(a) $V = 9 \times 10^{9}.\frac{p}{r^{2}}$ $= 9 \times 10^{9} \times \frac{(1.6 \times 10^{- 19}) \times 1.28 \times 10^{- 10}}{\left( 12 \times 10^{- 10} \right)^{2}} = 0.13V$
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