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 :
E and V at a point P that lies on the axis of ring -
Potential energy Per unit charge -
S.I unit is .
Use energy conservation