Bài 3:
\(MCD:R1nt\left(R2//R3\right)\)
\(=>R=R1+\dfrac{R2\cdot R3}{R2+R3}=30+\dfrac{15\cdot10}{15+10}=36\Omega\)
\(I=I1=I23=\dfrac{U}{R}=\dfrac{24}{36}=\dfrac{2}{3}A\)
\(U23=U2=U3=I23\cdot R23=\dfrac{2}{3}\cdot\dfrac{15\cdot10}{15+10}=4V=>\left\{{}\begin{matrix}I2=U2:R2=4:15=\dfrac{4}{15}A\\I3=\dfrac{U3}{R3}=4:10=\dfrac{2}{5}A\end{matrix}\right.\)
\(A=UIt=24\cdot\dfrac{2}{3}\cdot5\cdot60=4800J\)
Bài 4:
\(R=\dfrac{U^2}{P}=\dfrac{220^2}{880}=55\Omega\)
\(I=\dfrac{P}{U}=\dfrac{880}{220}=4A\)
\(A=UIt=220\cdot4\cdot5\cdot60=264000J\)
Bài 5:
\(Q=I^2Rt=2,5^2\cdot80\cdot15=7500J\)
\(Q_i=mc\Delta t=4200\cdot1,5\cdot75=472500J\)
\(Q'=I^2Rt'=2,5^2\cdot80\cdot25\cdot60=750000J\)
\(=>H=\dfrac{Q_i}{Q'}100\%=\dfrac{472500}{750000}100\%=63\%\)
