Question

Two equally charged identical small balls kept some fixed distance apart exert a repulsive force F on each other. A similar uncharged ball, after touching one of them is placed at the mid-point of line joining the two balls. Force experienced by the third ball is :

• A. $4F$
• B. $2F$
• C. $F$
• D. $\frac{F}{2}$

Answer 1

Answer: (C)

$F$

Then a similar uncharged ball, after touching one of them is placed at centre then force experienced by $3$rd ball.

solution: The charge will be equally distributed an $3$rd body, after touching to $1$st body.

$F=\frac{k{Q}^2}{r^2}$

$F_1=\frac{k{Q}^2/2}{{\left(\frac{r}{2}\right)}^2}=\frac{2k{Q}^2}{r^2}$

$F_2=\frac{k{\left(\frac{Q}{2}\right)}^2}{{\left(\frac{r}{2}\right)}^2}=\frac{k{Q}^2}{r^2}$

$F_{net}=F_1-F_2=\frac{2k{Q}^2}{r^2}-\frac{k{Q}^2}{r^2}=\frac{k{Q}^2}{r^2}=F$.