Bài 1:
\(MCD:R_1nt\left(R_2//R_3\right)\)
\(\Rightarrow R_{td}=R_1+\dfrac{R_2R_3}{R_2+R_3}=15+\dfrac{10\cdot10}{10+10}=20\Omega\)
\(\Rightarrow I=I_1=I_{23}=\dfrac{U}{R}=\dfrac{12}{20}=0,6A\)
Ta có: \(U_{23}=U_2=U_3=I_{23}R_{23}=0,6\cdot5=3V\)
\(\Rightarrow\left\{{}\begin{matrix}I_2=\dfrac{U_2}{R_2}=\dfrac{3}{10}=0,3A\\I_3=I_2=0,3A\end{matrix}\right.\)
Bài 2:
Ta có: \(\dfrac{I_1}{I_2}=\dfrac{U_1}{U_2}\)
\(\Rightarrow I_2=\dfrac{I_1U_2}{U_1}=\dfrac{0,2\cdot\left(3+12\right)}{12}=0,25A\)
Bài 3:
Ta có: \(P=UI\)
\(\Rightarrow I=\dfrac{P}{U}=\dfrac{3}{6}=0,5A\)
Vậy với cường độ dòng điện là 0,5A thì đèn sáng bình thường.
Chọn C
Bài 4:
Ta có: \(\dfrac{R_1}{R_2}=\dfrac{S_2}{S_1}\)
\(\Rightarrow S_2=\dfrac{R_1S_1}{R_2}=\dfrac{8,5\cdot0,5}{127,5}\approx0,0\left(3\right)\left(mm^2\right)\)
Bài 5:
\(MCD:R_1ntR_2ntR_3\)
\(\Rightarrow R=R_1+R_2+R_3=10+20+30=60\Omega\)
Ta có: \(I=I_1=I_2=I_3=1,5A\)
\(\Rightarrow\left\{{}\begin{matrix}U_1=I_1R_1=1,5\cdot10=15V\\U_2=I_2R_2=1,5\cdot20=30V\\U_3=I_3R_3=45V\end{matrix}\right.\)
