CGPET 2021 Solved Paper

Consider charges −q,Q‌and−q are placed at equal distance a as shown in figure

For two charges q1 and q2, electrostatic potential energy is given by
U=

kq1q2

r

(r = distance between two charges)
So, the potential energy of the given system of charges,
U=−

kqQ

a

−

kQq

a

+

kq.q

2a

According to the problem, U=0
∴ −2‌qQ−2‌Qq+q2=0⇒−2Q−2Q+q=0
∴

Q

q

=

1

4

⇒Q:q=1:4