Đáp án:
$\begin{array}{l}
a.\dfrac{{{Q_1}}}{{{Q_2}}} = \dfrac{{{l_1}}}{{{l_2}}}\\
b.\dfrac{{{Q_1}}}{{{Q_2}}} = \dfrac{{{l_2}}}{{{l_1}}}
\end{array}$
Giải thích các bước giải:
a. Khi mắc nt thì:
$\left\{ \begin{array}{l}
{Q_1} = {I^2}.{R_1}.t = {I^2}.\rho .\dfrac{{{l_1}}}{S}.t\\
{Q_2} = {I^2}.{R_2}.t = {I^2}.\rho .\dfrac{{{l_2}}}{S}.t
\end{array} \right. \Leftrightarrow \dfrac{{{Q_1}}}{{{Q_2}}} = \dfrac{{{I^2}.\rho .\dfrac{{{l_1}}}{S}.t}}{{{I^2}.\rho .\dfrac{{{l_2}}}{S}.t}} = \dfrac{{{l_1}}}{{{l_2}}}$
b. Khi mắc // thì:
$\left\{ \begin{array}{l}
{Q_1} = \dfrac{{{U^2}}}{{{R_1}}}.t = \dfrac{{{U^2}}}{{\rho .\dfrac{{{l_1}}}{{{S_1}}}}}.t\\
{Q_2} = \dfrac{{{U^2}}}{{{R_2}}}.t = \dfrac{{{U^2}}}{{\rho .\dfrac{{{l_2}}}{S}}}.t
\end{array} \right. \Leftrightarrow \dfrac{{{Q_1}}}{{{Q_2}}} = \dfrac{{\dfrac{{{U^2}}}{{\rho .\dfrac{{{l_1}}}{S}}}.t}}{{\dfrac{{{U^2}}}{{\rho .\dfrac{{{l_2}}}{S}}}.t}} = \dfrac{{{l_2}}}{{{l_1}}}$
