In the Millikan oil droplet experiment, the oil is sprayed from an atomizer into a chamber.

Question

The droplets are allowed to pass through the hole into the chamber so that their fall can be observed. The top and bottom of the chamber consist of electrically charged plates. The upper plate is positively charged, and the lower plate is negatively charged. X rays are introduced into the chamber so that when they strike the oil droplets, the droplets will acquire one or more negative charges. The electric field (voltage) is applied to the metal plates.

 

 

Which of the following applies? Can have more than one answer.
In the presence of an electric field, the negatively charged oil droplet moves freely toward the negatively charged plate.

In the absence of an electric field, the oil droplet falls freely due to the gravitational force.

If Fe is greater than Fg , the negatively charged oil droplet will move freely toward the negatively charged plate.

If Fe is increased until it is equal to Fg , the negatively charged oil droplet will remain stationary.

Answer 1

In this experiment, you end up with little droplets that are negatively charged falling down because of gravity. If gravity were the only force, the droplets would eventually fall to the bottom of the apparatus.

 

But, the apparatus is designed so that the bottom plate is negatively charged. If you recall, like repels like, and the closer the negatively charged drop approaches the negatively charged plate, the stronger the repulsion.

 

So, you have two forces

1) gravity pulls the droplets down

2) electrical repulsion pushes the droplets back up

 

If the two forces are equal, the droplets will be stationary, as equal and opposite forces cancel each other out.

 

If you were to increase the negative charge (either on the droplet or the plate), the repulsion would increase and the droplets would be pushed back up.

 

If you were to decrease the negative charge (either on the droplet or the plate), the repulsion would weaken and the droplets would fall further down.

 

If you were to turn off the plate, then the repulsion would cease and the only acting force would be gravity and the droplets would fall to the bottom plate.

 

I hope this clears up this experiment. You might want to check YouTube. There are several videos there that explain Milikan's oil drop experiment in detail with good diagrams.