Four identical particles, each having charge +q, are fixed at the corners of a square of side L. A fifth point charge -Q lies a distance z along the line perpendicular to the plane of the square and passing through the center of the square (Fig. P23.66)
(a) Show that the force exerted by the other four charges on -Q is Note that this force is directed toward the center of the square whether z is positive (-Q above the square) or negative (-Q below the square).
(b) If z is small compared with L, the above expression reduces to F = - (constant) zk. Why does this imply that the motion of the charge -Q is simple harmonic, and what is the period of this motion if the mass of -Q is m?

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+9 +9 I. +4