a, ta có \(F=k.\dfrac{\left|q_1q_2\right|}{r^2}\)
\(\Leftrightarrow0,288=9.10^9.\dfrac{\left|q_2.2.10^{-7}\right|}{0,05^2}\Rightarrow q_2=4.10^{-7}\)
q2 dương vì 2 điện tích đẩy nhau
b, \(\dfrac{0,288}{4}=9.10^9.\dfrac{2.10^{-7}.4.10^{-7}}{0,05^2.\varepsilon}\Rightarrow\varepsilon=4\)