1. The problem statement, all variables and given/known data A square of side a located in the x-y plane and centered on the origin carries a total charge Q uniformly distributed over its circumference. (a) What is the electric eld at any point on the z-axis? How does the eld behave far from the square? (b) An electron constrained to move along the z-axis near the center of the square is seen to exhibit small oscillations above and below the plane of the square. What is the frequency of these oscillations? (assume Q is positive) 2. Relevant equations So for the first part I got E=KQz/[(x^2+y^2)^2sqrt((x^2+y^2)^2+(a/2)^2)] For the behavior far from the square I made z>>a/2 and got back E=kQ/b^2 I am stuck on part B. The force is just q times the field, and z<<a/2 but I don't know where to go to get the period. I appreciate any help.