Bài 3:
\(\left\{{}\begin{matrix}R=\dfrac{U^2}{P}=\dfrac{220^2}{1000}=48,4\Omega\\I=\dfrac{P}{U}=\dfrac{1000}{220}\approx4,54A\end{matrix}\right.\)
\(H=\dfrac{Q_{thu}}{Q_{toa}}100\%=>Q_{toa}=\dfrac{Q_{thu}}{H}100\%=\dfrac{5\cdot4200\cdot75}{80}100\%=1968750J\)
Ta có: \(Q_{toa}=A=Pt=>t=\dfrac{Q_{toa}}{P}=\dfrac{1968750}{1000}=1968,75\left(s\right)\)
Bài 4:
\(\left\{{}\begin{matrix}I=\dfrac{P}{U}=\dfrac{1100}{220}=5A\\R=\dfrac{U^2}{P}=\dfrac{220^2}{1100}=44\Omega\end{matrix}\right.\)
\(Q_{thu}=mc\Delta t=10\cdot4200\cdot80=3360000J\)
\(H=\dfrac{Q_{thu}}{Q_{toa}}1005=>Q_{toa}=\dfrac{Q_{thu}}{H}100\%=\dfrac{3360000}{85\%}100\%=3952941,176J\)
Ta có: \(Q_{toa}=A=Pt=>t=\dfrac{Q_{toa}}{P}=\dfrac{3952941,176}{1100}\approx3593,6\left(s\right)\)
\(A=Pt=1100\cdot1\cdot30=33000\)Wh = 33kWh
\(=>T=A\cdot1500=33\cdot1500=49500\left(dong\right)\)
Bài 5:
\(\left\{{}\begin{matrix}I=\dfrac{P}{U}=\dfrac{1000}{220}=\dfrac{50}{11}A\\R=\dfrac{U^2}{P}=\dfrac{220^2}{1000}=48,4\Omega\end{matrix}\right.\)
\(Q_{toa}=A=Pt=1000\cdot15\cdot60=900000J\)
\(H=\dfrac{Q_{thu}}{Q_{toa}}100\%=>Q_{thu}=Q_{toa}\cdot H\cdot100\%=900000\cdot85\%\cdot100\%=765000J\)
Ta có: \(Q_{thu}=mc\Delta t\Leftrightarrow765000=m\cdot4200\cdot70=>m\approx2,6\left(kg\right)\approx2,6\left(l\right)\)
\(A=Pt=1000\cdot1\cdot30=30000\)Wh = 30kWh
\(=>T=A\cdot1500=30\cdot1500=45000\left(dong\right)\)
