b
થતું કાર્ય \( = \,\,\Delta U\,\, = \,\,\frac{1}{{4\pi \,\,{ \in _0}}}\,\,\left[ {\frac{{{q_2}{q_3}}}{{{r_2}}}\,\, + \,\,\frac{{{q_1}{q_3}}}{{{r_1}}}} \right]\)

\( = \,\,\frac{1}{{4\pi \,\,{ \in _0}}}\,\,\left[ {\frac{{1\,\, \times \,\,1}}{1}\,\, + \,\,\frac{{( - 1)\,\, \times \,\,(1)}}{2}} \right]\,\, \times \,{10^{ - 12}}\,\,\)

\(= \,\,9\,\, \times \,\,{10^9}\, \times \,\,\frac{1}{2}\,\, \times \,\,{10^{ - 12}}\,\, = \,\,4.5\,\, \times \,\,{10^{ - 3}}\,J\)