Two positive point charges of $12\,\mu C$ and $8\,\mu C$ are $10\,cm$ apart. The work done in bringing them $4\, cm$ closer is
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(c) $W = {U_f} - {U_i} = 9 \times {10^9} \times {Q_1}\,{Q_2}\left[ {\frac{1}{{{r_2}}} - \frac{1}{{{r_1}}}} \right]$
$==>$ $W = 9 \times {10^9} \times 12 \times {10^{ - 6}} \times 8 \times {10^{ - 6}}\left[ {\frac{1}{{4 \times 10{\,^{ - 2}}}} - \frac{1}{{10\, \times 10{\,^{ - 2}}}}} \right]$ $= 12.96 \,J ≈ 13 \,J$
$==>$ $W = 9 \times {10^9} \times 12 \times {10^{ - 6}} \times 8 \times {10^{ - 6}}\left[ {\frac{1}{{4 \times 10{\,^{ - 2}}}} - \frac{1}{{10\, \times 10{\,^{ - 2}}}}} \right]$ $= 12.96 \,J ≈ 13 \,J$
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