Bài 1:
a. \(\left\{{}\begin{matrix}R=\dfrac{U^2}{P}=\dfrac{220^2}{1760}=27,5\Omega\\P=P_{dm}=1760W\end{matrix}\right.\)
b. Ta có: \(\left\{{}\begin{matrix}Q_{thu}=mc\Delta t=4\cdot4200\cdot70=1176000\left(J\right)\\Q_{toa}=A=Pt=1760\cdot14\cdot60=1478400\left(J\right)\end{matrix}\right.\)
\(\Rightarrow H=\dfrac{Q_{thu}}{Q_{toa}}100\%=\dfrac{1176000}{1478400}100\%\approx79,54\%\)
c. Điện năng:
\(A=Pt=1760\cdot\dfrac{14}{60}\cdot30=12320\)Wh = 12,32kWh
Tiền điện:
\(1280\cdot12,32=15769,6\left(dong\right)\)
Bài 4:
Chiều dài:
\(R=p\dfrac{l}{S}\Rightarrow l=\dfrac{RS}{p}=\dfrac{50\cdot0,33\cdot10^{-6}}{1,1\cdot10^{-6}}=15m\)
Ta có: \(\left\{{}\begin{matrix}U_b=U-U_d=12-6=6V\\I_b=I_d=I=0,4A\end{matrix}\right.\)
\(\Rightarrow R_b=\dfrac{U_b}{I_b}=\dfrac{6}{0,4}=15\Omega\)