Bài 1:

Ta có: 

\(\begin{array}{l}
{U_{BA}} = E.AB = E.BC.\cos 60\\
 \Rightarrow 120 = E.0,12.\dfrac{1}{2}\\
 \Rightarrow E = 2000\left( {V/m} \right)
\end{array}\)

Bài 2: 

a) Ta có: 

\(\begin{array}{l}
{R_{12}} = \dfrac{{{R_1}{R_2}}}{{{R_1} + {R_2}}} = \dfrac{{6.30}}{{6 + 30}} = 5\Omega \\
R = {R_{12}} + {R_3} = 5 + 5 = 10\Omega \\
I = \dfrac{\xi }{{R + r}} = \dfrac{{24}}{{10 + 2}} = 2A
\end{array}\)

b) Ta có: 

\(\begin{array}{l}
{R_d} = \dfrac{{U_{dm}^2}}{{{P_{dm}}}} = \dfrac{{{{12}^2}}}{6} = 24\Omega \\
{R_{1d}} = \dfrac{{{R_1}{R_d}}}{{{R_1} + {R_d}}} = \dfrac{{6.24}}{{6 + 24}} = 4,8\Omega \\
R = {R_{1d}} + {R_3} = 4,8 + 5 = 9,8\Omega \\
I = \dfrac{E}{{R + r}} = \dfrac{{24}}{{9,8 + 2}} = 2,03A\\
{Q_3} = {I^2}{R_3}t = 2,{03^2}.5.3600 = 74461,36J
\end{array}\)