# Millikan's oil drop experiment-terminal velocity.

• Jenkz
In summary, the conversation discusses finding the terminal velocity and time taken for an oil drop with a radius of 1x10^-6 m to fall 5mm under zero field conditions. The equations used are Stokes' law for drag, weight = mass x gravity, and time = distance/velocity. The calculated results raise some doubt due to the tendency of small particles to fly in air rather than drop down like stones.
Jenkz

## Homework Statement

Find the terminal velocity of an oil drop with radius 1x10^-6 m at 20 degress under zero field conditions. How long would it take to fall 5mm (range of microscope graticule)?

## Homework Equations

Terminal velocity; Weight = Drag
Weight = mg

Assuming Stokes law holds; Drag = 6(pi)rηv
v - terminal velocity
η -viscosity of air, 1.832×10–5 Pa s (at 20 degrees)

Mass, m = (4/3) pi (r^3) $$\rho$$
$$\rho$$ = Density of oil, 874 kgm^-3 (at 20 degrees)

time= distance/ velocity

## The Attempt at a Solution

Mass = 3.66 x 10^-15 Kg
v = (mg) / (6(pi)rη) = 1.04 x10^-4 ms^-1

time = 48s

I'd say the result is theorically true.
Small particles tends to "fly" in air rather than drop down like stones.

In real world the particle will actually fly away following some slow air flow generated eg. from one near hot lamp, or by the body of someone near.

## 1. What is Millikan's oil drop experiment?

The Millikan's oil drop experiment was a scientific experiment conducted by Robert Millikan in 1909 to determine the charge of an electron. He used tiny oil droplets and observed their motion in an electric field.

## 2. How did Millikan measure the charge of an electron in his experiment?

Millikan used an atomizer to spray oil droplets into a chamber filled with air. He then applied an electric field to the chamber and observed the motion of the droplets. By adjusting the strength of the electric field, he could balance the force of gravity acting on the droplets and determine their terminal velocity. From this, he was able to calculate the charge on the droplets and, therefore, the charge of an electron.

## 3. What is terminal velocity in Millikan's oil drop experiment?

Terminal velocity is the constant speed that an object reaches when the forces acting on it are balanced. In Millikan's experiment, this is the speed at which the upward force of the electric field equals the downward force of gravity on the oil droplets.

## 4. What is the significance of Millikan's oil drop experiment?

Millikan's experiment was significant because it provided the first direct measurement of the charge of an electron. This was a crucial step in understanding the structure of the atom and the behavior of subatomic particles.

## 5. What are some modern-day applications of Millikan's oil drop experiment?

Millikan's oil drop experiment is still relevant today and has been used to measure the charge of other particles, such as ions and molecules. It is also the basis for many modern-day experiments and technologies, such as mass spectrometry, which is used in various fields like chemistry, biology, and environmental science.

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