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A uniformly charged ring of radius 3a and total charge q is placed in xy-plane centred at 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 :

1. 2 / 1 0 2 a 4 q 5 1 m 2 ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ πε
2. 2 / 1 0 2 a 4 q 15 2 m 2 ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ πε
3. 2 / 1 0 2 a 4 q 15 1 m 2 ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ πε
4. 2 / 1 0 2 a 4 q 15 4 m 2 ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ πε

Correct answer: 2 / 1 0 2 a 4 q 15 2 m 2 ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ πε

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