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 )**