Two thin coaxial rings,each of radius $a$ and having charges $+Q$ and $-Q$ respectively,are separated by a distance $s$. The potential difference between the centres of the two rings is:

  • A
    $\frac{Q}{2 \pi \varepsilon_{0}}\left[\frac{1}{a}+\frac{1}{\sqrt{s^{2}+a^{2}}}\right]$
  • B
    $\frac{Q}{4 \pi \varepsilon_{0}}\left[\frac{1}{a}+\frac{1}{\sqrt{s^{2}+a^{2}}}\right]$
  • C
    $\frac{Q}{4 \pi \varepsilon_{0}}\left[\frac{1}{a}-\frac{1}{\sqrt{s^{2}+a^{2}}}\right]$
  • D
    $\frac{Q}{2 \pi \varepsilon_{0}}\left[\frac{1}{a}-\frac{1}{\sqrt{s^{2}+a^{2}}}\right]$

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