During the Millikan experiment used to determine the charge on the electron, a charged polystyrene microsphere is released in still air in a known vertical electric field. The charged microsphere will accelerate in the direction of the net force until it reaches terminal speed. The charge on the microsphere is determined by measuring the terminal speed. During one such experiment, the microsphere has radius r = 5.90 10-7 m, and the field has a magnitude E = 4.825 104 N/C. The magnitude of the drag force on the sphere is given by FD = 6??rv where v is the speed of the sphere and ? is the viscosity of air (? = 1.8 10-5 N · s/m2). Polystyrene has density 1.05 103 kg/m3.
(a) If the electric field is pointing down and the polystyrene microsphere is rising with a terminal speed of 1.10 10-4 m/s, what is the charge on the sphere?
C
(b) How many excess electrons are on the sphere?
electrons
(c) If the direction of the electric field is reversed but its magnitude remains the same, what is the new terminal speed?
mm/s