Câu hỏi 22

Ta có:

\(\begin{array}{l}\left\{ {\begin{array}{*{20}{l}}{{P_1} = {I_1}^2.R = \frac{{{U_1}^2}}{R} = 100W}\\{{P_2} = {I_2}^2.R = \frac{{{U_2}^2}}{R} = 400W}\end{array}} \right.\\\frac{{{U_2}}}{{{U_1}}} = \frac{{U.\frac{{{N_1}}}{{{N_2}}}}}{{U\frac{{{N_2}}}{{{N_1}}}}} = {\left( {\frac{{{N_1}}}{{{N_2}}}} \right)^2} \Rightarrow \frac{{{P_2}}}{{{P_1}}} = {\left( {\frac{{{U_2}}}{{{U_1}}}} \right)^2} = {\left( {\frac{{{N_1}}}{{{N_2}}}} \right)^4} = 4\\ \Rightarrow \frac{{{N_1}}}{{{N_2}}} = \sqrt 2  \Rightarrow {U_2} = \sqrt 2 .U \Rightarrow P = \frac{{{U^2}}}{R} = \frac{{U_2^2}}{{2.R}} = \frac{{400}}{2} = 200{\rm{W}}\end{array}\)

Chọn B