Đáp án:

\(4,29\left( {V/m} \right)\)

Giải thích các bước giải:

Ta có:

\(\begin{array}{l}
{E_1} = \dfrac{{kQ}}{{O{A^2}}}\\
{E_2} = \dfrac{{kQ}}{{O{B^2}}}\\
 \Rightarrow \dfrac{{{E_1}}}{{{E_2}}} = {\left( {\dfrac{{OB}}{{OA}}} \right)^2} \Rightarrow 3,125 = {\left( {\dfrac{{OB}}{{OA}}} \right)^2}\\
 \Rightarrow \dfrac{{OB}}{{OA}} = \dfrac{{5\sqrt 2 }}{4} \Rightarrow OB = \dfrac{{5\sqrt 2 }}{4}.OA\\
{E_M} = \dfrac{{kQ}}{{O{M^2}}} = \dfrac{{kQ}}{{{{\left( {OA + \dfrac{{AB}}{2}} \right)}^2} + O{A^2}}}\\
 \Rightarrow {E_M} = \dfrac{{kQ}}{{{{\left( {OA + \dfrac{{OB - OA}}{2}} \right)}^2} + O{A^2}}} = \\
 \Rightarrow {E_M} = \dfrac{{32}}{{65 + 20\sqrt 2 }}.\dfrac{{kQ}}{{O{A^2}}} = 0,343{E_1} = 4,29\left( {V/m} \right)
\end{array}\)