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Two charges are moved so that the distance between them becomes three times the original value. The new electrostatic force is

A$9F$
B$3F$
C$F/3$
D$F/9$
Answer & Solution
Correct answer: D. $F/9$
Using Coulomb's law, $F \propto \dfrac{1}{r^2}$. If the distance becomes $3r$, then $F' = \dfrac{F}{3^2} = \dfrac{F}{9}$. **Hence the new force is $F/9$.**
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