# Millikan oil drop problem

• BobRoss
In summary, the problem involves determining the magnitude of the charge on an oil drop suspended between two horizontal plates with a potential difference of 980 V and a distance of 9.40 cm. The electric field strength and net force are calculated, and the charge is found to be 1.16x10-17 C, which may not be an exact multiple of the elementary charge due to experimental error. The question of why the oil drop is suspended between opposite charged plates in the Millikan experiment is raised, prompting the suggestion to draw a Free Body Diagram to consider the forces at play.

## Homework Statement

A 1.50x10-14 kg oil drop accelerates downwards at a rate of 1.80 m/s2 when placed between two horizontal plates that are 9.40 cm apart. The potential difference between the two plates is 980 V. Determine the magnitude of the charge on the oil drop.

|E|=V/d
Fnet=ma
Fg=mg
Fe=q|E|

## The Attempt at a Solution

So first I find the strength of the electric field.

|E|=V/d
=(980 V)/(0.094 m)
|E|=1.04x104V/m

Then I find the net force.
Fnet=ma
=(1.50x10-14kg)(-1.80 m/s2)
=-2.70x10-14N

Then I find the force of gravity on the drop.
Fg=mg
=(1.50x10-14kg)(-9.81m/s2)
=-1.47x10-13N

Here is where I think I am going wrong. I try to find the electric force.
Fnet=Fg+Fe
Fe=fnet-Fg
=(-2.70x10-14N)-(-1.4715x10-13N)
=1.20x10-13N

Then I find the charge.
Fe=q|E|
q=Fe/|E|
=(1.2015x10-13N)/(1.04x104N/C
=1.16x10-17 C

Now this answer is obviously wrong. Where am I messing up?

Doesn't the charge have to be an integer multiple of the elementary charge (1.60x10-19C) ?

BobRoss said:
Doesn't the charge have to be an integer multiple of the elementary charge (1.60x10-19C) ?

Sure. But what experiment has perfect accuracy?

Divide by the fundamental charge and see how close you come to a round number.

Okay, so then my original answer is correct?

BobRoss said:
Doesn't the charge have to be an integer multiple of the elementary charge (1.60x10-19C) ?

well if you will ever do the actual Millikan experiment you will find that you will not exactly get 1.6x10^-19, but around 1.6x10^-19 ± some error x10^-17

BobRoss said:
Okay, so then my original answer is correct?

Your method and numbers look fine to me.

Great, thanks for the help!

What happen If we place negative charge between parallel opposite charge plate ?
answer is negative charge attract toward positive plate...

Then why Oil drop is suspended btw opposite plates in Millikan's oil drop experiment??

What happen If we place negative charge between parallel opposite charge plate ?
answer is negative charge attract toward positive plate...

Then why Oil drop is suspended btw opposite plates in Millikan's oil drop experiment??

What forces are operating on the oil drop? Draw a Free Body Diagram.

## 1. What is the Millikan oil drop problem?

The Millikan oil drop problem, also known as the Millikan experiment, is a famous physics experiment conducted by Robert Millikan in 1909. It involved measuring the charge of an electron by observing the motion of charged oil droplets in an electric field.

## 2. Why is the Millikan oil drop problem significant?

The Millikan oil drop problem is significant because it provided the first accurate measurement of the charge of an electron. This was an important step in understanding the fundamental structure of atoms and paved the way for further research in quantum mechanics.

## 3. How did Millikan conduct the experiment?

Millikan used a device called an oil drop apparatus, which consisted of two parallel metal plates with a small hole in the top plate. He sprayed oil droplets into the apparatus and then applied an electric field to the plates. By measuring the speed of the droplets as they fell, he was able to calculate the charge of each droplet and determine the charge of an electron.

## 4. What challenges did Millikan face in his experiment?

One of the main challenges Millikan faced was ensuring that the oil droplets were truly charged and that the charge was constant throughout the experiment. He also had to deal with air currents and fluctuations in temperature, which could affect the motion of the droplets.

## 5. Has the Millikan oil drop problem been replicated and confirmed?

Yes, the Millikan oil drop problem has been replicated and confirmed by numerous scientists over the years. However, there have been some criticisms of Millikan's methods and calculations, and more recent experiments have refined the value of the charge of an electron to a more accurate measurement.