Bài 4:
\(R_{tđ}=\dfrac{R_1.R_2}{R_1+R_2}=\dfrac{40.60}{40+60}=24\left(\Omega\right)\)
Do mắc song song nên \(U=U_1=U_2=I.R_{tđ}=0,5.24=12\left(V\right)\)
\(\left\{{}\begin{matrix}I_1=\dfrac{U_1}{R_1}=\dfrac{12}{40}=0,3\left(A\right)\\I_2=\dfrac{U_2}{R_2}=\dfrac{12}{60}=0,2\left(A\right)\end{matrix}\right.\)
\(P=U.I=12.0,5=6\left(W\right)\)
\(A=P.t=6.60.60=21600\left(J\right)\)
Bài 5:
\(R_{tđ}=\dfrac{R_1.R_2}{R_1+R_2}=\dfrac{3.6}{3+6}=2\left(\Omega\right)\)
Do mắc song song nên \(U=U_1=U_2=18V\)
\(\left\{{}\begin{matrix}I=\dfrac{U}{R_{tđ}}=\dfrac{18}{2}=9\left(A\right)\\I_1=\dfrac{U_1}{R_1}=\dfrac{18}{3}=6\left(A\right)\\I_2=\dfrac{U_2}{R_2}=\dfrac{18}{6}=3\left(A\right)\end{matrix}\right.\)
\(R_2=\rho\dfrac{l}{S}\Rightarrow S=\dfrac{\rho.l}{R}=\dfrac{1,6.10^{-6}.10}{6}\approx2,67.10^{-6}\left(m^2\right)\)
