a
(a) Change in potential energy (\(\Delta U\))\( = Uf -Ui\)
\(==>\) \(\Delta U = \frac{1}{{4\pi {\varepsilon _0}}}\left[ {\left( {\frac{{{q_1}{q_3}}}{{0.4}} + \frac{{{q_2}{q_3}}}{{0.1}}} \right) - \left( {\frac{{{q_1}{q_3}}}{{0.4}} + \frac{{{q_2}{q_3}}}{{0.5}}} \right)} \right]\)
\(==>\) \(\Delta U = \frac{1}{{4\pi {\varepsilon _0}}}[8{q_2}{q_3}] = \frac{{{q_3}}}{{4\pi {\varepsilon _0}}}(8{q_2})\)
\(k = 8q_2\)
