A ring of radius R has charge −q distributed uniformly over it. Calculate the charge that should be placed at the centre of the ring such that the electric field becomes zero at a point on the axis of the ring at a distance R from the centre of the ring.
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Electric field point at point P due to charge of ring is E=(R2+x2)3/2kQx At x=R;E=22R2kQ, directed towards the centre Electric field at P due to charge at centre: R2kq For net field to be zero at P: R2kq=22R2kQ⇒q=22Q