A spring with an unstrained length of 0.073 m and a spring constant of 2.2 N/m hangs vertically downward from the ceiling. A uniform electric field directed upward fills the region containing the spring. A sphere with a mass of 5.1 x 102 kg and a net charge of +7.1 uC is attached to the lower end of the spring. The spring is released slowly, until it reaches equilibrium. The equilibrium length of the spring is 0.063 m. What is the magnitude of the external electric field? L f [.D