Consider a uniformly charged ring in the xy plane, centered at the origin. The ring has radius a and positive charge q distributed evenly along its circumference. What is the direction of the electric field at any point on the z axis?

Figure

What is the magnitude of the electric field along the positive z axis? Use k in your answer, where k=14πϵ0.

Imagine a small metal ball of mass m and negative charge −q0. The ball is released from rest at the point (0,0,d) and constrained to move along the z axis, with no damping. If 0<d≪a, what will be the ball's subsequent trajectory?
 

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