải:

câu 1: 
\[{Q_1} = \frac{{{U^2}}}{{{R_1}}} = \frac{{{U^2}}}{{\rho .\frac{l}{s}}} = \frac{{{U^2}}}{{\rho .\frac{l}{{\pi .{{(\frac{d}{2})}^2}}}}}\]

\[{Q_2} = \frac{{{U^2}}}{{{R_2}}} = \frac{{{U^2}}}{{\rho .\frac{{{l_2}}}{{{s_2}}}}} = \frac{{{U^2}}}{{\rho .\frac{{\frac{l}{2}}}{{\pi .{{(\frac{{2d}}{2})}^2}}}}}\]

=>
\[\frac{{{Q_1}}}{{{Q_2}}} = \frac{{{R_2}}}{{{R_1}}} = 2\]

câu 2: 
\[{I_1} = \frac{U}{{{R_1}}} = \frac{U}{{{\rho _1}.\frac{{{l_1}}}{{{s_1}}}}}\]
\[{I_2} = \frac{U}{{{R_2}}} = \frac{U}{{{\rho _2}.\frac{{{l_2}}}{{{s_2}}}}}\]

=>
\[\frac{{{I_1}}}{{{I_2}}} = \frac{{{R_2}}}{{{R_1}}} = \frac{{{\rho _2}.\frac{{{l_2}}}{{{s_2}}}}}{{{\rho _1}.\frac{{{l_1}}}{{{s_1}}}}} = \frac{{0,{{4.10}^{ - 6}}}}{{0,{{5.10}^{ - 6}}}}.\frac{{\frac{l}{{\frac{5}{4}{s_2}}}}}{{\frac{l}{{{s_2}}}}} = 0,64\]