Problem 4.4: Determine the charge on a drop in the Millikan Oil Drop Apparatus

Voltage Control:

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The animation illustrates a charged particle in a Millikan oil drop apparatus (position is given in mm and time in seconds).  When the voltage between the plates is off, the drop falls freely with a constant speed because it quickly reaches terminal velocity due to air resistance.  When the voltage is on, the droplet is suspended between the plates. Some constants used in this animation that may be useful: density of the oil is 875 kg/m3, density of air is 1 kg/m3, and the viscosity of air in this animation is 7 × 10−6 N·s/m2. The droplet is not shown to scale to make it visible. Restart.

  1. If the voltage required to suspend the drop is 280 V, how many excess electrons does the drop have?
  2. If the same drop were to only have one excess electron, what voltage would be required?