Câu 1: 

Ta có: 

\(\begin{array}{l}
{q_1} + {q_2} = {5.10^{ - 6}}C \Rightarrow {q_1} = {5.10^{ - 6}} - {q_2}\\
F = \dfrac{{k\left| {{q_1}{q_2}} \right|}}{{{r^2}}} \Rightarrow 5,4 = \dfrac{{{{9.10}^9}.{q_2}\left( {{{5.10}^{ - 6}} - {q_2}} \right)}}{{0,{1^2}}}\\
 \Rightarrow {6.10^{ - 12}} = {q_2}\left( {{{5.10}^{ - 6}} - {q_2}} \right)\\
 \Rightarrow \left[ \begin{array}{l}
{q_2} = {3.10^{ - 6}}C\\
{q_2} = {2.10^{ - 6}}C
\end{array} \right. \Rightarrow \left[ \begin{array}{l}
{q_1} = {2.10^{ - 6}}C\\
{q_1} = {3.10^{ - 6}}C
\end{array} \right.
\end{array}\)

Câu 2: 

Ta có: 

\(\begin{array}{l}
{E_A} = \dfrac{{k\left| q \right|}}{{r_A^2}}\\
{E_B} = \dfrac{{k\left| q \right|}}{{r_B^2}}\\
 \Rightarrow \dfrac{{{E_A}}}{{{E_B}}} = {\left( {\dfrac{{{r_B}}}{{{r_A}}}} \right)^2} \Rightarrow \dfrac{{{{10}^4}}}{{5625}} = {\left( {\dfrac{{{r_B}}}{{{r_A}}}} \right)^2}\\
 \Rightarrow \dfrac{{{r_B}}}{{{r_A}}} = \dfrac{4}{3} \Rightarrow {r_B} = \dfrac{4}{3}{r_A}\\
AB = \sqrt {r_A^2 + r_B^2}  = \dfrac{5}{3}{r_A}\\
{r_M} = \dfrac{{AB}}{2} = \dfrac{5}{6}{r_A}\\
{E_M} = \dfrac{{k\left| q \right|}}{{r_M^2}}\\
 \Rightarrow \dfrac{{{E_M}}}{{{E_A}}} = {\left( {\dfrac{{{r_A}}}{{{r_M}}}} \right)^2} = {\left( {\dfrac{{{r_A}}}{{\dfrac{5}{6}{r_A}}}} \right)^2} = {\left( {\dfrac{6}{5}} \right)^2}\\
 \Rightarrow {E_M} = 14400\left( {V/m} \right)
\end{array}\)

Câu 3: 

a) Số mol đồng trong \(1{cm^3}\) là: 

\({n_{mol}} = \dfrac{{DV}}{A} = \dfrac{{8,94.1}}{{63,546}} = 0,141\left( {mol} \right)\)

Mật độ e là: 

\(\begin{array}{l}
n = {n_{mol}}.{N_A} = 0,141.6,{023.10^{23}} = 8,{49.10^{22}}\left( {e/c{m^3}} \right)\\
 \Rightarrow n = 8,{49.10^{28}}\left( {e/{m^3}} \right)
\end{array}\)

b) Vận tốc trôi là: 

\(\begin{array}{l}
v = \dfrac{I}{{n\left| e \right|S}} = \dfrac{1}{{8,{{49.10}^{28}}.1,{{6.10}^{ - 19}}.3,14.{{\left( {\dfrac{{{{2.10}^{ - 3}}}}{2}} \right)}^2}}}\\
 \Rightarrow v = 2,{34.10^{ - 5}}\left( {m/s} \right)
\end{array}\)