$A$ uniformly charged ring of radius $3a$ and total charge $q$ is placed in the $xy$-plane centered at the origin. $A$ point charge $q$ is moving towards the ring along the $z$-axis and has speed $v$ at $z = 4a$. The minimum value of $v$ such that it crosses the origin is

  • A
    $\sqrt {\frac{2}{m}} {\left( {\frac{1}{5}\frac{{{q^2}}}{{4\pi { \in _0}a}}} \right)^{1/2}}$
  • B
    $\sqrt {\frac{2}{m}} {\left( {\frac{1}{15}\frac{{{q^2}}}{{4\pi { \in _0}a}}} \right)^{1/2}}$
  • C
    $\sqrt {\frac{2}{m}} {\left( {\frac{4}{15}\frac{{{q^2}}}{{4\pi { \in _0}a}}} \right)^{1/2}}$
  • D
    $\sqrt {\frac{2}{m}} {\left( {\frac{2}{15}\frac{{{q^2}}}{{4\pi { \in _0}a}}} \right)^{1/2}}$

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