Doubt from Electrostatics

Two thin wire rings each having a radius R are placed at a distance d apart with their axes coinciding. The charges on the two rings are +q and -q. The potential difference between the centers of the two rings is

JEE Main 2005

( A ) QR/4\pi\epsilon_0 d^2
( B ) \frac{Q}{2\pi\epsilon_0}[\frac{1}{R}-\frac{1}{\sqrt{R^2+d^2}}]
( C ) zero
( D ) \frac{Q}{4\pi\epsilon_0}[\frac{1}{R}-\frac{1}{\sqrt{R^2+d^2}}]

Answer: ( B )

A cone made of insulating material has a total charge Q spread uniformly over it’s sloping surface . Calculate work done in bringing a test charge q from infinity to the apex of the cone , the cone has slope length L .
@PrathamChopra @LaveshGupta @Gaurav_Singh @Aastik_Guru

We just have to calculate the potential at A by integration to get the answer , trying
Answer : qQ/2πe°L
e° = epsilon

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