Điện trở mỗi đèn: \(P=\dfrac{U^2}{R}\Rightarrow\left\{{}\begin{matrix}R_1=\dfrac{U_1^2}{P_1}=\dfrac{12^2}{6}=24\Omega\\R_2=\dfrac{U_2^2}{P_2}=\dfrac{12^2}{9}=16\Omega\end{matrix}\right.\)
\(MCD:R_1//R_2\)
\(\Rightarrow R=\dfrac{R_1R_2}{R_1+R_2}=\dfrac{24\cdot16}{24+16}=9,6\Omega\)
\(\Rightarrow I=\dfrac{U}{R}=\dfrac{12}{9,6}=1,25A\)
Ta có: \(\left\{{}\begin{matrix}I_1=\dfrac{U_1}{R_1}=\dfrac{12}{24}=0,5A\\I_2=\dfrac{U_2}{R_2}=\dfrac{12}{16}=0,75A\end{matrix}\right.\)\(\Rightarrow\) Đèn 2 sáng hơn đèn 1
\(MCD:R_1ntR_2\)
\(\Rightarrow R'=R_1+R_2=24+16=40\Omega\)
\(\Rightarrow I=I_1=I_2=\dfrac{12}{40}=0,3A\)
Ta có: \(\left\{{}\begin{matrix}I_1=\dfrac{P_1}{U_1}=\dfrac{6}{12}=0,5A\\I_2=\dfrac{P_2}{U_2}=\dfrac{9}{12}=0,75A\end{matrix}\right.\)\(\Rightarrow\) cả 2 đèn đều sáng quá mức
\(P=UI=12\cdot0,3=3,6\)W
