(b) $F = kq_{1}\ q_{2}/r^{2},i.e.,$ $F = \frac{9 \times 10^{9} \times 1.6 \times 10^{- 19} \times 1.6 \times 10^{- 19}}{\left( 2.5 \times 10^{- 11} \right)^{2}} = 3.7 \times 10^{- 7}N$
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Ph.D. in Physics with 19+ years of teaching experience.