\(I=\dfrac{U}{R}=\dfrac{90}{60}=1,5A\)

Chọn B

B

Câu 13:

\(\left\{{}\begin{matrix}U1=I1.R1=4.15=60V\\U2=I2.R2=3.30=90V\end{matrix}\right.\)

\(\Rightarrow U_{max}=U1+U2=60+90=150V\)

Câu 8: \(I=\dfrac{U}{R}=\dfrac{U}{\dfrac{R1.R2}{R1+R2}}=\dfrac{24}{\dfrac{20.30}{20+30}}=2A\)

Câu 9: \(R_{12}=\dfrac{R1.R2}{R1+R2}=\dfrac{12.18}{12+18}=7,2\Omega\)

a. \(I=I1=I2=\dfrac{U}{R}=\dfrac{220}{200}=1,1A\left(R1ntR2\right)\)

\(\Rightarrow\left\{{}\begin{matrix}Q_1=I_1^2R_1t=1,1^2.120.1.3600=522720\left(J\right)\\Q_2=I_2^2R_2t=1,1^2.80.1.3600=348480\left(J\right)\end{matrix}\right.\)

b. \(Q=UIt=220.1,1.1.3600=871200\left(J\right)\)

\(Q>Q_1>Q_2\)

\(20cm=0,2m\)

\(\dfrac{R1}{R2}=\dfrac{l1}{l2}\Rightarrow l2=\dfrac{R2.l1}{R1}=\dfrac{15.0,2}{5}=0,6\left(m\right)\)

\(I=I1=I2=\dfrac{U}{R}=\dfrac{22,5}{45}=0,5A\left(R1ntR2\right)\)

\(\Rightarrow\left\{{}\begin{matrix}U1=I1.R1=0,5.30=15V\\U2=I2.R2=0,5.15=7,5V\end{matrix}\right.\)

\(300mA=0,3A\)

\(\Rightarrow R=\dfrac{U}{I}=\dfrac{18}{0,3}=60\Omega\)

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B

\(R=\dfrac{U}{I}=\dfrac{12}{2,4}=5\Omega\)

\(R=p\dfrac{l}{S}\Rightarrow S=\dfrac{p.l}{R}=\dfrac{2,8.10^{-8}.200}{5}=1,12.10^{-6}\left(m^2\right)=1,12\left(mm2\right)\)

Chọn A