# Millikan's Oil-Drop Experiment: Is my reasoning correct?

• Mason Smith
In summary, to show that the electric field needed to make the rise time of the oil drop equal to its field free time is ε = (2mg)/q, the forces of gravity and buoyancy were added for the oil drop falling when the electric field is "off." When the electric field is "on," the sum of the forces must be equal to the force of gravity.
Mason Smith

## Homework Statement

Show that the electric field needed to make the rise time of the oil drop equal to the its field free time is
ε = (2mg)/q

## Homework Equations

Newton's second law F = ma
The force of gravity FG = mg, where g = 9.8 ms-2
The force of buoyancy Fb = bv
The force of an electric field on a charge ε = F/q, where ε = the strength of the electric field and q = the charge of the particle

## The Attempt at a Solution

First, I added the forces for the oil drop falling when the electric field is "off." I added the buoyant force that results from terminal velocity and the force of gravity.
F = ma = 0 = bv - mg = 0 [1]
This can be rearranged to
bv = mg [2]

Second, I reasoned that when the electric field is "on," the sum of the forces must be
F = ma = εq - mg = mg [3]
I rewrote this as
ε = (2mg)/q

Was my approach correct for solving this problem?

The "force from the velocity" is not related to buoyancy, it is just drag. If you take it into account for the downwards motion you should also do it for the upwards motion to keep the symmetry.

## 1. How does Millikan's oil-drop experiment work?

The experiment involved suspending tiny oil droplets between two charged plates and observing their behavior under the influence of an electric field. By measuring the droplets' movement, Millikan was able to calculate the charge of an individual electron.

## 2. Why is Millikan's oil-drop experiment important in the history of science?

Millikan's experiment is considered one of the most important in the history of science because it provided the first direct measurement of the charge of an electron, which was a crucial step in understanding the structure of atoms and the concept of electric charge.

## 3. Can Millikan's oil-drop experiment be replicated today?

Yes, Millikan's experiment has been replicated numerous times by scientists and students alike. With modern technology and equipment, the experiment can be performed with even greater accuracy and precision.

## 4. What were the criticisms of Millikan's oil-drop experiment?

Some of the criticisms of Millikan's experiment included his use of a limited number of droplets, the assumption that all droplets had the same mass, and the possibility of systematic errors in his measurements. However, these criticisms have been addressed and do not discredit the overall significance of the experiment.

## 5. How did Millikan's oil-drop experiment contribute to our understanding of the atomic structure?

Millikan's experiment provided evidence for the existence of subatomic particles, specifically electrons, and their charge. This helped to confirm the modern atomic model of a nucleus surrounded by orbiting electrons, and also paved the way for further discoveries in the field of particle physics.

Replies
52
Views
4K
Replies
6
Views
2K
Replies
12
Views
1K
Replies
3
Views
2K
Replies
2
Views
3K
Replies
10
Views
5K
Replies
5
Views
2K
Replies
26
Views
2K
Replies
7
Views
3K
Replies
1
Views
1K