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(B) Let the charges on the two balls be $q$ and $q$,and the distance between them be $r$. The initial force is $F = k \frac{q^2}{r^2}$.When an uncharg

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$F$

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(B) Let the charges on the two balls be $q$ and $q$,and the distance between them be $r$. The initial force is $F = k \frac{q^2}{r^2}$.When an uncharged ball touches one of the charged balls,the charge $q$ is shared equally between them. Thus,the charge on the first ball becomes $q/2$ and the charge on the third ball becomes $q/2$.The third ball is placed at the midpoint,so its distance from both balls is $r/2$.The force on the third ball due to the first ball is $F_1 = k \frac{(q/2)(q/2)}{(r/2)^2} = k \frac{q^2/4}{r^2/4} = k \frac{q^2}{r^2} = F$ (directed away from the first ball).The force on the third ball due to the second ball is $F_2 = k \frac{(q/2)(q)}{(r/2)^2} = k \frac{q^2/2}{r^2/4} = 2k \frac{q^2}{r^2} = 2F$ (directed away from the second ball).Since these forces are in opposite directions,the net force on the third ball is $F_{net} = |F_2 - F_1| = |2F - F| = F$.

Two equally charged small balls placed at a fixed distance experience a force $F$. $A$ similar uncharged ball after touching one of them is placed at the middle point between the two balls. The force experienced by this ball is:

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