Final answer:
The Millikan oil-drop experiment assumes that the oil drop is a perfect sphere and that the oil density is uniform, which are necessary for calculations related to the drop's mass, volume, and surface area.
Step-by-step explanation:
The question refers to the estimations made in the Millikan oil-drop experiment to determine the charge of an electron. In this experiment, two common assumptions related to the thickness of an oil drop are made:
- The oil drop is assumed to be a perfect sphere, which simplifies the calculation involving its volume and surface area.
- The density of the oil is considered to be uniform, allowing the mass to be estimated from the known volume and density.
These assumptions are critical for calculating the electric force required to balance gravity and suspend the oil drop motionless between two electric plates. Furthermore, by knowing the force applied, the charge on the oil drop can be calculated.