Question

The direction of the electric field in Millikan's oil drop experiments acts.

• A. Downwards
• B. Upwards
• C. First upwards then downwards
• D. First downwards, then upwards

Answer 1

Answer: (C)

First upwards then downwards

In Millikan's oil drop experiment, the motion of oil drop is due to gravitational force and then the motion is due to electric field.

In the first case, when the oil drop moves downwards due to gravitational force, the viscous force acts in upward direction opposing its motion.

When the electric field is switched on, and if the upper plate is positively charged and the lower plate is negatively charged(equivalently electric field is downwards), the oil drop will move upward, assuming the charge of oil drop to be negative.

The voltage was adjusted until the drops were suspended in mechanical equilibrium, indicating that the electrical force and the gravitational force were balanced. Now using the known electric field, Millikan and Fletcher could determine the charge on the oil droplet.