Question

Three charges $-q, \, Q$ and $-q$ are placed at equal distances on a straight line. If the potential energy of the system of three charges is zero, then what is the ratio of $Q:q$

• A. $1:1$
• B. $1:2$
• C. $1:3$
• D. $1:4$

Answer 1

Answer: (D)

$U=\frac{- k Q q}{r}\times 2+\frac{k q^{2}}{2 r}=0$
$2\times \frac{k Q q}{r}=\frac{k q^{2}}{2 r}$
$2\times \frac{Q}{1}=\frac{q}{2}$
$\frac{Q}{q}=\frac{1}{4}$