Ta có: F=k.\(\dfrac{q_1q_2}{r^2}\)
<=> 0,1=9.109.\(\dfrac{10^{-7}.4.10^{-7}}{r^2}\)
<=> R = 0,06m= 6cm
Ta có: F=qE <=> 2.10-4 =q.25
=> q = 8.10-6
\(F=k.\dfrac{\left|q_1.q_2\right|}{r^2}\Leftrightarrow9.10^9.\dfrac{\left|10^{-7}.4.10^{-7}\right|}{r^2}=0,1\)
\(\Leftrightarrow r=0.06m=6cm\)