a) Công thức tính độ lớn lực tương tác:
\(F = {{k.\left| {{q_1}{q_2}} \right|} \over {\varepsilon {r^2}}}\)
\(\eqalign{
& \left\{ \matrix{
\left| {{q_1}} \right| = \left| {{q_2}} \right| = q \hfill \cr
\varepsilon = 1 \hfill \cr} \right. \Rightarrow F = {{k.{q^2}} \over {{r^2}}} \cr
& \Rightarrow q = \sqrt {{{F.{r^2}} \over k}} = \sqrt {{{{{1,8.10}^{ - 4}}{{.0,1}^2}} \over {{{9.10}^9}}}} = \sqrt 2 {.10^{ - 8}}C \cr} \)
b)
\(\eqalign{
& \left\{ \matrix{
F = {{k.\left| {{q_1}{q_2}} \right|} \over {r_1^2}} = {1,8.10^{ - 4}}N \hfill \cr
F' = {{k.\left| {{q_1}{q_2}} \right|} \over {r_2^2}} = {2,6.10^{ - 4}}N \hfill \cr} \right. \cr
& \Rightarrow {F \over {F'}} = {{r_2^2} \over {r_1^2}} = {9 \over {13}} \Rightarrow {r_2} = {3 \over {\sqrt {13} }}.{r_1} \cr
& \Rightarrow {r_2} = {3 \over {\sqrt {13} }}.10 = {{30} \over {\sqrt {13} }}cm = 8,32cm \cr} \)
