Câu 15: B

Mạch gồm: R1 // (R4 nt (R2 // (R3 nt R5)))

Ta có: 

\(\begin{array}{l}
{R_{35}} = {R_3} + {R_5} = 10 + 20 = 30\Omega \\
{R_{235}} = \dfrac{{{R_2}{R_{35}}}}{{{R_2} + {R_{35}}}} = \dfrac{{20.30}}{{20 + 30}} = 12\Omega \\
{R_{2345}} = {R_{235}} + {R_4} = 12 + 20 = 32\Omega \\
{U_{35}} = {I_3}{R_{35}} = 2.30 = 60V\\
{I_{2345}} = \dfrac{{{U_{35}}}}{{{R_{235}}}} = \dfrac{{60}}{{12}} = 5A\\
{U_1} = {U_{2345}} = {I_{2345}}.{R_{2345}} = 5.32 = 160V
\end{array}\)

Câu 16: A

Khi mắc 3 điện trở nối tiếp:

\({R_1} + {R_2} + {R_3} = \dfrac{U}{{{I_{nt}}}} = \dfrac{9}{1} = 9\Omega \)

Khi mắc 3 điện trở song song: 

\(\begin{array}{l}
{I_{ss}} = \dfrac{U}{{{R_1}}} + \dfrac{U}{{{R_2}}} + \dfrac{U}{{{R_3}}} = 9\\
 \Rightarrow \dfrac{1}{{{R_1}}} + \dfrac{1}{{{R_2}}} + \dfrac{1}{{{R_3}}} = 1
\end{array}\)

\( \Rightarrow {R_1} = {R_2} = {R_3} = 3\Omega \)

Khi mắc (R1 // R2) nt R3:

Ta có: 

\(\begin{array}{l}
R = \dfrac{{{R_1}{R_2}}}{{{R_1} + {R_2}}} + {R_3} = \dfrac{{3.3}}{{3 + 3}} + 3 = 4,5\Omega \\
I = \dfrac{U}{R} = \dfrac{9}{{4,5}} = 2A
\end{array}\)