A charged dust particle of mass m = 10-8 g is in a uniform electrostatic field between two horizontal plates, of which the lower one is charged to a potential φ1 = 3 kV, and the upper one is charged to a potential φ2 = -3 kV. The distance between the plates is d = 5 cm. A speck of dust, being initially at a distance d0 = 1 cm from the lower plate, reaches the upper plate in time t = 0.1 s. Find the charge of a speck of dust. What charge must a speck of dust have in order to remain in balance?

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